3.1  THE VAALIAN AGE

 

The oldest rocks of the Vaalian age appear to be the Epupa Metamorphic Complex in the northern parts of the Kaokoveld in the north-western parts of Namibia which is intruded by the younger Kunene Anorthosite Complex with an approximate age of 2.100 millions of years (Ma). Complexes of similar kind and age occur as inliers in the Damara Orogen west of Outjo, in the Tsumeb-Grootfontein area, between Okahandja and Swakopmund as well as south and south-west of Windhoek. These metamorphic complexes consist largely of various intensely deformed granitic para- and orthogneisses. In places hornblende gneiss, amphibolite, marble, quartzite and schist can be found. These rocks, however, do not play a distinct role as building materials in road construction. The oldest rocks used as building materials are those from deposits from the Upper Mokolian age (Rehoboth and Sinclair Sequences), followed by the Namibian age (Damara Sequence). The next isotopic ages are the Cambrian, Carboniferous to Jurassic and Cretaceous ages (Karoo Sequence). The youngest isotopic ages are the Tertiary to Quaternary ages (Kalahari Group).

 

3.2  THE MOKOLIAN AGE

 

The rocks and their weathered derivatives in the Mokolian and Namibian ages are from strata aged some 1.860 to 570 Ma. Brink [7] reports that the larger part of Namibia is underlain by rocks of these Precambrian ages which are grouped into a number of litho-stratigraphic units. The most important systems of this period are as far as road building materials are concerned, the Damara Sequence and the Nama Group which are belonging to the same period than the less important Gariep Complex, all with syn- to post-tectonic granitic intrusions. All these units can be found in different areas of the country. The Damara Sequence is restricted to the northern and central while the Nama Group and Gariep Complex occur in the southern respective extreme south-western parts of Namibia. All rocks and derived materials discussed in this section are exposed to an arid climatic environment which favours mechanical disintegration as major form of weathering. Brink (1981) observed that the resulting residual soils are normally existing in thin layers with pebbly sand and silt which contain very often a large portion of mica depending on the bedrock. The formation of silcrete and calcrete is typical for these units although these pedocretic materials, formed by a pedogenic process, are situated in layers much younger than Damara and even Karoo Sequences. These pedogenic materials are of Tertiary origin with an age of between 39 and 32 Ma. The calcretes are the most important of all road construction materials in Namibia.

The oldest systems of the Mokolian age are various formations like the Abbabis, the Hohewarte, the Huab, the Grootfontein and the Neuhof formations. All these formations consist of paragneiss and various metasedimentary rocks which can be found in many parts of Namibia. Not very much is known about the road building properties of these rocks. A paragneiss of the Hohewarte Formation which can be found between Oamites and Brack has been used for the construction of the subbase of trunk road 1/5 between Rehoboth and Aris (Mho on Geological Map, 1980). The pre-Rehoboth lithostratigraphic units can be differentiated as follows:

Huab (Mhu), Grootfontein (Mgr), Abbabis (Mab), Hohewarte (Mho), Mooiriver Complexes (Mmo) as well as Neuhof Formation (Mnh).

 

3.2.1  THE KHOABENDUS GROUP

 

The Khoabendus Group which is older than 1.750 Ma is, however, younger than above oldest Mokolian formations. It is composed predominantly of a volcanic lower portion and a large sedimentary upper portion. These rocks can be found in many parts of the Kaokoveld, around Kamanjab and Otjovasandu. Granites of the Franzfontein Granitic Suite intrude the whole succession. The Elim Formation of the Rehoboth area consists of interbedded sedimentary rocks and lavas which have undergone low-grade metamorphism [8].

 

3.2.2  THE REHOBOTH AND SINCLAIR SEQUENCES

 

The Mokolian Rehoboth Sequence is ranging approximately from 1.670 Ma to 1.420 Ma of age and has three formations, the Marienhof, the Gaub Valley and the Billstein Formations. These formations consist of hard blue-grey fine quartzites, which can be found in many parts of the western Rehoboth district. Furthermore these formations are composed of phyllite, schist, conglomerate, quartz, porphyry, basalt and rhyolite. The rocks of the Rehoboth Sequence are very often intruded by younger rocks, mainly granite, syenite, diorite and gabbro. These gneissic-granitic injections into the older Rehoboth Sequence can be found in many parts of Namibia, as for instance in the Gamsberg area, at the Namib border around the farm Namibgrens and other. The Rehoboth Sequence lithostratigraphic units can be differentiated as follows: Marienhof (Mm), Gaub Valley (Mvy) and Billstein Formations (Mbi).

For road building purposes the rocks from the Rehoboth Sequence have been used only for randomly selected wearing-courses for gravel roads in the western parts of the Rehoboth area. The Rehoboth Sequence granites, which intruded much older formations, like for instance the Khoabendus Group and the para-gneisses of the pre-Khoabendus formations, have been used for some surfaced roads in the vicinity of Rehoboth. The trunk roads 1/4 from Rehoboth to Tsumis and 1/5 from Rehoboth to the Wortel junction have basecourses consisting of these old granites (Mgg (Gamsberg Suite) on the Geological Map, 1980).

The Sinclair Sequence is the youngest sequence in the Mokolian age and reaches into the early Namibian age. This sequence accumulated in the Helmeringhausen-Solitaire area during three broad cycles of volcanism, plutonism and sedimentation between about 1.300 Ma and 1.000 Ma ago. It consists of five different formations : the Nagatis (Mna), Kunjas (Mku), Barby (Mb),Guperas (Mg) and Aubures (Ma) Formations. The Aubures Formation is subdivided into three subformations: Grauwater (Mga), Doornpoort and Eskadron (Md) as well as Klein Aub (Mka). The four older formations consist mainly of rhyolite, conglomerate, quartzite, basalt, shale, andesite, arkose, felsite, tuff and porphyry.

These formations have not been tested systematically for any road construction material properties. Parallel to these formations the Namaqualand belt of metamorphism and granitisation with syntectonic ages of 1.200 Ma and cooling ages of 900 Ma is stretching.

The Mokolian Namaqua Complex (Mp, Mgb and Mgw) consists of granites and gneiss and can be found in the areas between Lüderitzbucht and Aus, between Warmbad and the Fish River and in the area around Grünau. This material has quite favourable road construction properties and the granites and gneiss of this complex have been used for the construction of the basecourse for trunk road 4/2 between Aus and Lüderitzbucht and for coarse concrete aggregate (Codes 11, 13 and 14 on Ann.Table 1; Mgw, Mgb and Mp on the Geological Map, 1980).

The youngest of the formations of the Sinclair Sequence is the Aubures Formation which contains mainly quartzite, conglomerate, argillite, shale, basalt, rhyolite and ignimbrite. The Aubures Formation consists of four sub-formations, the Grauwater, the Doornpoort, the Eskadron and the Klein Aub Sub-formations, the Sinclair equivalents in the Rehoboth-Witvley area (Geological Map: 1980).

The Doornpoort Sub-formation consists of red quartzites and conglomerates. These rocks can be found in the area of Witvley and on the farm Gravenstein. The Doornpoort red quartzite has been used for crusher-run basecourse for trunk road 6/1 between Witvley and Gobabis and for coarse concrete aggregates (Code 40 on Ann.Table 1, Md on Geological Map, 1980).

 

3.3  THE NAMIBIAN AND CAMBRIAN AGES (DAMARA, GARIEP AND NAMA)

 

Brink (1981) reports that most of the rocks of the Namibian Damara Sequence are situated in the central and northern parts of Namibia between Windhoek and Tsumeb. The basal arenitic succession with local evaporitic rocks and alkyne ignimbrites of the Nosib Group, the oldest group in the Damara Sequence was laid down 850 and 700 Ma ago. Widespread carbonate deposition followed (Swakop Group, the second oldest group in the Damara Sequence) and interbedded mica and graphitic schist, quartzite as well as massflow deposits point to a variable, unstable condition south of a stable platform where mainly carbonates occur (Otavi Group, the northern equivalent to the Swakop Group). Near the southern margin of the orogen deep water-fans, facies equivalents of the northern carbonates were deposited on either side of an ocean forming the Auas Formation and the Tinkas Member. Thick schistose metagreywacke and metapelite (Kuiseb Formation) overlie the above rocks and contain a narrow 350 km-long zone of interbedded oceanic greenstones (Matchless Member). Deformation, apparently as a consequence of a continental collision about 650 Ma ago, was accompanied by:

1. Deposition of arenite and pelite of the Mulden Group (youngest group in the Damara Sequence) in the north above the Otavi carbonates and the upper parts of the Nama Group in the south.

2. Syntectonic intrusion of serpentinites along the southern margin.

3. Syn-to post-tectonic intrusion of Cambrian granites with ages of approximately 650 to 470 Ma.

Paired metamorphic belts, an asymmetric structure, an intensely thrusted southern margin and the Naukluft Nappe Complex, overlying Nama Group rocks, developed.

The succession, predominantly consisting of quartzites, schists, limestones, dolomites and marbles, is quite variable as far as its lithology is concerned. These varieties resulted in the subdivision of the Damara rocks into three major zones (Brink:1981):

1. The northern zone is stretching in an arc from Tsumeb over Outjo and Kamanjab to Ruacana at the border between Namibia and Angola.

2. The central zone is stretching between Otavi in the north and Okahandja in the south with a branch extending to the north-west along the Skeleton Coast to the harbour town of Namibe in Angola.

3. The southern zone is stretching from Okahandja to Windhoek with the Khomas Hochland between Windhoek and Swakopmund the main out-crop area of the Damara Sequence in this zone.

 

3.3.1  DAMARA SEQUENCE: NORTHERN ZONE

 

The Namibian Damara Sequence: Northern zone can be divided in three groups with further subgroups and formations:

1. Nosib Group (Nn) with two formations: Varianto (Nv) and Askevold (Nas)

2. Otavi Group (No, stratigraphy undifferentiated) with subgroups:

2.1 Abenab Subgroup (Na)

2.2 Tsumeb Subgroup (Nt) with one formation Chuos (Nc)

3. Mulden Group (Nm)

Brink (1981) reports that at the base the Nosib Group is prevailing and contains mainly arenaceous rocks and conglomerates. The overlying Otavi Group is composed of dolomite and limestone with intercalations of quartzite and shale. At the top the Mulden Group consists mainly of quartzite and conglomerate but also contains substantial intercalations of shale, marl and phyllite.

The Nosib Complex can be encountered in the Chuos Mountains south-east of Usakos and in the eastern parts of the Kaokoveld. This formation consists of quartzite, sandstone, limestone and arenaceous rocks. Nosib quartzite is for instance exposed in the excavations of main road 92 between the junction with main road 67 (Kamanjab-Ruacana) and the end of the road at the 'Hippo Pool' of the Kunene River. The redish rock is medium-grained to coarse-grained and contains layers and lenses of conglomerate. Despite a variable content of felspar this rock is little affected by weathering (Brink:1981). Because of this, extensive use of explosives had to be made during the construction of these excavations on main road 92 (Code 61 on Ann.Table 1; Nn on the Geological Map, 1980). Nosib quartz was used as road construction material for the fill and the selected layers as well as for concrete aggregates for the building of main road 67 between Kamanjab and Ruacana in 1969/70 ( Code 61 on Ann.Table 1; Nn on Geological Map, 1980). In the extreme north-east of Namibia a small deposit of Nosib quartzite forms the only potential source of basecourse crusher-run material, surfacing chips and coarse concrete aggregate in the Okavango and Western Caprivi Strip (Codes 50, 51, 52 and 55 on Ann.Table 1; Nn on Geological Map, 1980).

The dolomites of the Otavi Group are showing typical karst features such as the forming of caverns in the rock and the occurrence of dolines and sinkholes below the surface (Brink:1981). One characteristic occurrence is on the records of the Department of Transport. After the good rainy season 1973/74 some small sinkholes developed in the road reserve of trunk road 1/8 between Otavi and Tsumeb on farm Ubi Bene 768. The remedial measures to solve this sinkhole problem consisted of filling up the holes and to establish a proper surface drainage around them to avoid the accumulation of rain water in the road reserve.

The Otavi Group dolomite is also quarried at Tsumeb, Abenab, on the farm Gabus 52 north of Otavi, at Outjo and between Kamanjab and Opuwa (Codes 36, 37, 38, 44, 45, 46, 47, 48, 53, 54, 57, 58, 59, 60, 62, 63, 64 and 68 on Ann.Table 1; Na, Nc and Nt on Geological Map, 1980). This dolomite is used extensively as road building material, especially for surfacing chips and for coarse concrete aggregates, in the 'rockless' regions of Ovamboland, Okavango, Bushmanland and Hereroland where no suitable coarse aggregates are available from local sources. The physical properties of commercially quarried dolomite of the Otavi Group is shown in Table 1 (will be translated into HTM Language in the future):

 

TABLE 1  PHYSICAL PROPERTIES OF QUARRIED DOLOMITE: OTAVI GROUP

|==================================================================|
| LOCATION OF SOURCE | RELATIVE | AGGREGATE CRUSHING | FLAKINESS % |
|
                        |   DENSITY   |       VALUE %           | ON 19 mm SIEVE  |
|-------------------------|-------------|-------------------------|-----------------|
| Gabus (Otavi)           |  2,85-2,95  |       22 - 23           |      20 - 25    |
| Abenab (Grootfontein)   |  2,85-2,95  |       15 - 18           |      25 - 30    |
| Tsumeb (Mine)           |  2,85-2,95  |       15                |      15 - 30    |
| Outjo                   |  2,85-2,95  |       17 - 23           |       20 - 25   |
|===================================================================================|

NOTA to Table 1: Aggregate Crushing Value: TMH 1 [90]: Method B1: Aggregate Crushing Value of an aggregate is determined by crushing a prepared confined aggregate sample under a specified, gradually applied compressive load and determining the percentage of the material crushed finer than a specified size.

Flakiness Index: TMH 1 [90]: Method B3: Flakiness Index of a coarse aggregate is determined by gauging screened-out fractions with appropriate slots which are specified.

 

Brink (1981) argues rightly that the engineering properties of rocks of the Mulden Group are not so well known than those from the dolomites of the Otavi Group. The base consists of a thin layer of conglomerates followed by quartzites with some shales. On the top of the Mulden Group shale, slate and phyllite can be encountered. Weathered Mulden phyllite was predominantly used for the construction of trunk road 8/1 between Otavi and Kombat (Nm on the Geological Map, 1980). The road-building characteristics are given in Table 2 (see my remark onTable 1):

 

TABLE 2  ROAD-BUILDING PROPERTIES OF WEATHERED MULDEN PHYLLITE

|==================================================================|
|                             | Percentage smaller than |CBR at 95%|
|              | LL | PI | LS | 2 mm 0,425 mm 75µm 5 µm |MOD AASHO |
|--------------|--------------|-------------------------|----------|
| Maximum XM   | 60 | 37 | 12 |   82    74      60   36 |    84    |
| Minimum Xm   | 18 |  1 |  0 |   10      6      4    1 |     3    |
|              |    |    |    |                         |          |
| Mean x       | 39 | 16 |  6 |   33     26     18    7 |    30    |
| Number of    |    |    |    |                         |          |
| test data n  | 77 | 81 | 81 |   81    81      81   81 |    11    |
| Standard     |    |    |    |                         |          |
| deviation F  |11,3| 9,7| 3,4|   17,2  15,6   11,7  6,3|   29,2   |
| Coefficient  |    |    |    |                         |          |
| of variation |    |    |    |                         |          |
|     F/x      |0,29| 0,6|0,58|  0,52  0,61    0,66 0,93|   0,97   |
|==================================================================|

NOTA to Table 2: Plasticity Index "PI" is the numerical difference between the Liquid Limit "LL" and the Plastic Limit "PL".

 

3.3.2  DAMARA SEQUENCE: CENTRAL ZONE

 

The central and southern zones of the Damara Sequence have the same groups, subgroups and formations:

1. Nosib Group (Nn) with five formations: Blaubeker (Nb), Spencer Bay (Nsb), Duruchaus (Ndu), Kamtsas (Nka) and Naauwpoort (Nnp) Formations.

2. Swakop Group (Khomas, Kudis and Ugab Groups)

2.1 Ugab (Nu) and Kudis (Nku) Subgroups

2.2 Khomas Subgroup with four formations: Chuos (Nc), Karibib (Nkb), Auas (Nau) and Kuiseb Formations (Nk)

Nd is representing an enormous area of rocks: from Nosib, via Ugab and Kudis, via Chuos and Karibib resp. Auas to Kuiseb (stratigraphy undifferentiated). Nsc is representing from Ugab and Kudis via Chuos to Karibib resp. Auas (stratigraphy undifferentiated).

The largest area of the central zone of the Damara Sequence is underlain by many varieties of schist, by marble and some quartzite. Outcrops of rocks of the Nosib Group can be found in the south western parts of the area and those of the Mulden Group only in the northern parts. Volumetrically the central zone is composed of marble, schist and granite in approximately equal proportions.

Brink (1981) reports further that the engineering properties of the schist vary considerably with the metamorphic grade. Different schists can be established from low-grade to medium-grade biotite schist mainly in the north-east to medium-grade cordierite schist grading into gneiss at places in the south-west of the area. The properties regarding strength, permeability and weathering characteristics are a function of their foliation. The higher-grade schists have a more homogeneous to massive appearance whose engineering properties can approach those of gneiss, even of quartzite. The deposits of calcareous and dolomitic marbles of Karibib and the Rössing Formations in the region of Karibib, Usakos and Swakopmund are used extensively for concrete aggregates and road building materials ( Codes 28, 30, 31 of Ann.Table 1; Nd, Nkb and Nsc on Geological Map, 1980). The three main groups of the Cambrian granites which are intruding all formations of the Damara Sequence are Salem Granite (Cgs), the Sorris-Sorris Granite (Cgss)and the Donkerhuk Granite (Cgdh). The granites are susceptible to weathering. Weathered Salem Granite of a pegmatitic type was used for the building of trunk road 2/3 between Karibib and Omaruru (Code 32 on Ann.Table 1; Kgr and Cgs on Geological Map, 1980). The road building properties of this granite are listed in Table 3 (see my remark on Table 1:

 

TABLE 3  ROAD BUILDING PROPERTIES OF WEATHERED SALEM GRANITE

=====================================================================|
|                              | Percentage smaller than  |CBR at 95%|
|             | LL | PI | LS   | 2 mm 0,425 mm 75µm 5 µm  |Mod AASHO |
|-------------|----|----|------|--------------------------|----------|
| Maximum XM  | 41 | 23 |  8   |   80        45   20    5 |    110   |
| Minimum Xm  | 21 |  1 |  0   |   18         6    4    1 |     10   |
| Mean x      |27,5| 9,8| 3,4  |   46,9      22,2  8,5 2,3|     42,8 |
| Number of   |    |    |      |                          |          |
| test data n | 115| 135| 135  |   135      135   135 135 |     55   |
| Standard    |    |    |      |                          |          |
| deviation F | 4,9| 5,7|  1,9 |    14,0      9,0  3,9 1,3|     20,2 |
|Coefficient: |    |    |      |                          |          |
|variation F/x|0,18|0,58|  0,55|    0,30    0,41 0,46 0,58|      0,47|
|====================================================================|

Quartzites are concentrated in the Nosib Group at the base of the sequence but are also found in layers higher up. A commercial quarry at the foot of the Rössing Mountain east of Swakopmund is producing a fine-grained massive felspar-pyroxene-hornblende quartzite of the Khan Formation which is used for railway ballast, concrete aggregate and for crusher-run basecourse material (Code 29 on Ann.Table 1; Nn on the Geological Map, 1980 ). In comparison with the quartzites of the Nosib Group the quartzites of the Swakop Group often grade into quartz schist and quartz-mica schist which accounts for the lower strength of these rocks (Brink:1981).

 

3.3.3  DAMARA SEQUENCE: SOUTHERN ZONE

 

The southern zone of the Damara Sequence consists mainly of schist and quartzite which makes up 60 % respective 30 %. Other rocks in this succession are granite, marble, amphibolite and other rock types (Brink:1981).

Brink (1981) reports further that quartzite is largely restricted to the Nosib Group at the base of the sequence although layers of quartzite are also found in the Auas Formation. Thinner quartzite layers which often grade into quartz mica schist can also be encountered in the other formations of the Swakop Group. East of Witvley virtually all Damara outcrops are belonging to the Kamtsas Formation which consists of mainly quartzite and arcose with smaller intercalations of conglomerate and shale. The schist in this group is generally biotite bearing and has undergone low-grade to medium-grade metamorphism and is strongly foliated with the resulting variable physical properties such as permeability and strength in the different directions of the rock. The quartz content varies considerably vertically and laterally. The quartzitic component is more resistant to erosion than the mica schist component with the resulting typical ridges in this material. In spite of the high quartzitic content in it the rock seldom reaches the strength of a true quartzite. The quartzites of the Auas Formation are generally quite pure and are in spite of their high mica content extensively used as building material in the Windhoek region (Code 70 on Ann.Table 1, Nau on the Geological Map, 1980).

The biotite schist of the Kuiseb Formation is found all over the Windhoek region. It can be seen in many of the road excavations around the city as for instance on trunk road 1/5: Windhoek-Aris, trunk road 6/1: Windhoek-Seeis, main road 49: Windhoek-Haris and many other roads in the vicinity. These excavations are usually stable except for some minor wedge failures due to the low rainfall in this area. The biotite schist is known for its rapid weathering property. In the above mentioned road cuts it can be observed that a complete discolouration of the un-weathered schist is taking place in less than twenty years. In some depth beneath the open surface this weathering process is considerably slowing down or is coming to a complete hold (Brink:1981).

Due to the absence of other suitable road building materials biotite mica schist of the Khomas Subgroup has been used extensively as road construction material in the Windhoek area. This material can be easily compacted to 90 % Modified AASHO in up to 600 mm thick layers with 3 to 5 passes of a 15 ton vibrating roller. But this material must be met with caution. The Namibian Department of Transport has learnt some quite expensive lessons in using mica schist as road building material. For instance, during the construction of main road 52 between Windhoek and the Matchless Mine in 1971, a slightly plastic, rapid-weathering schist containing about 30 % of biotite and muscovite, was used for fill construction in a large number of fills from between six and twenty metres in height in this mountainous area west of Windhoek. Soon after the completion of the construction of the road extensive cracking occurred, some of which was found to be due to slope instability after the normal rainy season 1971/72. These cracks mixed with a variety of stabilisation cracks of the lime-stabilised basecourse which normally cracks in a very distinct block pattern. The slope-instability cracks took the form of longitudinal cracks up to 20 mm wide on the road shoulders just outside or close to the edges of the bituminous pavement.

The maximum dry density of the schist was to be found 2.007 kg/m3 at Modified AASHO compaction with optimum moisture contents of 8,5 % and a minimum air void content of 12 %. Samples compacted at less than 90 % Modified AASHO showed significant collapse settlement on inundation in laboratory oedometer tests. Analyses of many of the failed embankments using Bishop's method of slices for circular failure at zero pore water-pressure revealed that the slope stability was sufficient for a density of 90 %, but the factor of safety was too low in cases where the densities reached only 80 %. Remedial measures adopted were the removal of all materials deep into the inside of the road prism, cutting out all cracked sections and benching down to the roadbed. The sides have then be reconstructed to the specified densities and side slopes of 1:1,5. No further problems have been encountered to date.

For many road projects in the Windhoek area mica schist has been used successfully as selected subgrade and mixed with quartz gravel, even for subbase, but it is avoided for basecourse. Mica schist has also been quite successfully used in many wearing-courses for gravel roads where it has been found that mica schist needs less grader maintenance than the in abundance available quartz gravels. To establish the road building properties of mica schist and to compare it with quartz gravel, some gravel road test sections have been constructed in the vicinity of Windhoek in recent years [9].

 

3.3.4  THE GARIEP COMPLEX

 

The Namibian Gariep Complex (Ng) has the same age than the Damara Sequence. The Gariep Complex with an age of between 650 Ma and 500 Ma of years, comprises a sequence of sediment and volcanic rocks which are accumulated in a basin that formed on rocks of the Namaqua Mobile Belt, the Orange River Group, the Vioolsdrift Suite and the Richtersfeld Intrusive Complex. Miogeosynclinal deposits such as quartzites, conglomerates, dolomites and shales, together with volcanic tuffs and felsite, occur mainly in the northern and eastern parts of above mentioned basin. Phyllites, schists, amphibole-epodote rocks, porphyric lavas and minor quartzites, arcoses and carbonaceous rocks are common in the areas around Oranjemund and Alexander Bay. The Rosh Pinah Formations (Nrp) are forming the base of this complex (Brink:1981).

Grade of metamorphism is low, a high pressure facies may exist in the western parts of the complex. Deformation has been intense, complicating recognition of facies relationships. Intrusive granites range from 600 Ma to 500 Ma in age. Metamorphic cooling ages of about 480 Ma are recorded.

As a result of the remoteness of this area in the extreme south-western corner of Namibia and of the fact that due to very obvious, South Africa related economic-political reasons, no connecting roads between Oranjemund and the Namibian interior do exist, very little is known about the engineering characteristics of these rocks. Some gravel for the gravelling of district road 716 between Aus and Rosh Pinah has been used on a random basis but no testing has been done on these materials.

 

3.3.5  THE NAMA GROUP

 

 The Nama Group consists of three Subgroups: Kuibis (Nks), Schwarzrand (Ns) and the Fish River Subgroup. The Fish River Subgroup is subdivided into four formations: Stockdale (Cs), Breckhorn (Cb), Nababis (Cn) and Gross Aub (Cg) Formations.

The Nama Group is about 600 Ma to 450 Ma of age and is situated in the southern and south-western parts of Namibia. The early era of the Nama Group reaches still into the late Namibian age but the largest part of this group is taken by the Cambrian age. It consists mainly of a virtually unmetamorphosed succession of flat-lying marine, sedimentary rocks overlying a stable platform. A transgressive basal white quartzite is overlain successively by black limestone, green shale and sandstone, a second black limestone horizon, and red sandstone and shale. The red beds were derived from the rising Damara and Gariep Orogens to the north and west but lower stratigraphic units were derived from the east. Several intrusions of granite, syenite, bostonite, alnoitic tuffisite and carbonatite with an age of about 500 Ma cut Nama rocks in places and form a north-easterly line south-west of Grünau (Geological Survey:1982). In the eastern part of the Nama Group these rocks are partly overlain by the Dwyka Formations of the younger Karoo Sequence and partly covered by the unconsolidated to partially consolidated sediments of the Kalahari Group (Brink:1981).

Brink (1981) writes that not very much is known about the engineering properties of the Nama Group rocks. In areas underlain by rocks which are resistant to weathering, such as sandstones and quartzites, it is a typical appearance that the rivers are flowing in narrow steep-sided gorges which are giving a clear picture of the strata of the Nama Group rocks. To the south of Rehoboth the grey to greenish quartzitic sandstone of the Niederhagen Member is thickly bedded and folded and forms prominent ridges. The grey to green quartzitic sandstone of the Nasep Member is more friable and more slabby than the Niederhagen quartzite (Brink:1981). Rock of poor building characteristics can be expected where zones of alternating bands of sandstone, siltstone and shale are encountered. The high N-values in this region are responsible that the mudstones are rather disintegrating and don't decompose. Both the quartzite and the sandstone are suitable for coarse concrete aggregate. The Niederhagen quartzite and some of the more quartz-rich varieties of siltstone can be used for rockfills in road construction. Due to the slabby nature of the Nasep sandstone it is less suitable for road construction purposes.

Brink (1981) argues further that the properties of the shales, sandstones and siltstones of the Fish River Subgroup are similar to those of the older Nama sediments which can easily be distinguished on ground of their different colours. In the Haseweb and the Haribes Members of the Fish River Subgroup good quality red sandstones can be encountered which are suitable for coarse concrete aggregates. The Rosenhof Member of the Gross Aub Formation consists mainly of red shales interbedded with red sandstones. Although slightly slabby, the rocks have been found suitable for rockfill material and coarse concrete aggregates. The availability of fine aggregate is problematic in areas underlain by the Nababis and Gross Aub Formations (Brink:1981). Although alluvium in the Fish River has been used successfully as fine concrete aggregate, for instance, for the bridges and drainage structures on main road 34 between Mariental and Maltahöhe, the sand in most of the other rivers is usually too fine-graded and has a too high clay content. Clay and silt are quite common constituents of the alluvial terraces of the rivers in this southern regions of Namibia. Salt encrustations on the surface are indicating the presence of a high content of soluble salts in these formations which are endangering buried metal drainage pipes, as has been experienced on trunk road 14/2 between Solitaire and Nomtsas.

Solid sandstones and shales of the Fish River Subgroup of the Nama Group have been used as basecourse material and coarse concrete aggregates for the construction of trunk road 4/1 between Seeheim and Goageb (Code 16 on Ann.Table 1; Cb, Cn and Cs on the Geological Map, 1980). For the building of main road 34 between Mariental and Maltahöhe Nama Group sandstones and shales of the Fish River Subgroup have been used for basecourse material (Cb ,Cn and Cs on the geological Map, 1980) [10].

The rocks of the Schwarzrand Subgroup are delivering an excellent wearing-course for gravel roads in the vicinity of the south-eastern edge of the Naukluft via Maltahöhe to Bethanien. This road building material is especially favourable under the dry climatic conditions with very high N-values of this area but tends to become slippery in the rainy season. It consists of green and red shale, sandstone and mud rocks. The same material has also been used for the basecourse of trunk road 3/1 between Karasburg and Ariamsvley (Ns on the Geological Map, 1980).

 

3.4  THE CARBONIFEROUS TO THE CRETACEOUS AGES: KAROO SEQUENCE

 

The Karoo Sequence consists of one subgroup and nine formations. The Ecca Subgroup (CTR: stratigraphy undifferentiated) consists of four formations: Prince Albert (Pp), Whitehill (Pw), Aussenkjer (Pa) and Amibberg (Pm). Older than the Ecca Subgroup is the Dwyka Formation (Cd) and younger than above mentioned subgroup are the following formations: Gai-As (TRg) and Omingonde (TRo), further Etjo (Je), Kalkrand (Jk) and Etendeka (Ke).

The Karoo Sequence is younger than the Nama group and has a geological age of 400 Ma to 120 Ma. The main rocks encountered in this sequence are tillites, dolerites, mudrocks, shales and sandstones. Large parts of Namibia are underlain by this sequence. Basal glaciogenic rocks of the Dwyka Formation are overlain by shales, sandstones, mudstones and carboniferous shales of the Ecca Group. The succeeding red beds and aeolian sandstones are followed by basalts, latites and quartz latites with ages of 180 Ma near Mariental and 120 Ma in the north-west. Extensive dolerite sills and swarms of dykes are related to the volcanic rocks.

 

3.4.1  TILLITE AND OTHER STRATA OF THE DWYKA FORMATION

 

Brink [11] reports that tillite is the major component of the Carboniferous Dwyka Formation which is the oldest formation in the Karoo Sequence. Other components are boulder shales and warped shales. Sandstones, conglomerates and mudrocks are also present in places but rarely exposed and are of only local significance from a geotechnical viewpoint. In Namibia the tillite overlains a complex pre-Karoo topography and is predominantly of variable thickness.

Tillites and quartzitic sandstone from the Dwyka Formation have been used for example for basecourse during the re-building of trunk road 1/3 south of Mariental (Code 22 on Ann.Table 1, Cd on Geological Map, 1980). Hard tillite has been used for basecourse and coarse concrete aggregate on trunk road 4/1 between Keetmanshoop and Seeheim (Codes 9 and 12 on Ann.Table 1, Cd on Geological Map, 1980). The blue tillites encountered in Namibia have weak road construction properties and cannot be used for surfacing chips. Tillite is generally an easily worked quarry material but problems are often encountered with excessive flakiness which may usually eliminated by careful selection of the crushing equipment and the right crushing procedures. Tillite conglomerates have been used extensively for subgrade materials for the construction of trunk road 1/3 from Keetmanshoop to Mariental and the first section of trunk road 5/1 from Mariental to Stampriet. Highly weathered tillite is also suitable for gravel wearing courses, provided the stones in this material are not too large. The excellent wearing courses of main roads 27 between Keetmanshoop and Aroab and 28 between Seeheim and Holoog are good examples for the road building materials of the Dwyka Formation (Cd on Geological Map, 1980).

 

3.4.2  KAROO SANDSTONES AND MUDROCKS

 

A large portion of the Permian Ecca Group of the Karoo Sequence is comprising sandstones and mudrocks. This results in alternating arenaceous and argillaceous sediments which are very characteristic for this sequence. However, the different facies and local environments of deposition of the Karoo sediments have resulted in highly variable stratigraphic sequences in different parts of Namibia. This is particularly occurring within the Ecca Group where thin, poorly sorted sandstones may be found in the one area while thick quartzitic sandstone of the same age occur in the neighbouring region.

The Ecca Group consists, apart from the Carboniferous Dwyka Formation (Cd) which is the oldest one in this group, of four further Permian formations which all contain sandstones, mudrocks and shales:

1. Prince Albert Formation: Pp: Shale, sandstone and mudrock

2. Whitehill Formation: Pw: Coal-bearing shale

3. Aussenkjer: Pa: Shale, limestone and siltstone

4. Amibberg: Pm: Shale, sandstone and mudrock

5. Gai-As and Omingonde: TRg/TRo: Red mudstone, siltstone, sandstone, grit, conglomerate (younger than Ecca)

6. Etjo: Je: Aeolian sandstone (younger than Ecca)

The arenaceous members of the Ecca Group, though broadly categorised as sandstones, range in fact from arcose through subarcoses to greywackes, and average grain sizes range from 0,06 mm to 0,25 mm while between 50% to 70% of these rocks consist of quartz [12]. Most of the sandstones are poorly sorted, but some of the Ecca Group are fairly well sorted and contain less than 10% of clay matrix. Johnson reported the presence of substantial quantities of rock fragments in Ecca (30%-40%) sandstones [13].

Mudrocks include, however, all sedimentary rocks which are predominantly of silt-sized or smaller particles. Detailed classification of mudrocks is difficult owing to the fine-grained nature of these rocks. For simplicity it can be assumed, as Namibia's roadbuilder does, that mudstone or mudrock respective shale and even sandstone have all similar properties as far as road construction is concerned. For general purposes only, the terms mudrock or mudstone are used for the fissile variety and the term shale is used for the massive variety. Lundegard and Samuels point out that fissility is a weathering phenomena and therefore cannot be used to classify rock from below the surface [14]. They propose to use stratification or lamination rather than fissility to differentiate between shale and stone. Owing to the fine texture of mudrocks and the difficulty of disaggregating them, it is not easy to carry out grain-size distribution analyses or mineral separation analyses on them.

In Namibia Karoo-Sequence-sandstones are not used as coarse concrete aggregate, or river sand derived sandstones as fine aggregate due to their very unfavourable properties in reinforced concrete members which can lead to a serious deterioration of the concrete. In place of sandstone, Karoo dolerite is, as far as possible, used for coarse and fine concrete aggregates. The Namibian Karoo sandstones are mainly confined to the south-eastern parts of the country where very few bridge structures exist, and Keetmanshoop dolerite is easily available. Karoo baked shales ex the Dolerite Crushers at Keetmanshoop have been used for some isolated bridge structures before 1970 as for instance bridge 205 over the Wasser River and bridge 211 over the Bruckaros River on trunk road 1/3 between Keetmanshoop and Mariental. No problems with these shale aggregates on these and other structures have been experienced to date.

Karoo sandstone can be used as basecourse provided it has a 10 % FACT value greater than 140 kN and a soaked to dry ratio of 75 %, with the qualification if the cementing matrix is silica, the dry value may be reduced to 110 kN. Sandstones from the younger Omingonde Formation have been used for several roads in Ovamboland, as for instance for trunk road 1/11 between Ondangwa and Oshikango and main road 92 between Oshakati and Ombalantu as well as for the rehabilitation of some failed sections of trunk road 1/7 between Okahandja and Otjiwarongo in the surrounding of the Omatako Mountains for subbases and basecourses on surfaced roads as well as for the production of surfacing chips (indurated mudrocks)(TRo on Geological Map, 1980).

Mudrocks play an important role in road building in large parts of Namibia. In the south, for instance, a not very clearly defined conglomerate between sandstones, tillites and shales has been used on isolated sections on trunk road 1/3 between Keetmanshoop and Mariental. No problems have been encountered to date except a weak section around Itzawisis which has been rebuilt during 1978. The reasons for this failure in the foundation layers of the road are not very clearly established but it is presumed that the reason lies with weak shales in the subgrade and weak subsurface drainage of the road. Karoo mudrocks and shales have also been used for basecourses on trunk road 4/1 between Keetmanshoop and Seeheim as well as trunk road ½ between Keetmanshoop and Narubis. No problems have been encountered on these sections. Karoo sandstones, shales and mudrocks are used as subgrades on many surfaced road sections in the south around Keetmanshoop and on trunk road 3/1 between Karasburg and Ariamsvley (Cd, Pp and Pw on Geological Map, 1980).

Karoo sandstones have not been used so far to a great deal as gravel wearing courses in Namibia due to their low plasticity and lack of binder in the disintegrated material except for some very isolated spots on some gravel roads in the Gobabis (main road 39: Gobabis-Leonardville), Windhoek, Okahandja and Maltahöhe districts. Karoo mudrocks and shales have, however, been used successfully on many gravel roads in the south-eastern sector of the country, as for instance on main roads 39: Gobabis-Aranos; 25: Karasburg-Aroab; 30: Keetmanshoop-Koës).

Baked shales from a dolerite quarry (Code 7 on Ann.Table 1, Jd on Geological Map, 1980) in Keetmanshoop for surfacing chips have been used before 1970. Some test sections on trunk road 1/3 south of Kalkrand revealed their unfavourable properties regarding weak adherence between the surfacing aggregate and the bituminous binder. Since then only Karoo dolerites have been used for surfacing chips and concrete aggregates.

 

3.4.3  VOLCANIC ROCKS OF THE KAROO SEQUENCE

 

At the end of the period of the Karoo Sequence, during the transition of the Jurassic to the Cretaceous age, a marked magmatic ascent of high magnitude has taken place with the result that the whole of the sub-continent of Southern Africa became a magma province on its own (Brink:1983). These volcanic rocks of the Namibian Karoo Sequence are mainly represented by two formations, the Jurassic Kalkrand (Jk) and the Cretaceous Etendeka (Ke) Formation. These occurrences of both basic and acid varieties of volcanic rocks contain mainly basalt, andesite, rhyolite, dolerite dykes, Kaoko-lava, sandstones and quartzites. These different rocks can be found in the Kalkrand-Uhlenhorst-Hoachanas vicinity (Jk), the Erongo Mountains (Ke), in the western parts of Damaraland and the Kaokoveld as well as at the Atlantic coast of the Kaokoveld between the Ugab mouth and Cape Fria (Ke), in a climate with a N-value of more than 10. Where exposed to the atmosphere, these rocks disintegrate and rather coarse-grained, irregularly shaped gravels develop. Especially in the case of basalts and other more basic varieties, this process is often associated with strong development of calcretes.

A typical feature of the volcanic Karoo rocks is that they occur in the form of numerous lava flows superimposed on each other. The resultant rock is structurally not very uniform and the rocks at the bottoms and the tops are often weaker than those in the massive middle section with the consequence that these massive parts are the most favourable ones for quarrying (Brink:1983).

In a couple of cases volcanic Karoo rocks have been used as road aggregates like surfacing chips in Namibia. The engineering properties of these road building materials are those of the basic crystalline rocks like basalt and andesite and those of the acid crystalline rocks like rhyolite and various types of granites, as defined by Weinert (1980). In the case of using these rocks for surfacing chips caution should be exercised against polishing of the aggregates under traffic and against weak adherence to bituminous binders.

The problem with volcanic Karoo rocks is that they are either too good or too bad to be used as basecourse material. The good basalt rocks have been used for surfacing chips on surfaced roads in the vicinity of Mariental, as, for instance, on trunk road 5/1 between Mariental and Stampriet and main road 61 between Stampriet and Aranos. The surfacing chips have been quarried on the farms Dabib and Swartmodder along trunk road 5/1 (Jk on Geological Map, 1980 and Codes 21 and 56 on Ann.Table 1 (will be provided in due course)). The code 21 basalt has given slight weathering problems which are not serious and the code 56 basalt has been also judged as a phonolite or even a dolerite. This material is, however, too fine in texture to be a dolerite and it can't be a phonolite because phonolite is a tertiary rock which is much younger than the Jurassic basalt of the Karoo Sequence. Karoo basalt has been also used successfully as concrete aggregate for the drainage structures on trunk road 5/1 as well as main road 61.

Cretaceous Karoo high silica basalts (Ke) have been used very successfully as wearing course material in salt-gypsum roads along the Atlantic coast of the Damaraland and the Kaokoveld. Under the moist state of the Atlantic mist belt these basalts are weathering to an excellent wearing course material with a high PI because the felspar in the basalt is weathering under these circumstances. This material is as good as the salt-gypsum pavements further to the south. Where the Damara quartzitic system is encountered on district road 2302 between the Ugab mouth and Möwe Bay at the Atlantic coast of the Kaokoveld, the road must be covered with a salt-gypsum wearing course. But where the Karoo basaltic system is encountered this material is providing in itself an excellent wearing course. Along the coast the Karoo basalt is reacting chemically with the existing coastal salt and is improving the road building properties of this material, although this has not been tested in a systematic manner yet.

Karoo basalt in the Namibian interior, in the western parts of the Damaraland and the Kaokoveld as well as in the Erongo Mountains area, where the coastal mist belt is lacking, has not been used very successfully as wearing course material for gravel roads because this material is too hard and too coarse for a good wearing course.

 

3.4.4  KAROO DOLERITE

 

The Karoo dolerite (Jd) is of Jurassic age and is intruding the whole series of the Karoo Sequence from the Carboniferous Dwyka to the Jurassic Etjo Formation. Karoo dolerite can be found in many parts of Namibia, mainly in the south-east but also in the north.

The continuous outpouring of magma during the last phase of the Karoo era covered an even larger area than the early Karoo period. The magma solidified initially into basalt which increased tremendously in thickness until it prevented further extrusion of lava. In subsequent magmatic events the lava had to force its way into zones of weaknesses in the underlying rock, starting near the top of the sedimentary rocks and then proceeding into lower sedimentary layers. The dolerites are therefore generally younger than the overlying basalts but not in all cases, because the progressive break-up of Gondwanaland caused repeated developments of tension cracks and new activities of volcanism with the resulting intrusion of dolerites into the basalts. The age of dolerite is therefore broadly similar to that of basalt and can be established with 190 Ma to 150 Ma (Brink:1983).

Dolerite weathers in a similar manner than all crystalline rocks. Namibia's deposits of dolerites can be found in areas with N greater than 10, and disintegration is the sole mode of weathering. However, when weathered dolerite is used as natural road building material in the various layers of pavement, environmental conditions and the stage of weathering must receive careful attention.

Because of its sound properties and widespread occurrence in many areas of Namibia's south-eastern and southern regions extensive use has been made of dolerite as a concrete aggregate. Also extensive use has been made for dolerites, both fresh and weathered ones, in road construction. For bituminous surfacing, the adhesive properties of crushed fresh dolerite are generally satisfactory (See Code 7 on Ann.Table 1).

Dolerite has the same property than basalt, it is too good for basecourse or too weak. Because in most cases this rock is too good for basecourse, fresh dolerite has been used for surfacing chips (Codes 1, 2, 3, 4, 5, 7, 8 and 12 on Ann.Table 1, Jd on Geological Map, 1980).Most of the surfaced roads in the south have been surfaced with Keetmanshoop dolerite road aggregates, as for instance the trunk road 1 from Vioolsdrift at the South African border up to Mariental in the north and most of trunk road 4/1 from Keetmanshoop to Goageb as well as trunk road 3/1 from the South African border at Nakop to Grünau.

Only two cases are known where weathered Karoo dolerite has been used for the construction of a basecourse, namely for the rehabilitation of a section on trunk road 1/7 between Okahandja and Otjiwarongo on the farm Rimini during 1986 (Code 67 on Ann.Table 1, Je on Geological Map, 1980) and on one 15 km long section on trunk road 1/1 between Vioolsdrift and Grünau (Code 2 on Ann.Table 1, Jd on Geological Map, 1980). All the Karoo dolerites along trunk road 1/1 are suitable for basecourse crusher-run material (Codes 1, 2, 3, 4,5 on Ann.Table 1), but were except for above mentioned one case never used during the construction of this road during 1968/69 because it was more economical to use calcretes as natural basecourse material [15]. Weathered Karoo dolerite has also been extensively used on subbases and subgrade layers on many surfaced roads in the south and south-east of Namibia.

Karoo dolerite also gives an excellent wearing course for gravel roads, mainly in the districts of Karasburg and Keetmanshoop. Isolated examples are main road 30 from Keetmanshoop to Koës, main road 21 from Karasburg to Warmbad as well as main road 22 and district roads 545 and 609 in the Keetmanshoop district. Karoo rhyolite has been, however, never used as road building material to date. Another rock with properties similar to those of dolerite is the intrusive volcanic rock, phonolite, which must, however, be dated to a much younger age, i.e. the Tertiary age of about 39 Ma to 32 Ma. Phonolite is a hard rock which in its fresh state is extensively quarried at Aris, south of Windhoek. Phonolite is used as a good quality surfacing aggregate for most of the surfaced streets in Windhoek as well as for the rural roads as so far south as from Mariental to so far north as Sukses on trunk road 1/7 from Okahandja to Otjiwarongo (Code 26 on Ann.Table 1, Tr on Geological Map, 1980).

 

3.5  THE TERTIARY TO QUATERNARY AGES: KALAHARI SEQUENCE

 

The Kalahari Sequence of Tertiary to Quaternary (Recent) ages forms an extensive cover of terrestrial origin in the eastern and northern parts of the country. A lime-cemented sand and conglomerate or grit at the base is followed by green sandy clay, white, partly calcareous sand and the, for any road building purposes in Namibia, extreme important calcrete. Unconsolidated aeolian sand covers large areas of the Kalahari succession and forms stationary longitudinal dunes in many parts of the east of Namibia. In the west the sand seas of the Namib Desert contain both Tertiary and Quaternary dunes.

Due to the importance of sands and pedocrete materials from the Tertiary to Quaternary ages as road building materials they will be dealt with in two specific sections.

 

3.6  SAND AS IMPORTANT NAMIBIAN ROAD BUILDING MATERIAL

 

3.6.1  NAMIBIAN COASTAL DEPOSITS FROM TERTIARY TO RECENT

 

The deposits discussed in this section of the thesis are of a variety of ages and origins. They are grouped together because they predominantly consist of deposits originated from the action of littoral processes. Surely the most striking feature of the Namibian coast is the Namib Desert which forms a long, narrow zone west of the Great Escarpment between the Olifants River in the Cape Province in South Africa and the Carunjamba River in south-western Angola. This desert consists of spectacular aeolian deposits in the several sand seas with the highest sand dunes in the world which in places are climbing to 300 m of height (Tn and Qn on Geological Map, 1980). Less prominent aeolian accumulations of sand can be found banked up against 'inselbergs' and in local basins. In spite of the frequent storms accompanied by heavy dust clouds no loess deposits are known. The largest accumulation of sand covers some 34.000 km2 between the Lüderitzbucht/Koichab River area in the south and the Kuiseb River/Swakopmund area in the north [16]. Large parts of this central sand sea lie on a red aeolian rock known as the Tsondab Sandstone Formation which is dated to the Tertiary age and rests on a platform cut into the Cretaceous erosion surface.

The mostly westwards flowing Namibian rivers, which are all crossing or ending in the Namib Desert and which are flowing only in the short rainy season, have a Tertiary to Quaternary fill consisting of both coarse and fine fluviatile sediments and thick calcrete-cemented sands and conglomerates. Some gypcretes occur also in the western parts of the country. Several of these rivers have large fluvio-marine deltas which, in some places, contain layers of salt and gypsum-cemented sand (Brink:1985).

The coastal region of the Namib Desert possesses a high-energy wind regime, and the distribution of the wind-formed major dunes in the main Namib sand sea is more or less compatible with the west-east wind pattern (Brink:1985).

Brink (1985) reports further that the main components of the Namib dune sands consist of subrounded to sub-angular grains of quartz (70-90%) with some minor quantities of felspar (10-15%) and some heavy minerals. Surface sands are generally dry but become slightly moist with depth. The colour of the sand varies from yellowish-brown at the coast to red in the eastern sector of the Namib. The consistency is loose on the surface becoming medium dense with depth. Dune sands are normally uniformly graded and fine to medium grained without any cohesion.

But the significance of this material lies not so much in the properties of it but in the severe effects of windblown sands on roads and other infrastructural means. The two major access roads to Walvis Bay, trunk road 2/1 from Swakopmund to Walvis Bay and main road 36 from Walvis Bay to Rooikop, have continually serious maintenance problems by sand accumulations, deposited on the road during times of strong winds mainly from a south-easterly direction. During such sand storms more than half of the roadway can be completely covered with small sand dunes, sand ridges and hummocks which represent a serious traffic hazard. Still more seriously affected by the frequent strong to gale force south-south westerly winds is trunk road 4/2 between Lüderitzbucht and the railway station Grasplatz where this road traverses the major region of the northward-migrating crescentic dunes. This trunk road is notorious for the ill-famous sandstorms with its sand-blasting effect that can strip off the paint of a car and frost the glass of windscreens and headlights as well as the paint of road signposts.

Preventing the mobility of the Namib sands and controlling dune and sand migration have been attempted by the Namibian Department of Transport with varying degrees of success. Sand trapping fences and dune palisades from old bituminous binder drums have been erected to protect for instance trunk road 4/2 near Lüderitzbucht. Regular maintenance and clearing of these sand barriers is, however, required. The binding of the sand by covering of the dunes, after partial rounding-off and levelling them, with a layer of coarse gravel about 25 mm thick has been proved to be reasonable successful. Little or no success has been, however, achieved with vegetating the dunes, covering them with old diesel oil or spraying them with a chemical that forms a surface crust (Brink:1985).

At the design stage of a road some preventing measures can be effected by designing a proper geometrical crossection. This can be done by forming the crossection, as determined by model tests in a wind tunnel, like the wing of an aircraft to initiate a 'self-cleaning' effect of the road. Furthermore these roads are built on a high fill from which the sand tends to be blown. To increase this effect, the shoulders of the road are built to the same camber as the pavement. Within 50 m on each side of the road dunes are levelled off and stabilised with a layer of coarse gravel. All these preventing measures are working reasonably well as has been proved on trunk roads 2/1 and 4/2 as well as main road 36 in the vicinity of Walvis Bay respective Lüderitzbucht.

For road building material coastal sand is normally not used except for fine concrete aggregate and for the subgrade layers in road embankments. Brink (1985) reports that in the vicinity of Walvis Bay Lewis and Roussouw (1978) conducted a detailed analysis of local coastal sands in an attempt to find sources for fine concrete aggregates alternative to those of riverbeds, some 40 km to the east. It was found that beach and dune sands tended to be bunch-graded and required the addition of crushed aggregate to meet the required grading specifications. Sand from the upper levels of the coastal dunes has been found to be altogether too fine for use in concrete. While the mica content was within acceptable limits for fine aggregates, chloride content was well in excess of the specified maxima. It was consequently recommended to use purified sewage water to wash all coastal sands. In some cases it was additionally required to wash the sand in fresh water to meet the required 0,1 g/kg chloride specification for reinforced concrete. With the exception of a few samples from the upper slopes of dunes it was found that the sulphate content of all sands was generally within the specifications. Alluvial dune sands have been also used as fine concrete aggregate, although these dune sands tended to be on the too fine side. These sands have been used for small drainage structures on trunk roads 1/1 Vioolsdrift-Grünau, 3/1 Karasburg-Nakop and 4/2 Haalenberg-Aus (Codes 1, 2 and 3 on Ann.Table 2). Namib sand has also been successfully used for the construction of road fills in the vicinity of Walvis Bay and Lüderitzbucht, as, for instance, for the building of trunk road 2/1 between Swakopmund and Walvis Bay as well as for main road 36 between Walvis Bay and Rooikop respectively trunk road 4/2 between Lüderitzbucht and Haalenberg. Due to the fact that Namib dune sand has some grading and is not collapsible like Kalahari sands, it was sufficient to compact the fill layers to 90 % Modified AASHO which was easily achieved by means of a static rubber wheel compactor under optimum moisture with sea water. In most cases it was possible to achieve 100 % Modified AASHO without any effort.

 

3.6.2  NAMIBIAN INTERIOR DEPOSITS FROM TERTIARY TO RECENT

 

 The aeolian deposits of Namibia's interior all belong or have been derived from the uppermost formation of the Kalahari Group (Tk on Geological Map, 1980). The Kalahari Group comprises roughly the eastern third of Namibia and stretches from Ovamboland/ Okavango/ Caprivi Strip in the north via Bushman/ Hereroland and Gobabis district in the central region to the eastern parts of the Mariental/ Keetmanshoop/ Karasburg districts in the south.

In the northern sector, in Ovamboland and Okavango, the Kalahari Group varies in thickness from 225 to 500 m as reported by Brink (1985). Deep borehole investigations by the Namibian Directorate of Geological Survey indicate a succession which can be divided into a basal Beiseb Formation consisting of 15 to 30 m of reddish gritty or conglomeratic sandstone, more than 120 m of poorly consolidated reddish brown calcareous sandstone of the Olukonda Formation and the uppermost Andoni Formation consisting of greyish green sand or clayey sand with sandy facies towards the margins of the basin.

In the central sector of the Kalahari Group, in Bushmanland, Hereroland and the Gobabis district, the major omurambas (slowly flowing rivers in wide sandbeds ) are giving a clear picture of the exposing formations. The lowermost formation of carbonate-cemented sandy conglomerate, grit and variably cemented sand is known as the Tsumkwe Formation. The overlying silcrete-cemented quartz sand, silicified limestone and calcareous sandstone belong to the Eiseb Formation, which is again capped by ferricrete and ferruginous sandstone of the Omatako Formation (Brink:1985).

In the southern sector of the Kalahari Group, in the Mariental, Keetmanshoop and Karasburg districts, well exposed formations have been grouped into a single stratigraphic formation, the Weißrand Formation, where a basal conglomerate is overlain by grit and sandy limestone (Brink:1985).

The overlying sands of the Kalahari Group are currently stabilised except at the edges of rivers and omurambas where they are still mobile and a source of fresh sediments. These sands are mainly stabilised by vegetation, which in the north-eastern parts of Namibia is often thick bush or even semi-tropical forest.

The sedimentological characteristics of the Kalahari sands have been investigated inter alia by Baillieul [17]. Baillieul has differentiated between four different types of sand, three which can be encountered in Namibia:

1. This type will be mainly encountered in the northern sector of the Namibian Kalahari. It is a pure quartz sand with a red iron skin around its particles. This sand is found in the fossile dunes of the north Kalahari.

2. The next type is defined to the central sector of the Namibian Kalahari. It consists of sands of two distinct origins. One is an aeolian component of quartz grains of distant origin and the other one is a finer felspathic type derived from the underlying sandstone formation.

3. The third one is more or less restricted to the southern sector of the Namibian Kalahari. This is also a pure quartz sand similar to the second type without the felspathic component.

The dominant particle sizes of the Kalahari sand vary in the different dune sections in relation to the variable wind velocities across the dune profile. The finer particles can be found more prominently in the interior of dunes where they penetrate into the voids between coarser particles and are protected from further wind transport. These fine sand particles, with iron oxide skins on grain surfaces and a small but significant clay component formed by in situ weathering of felspathic and ferromagnesian minerals, are responsible for the development of a collapsible fabric in the Kalahari sands. In the Gobabis area, for instance, this type of collapsible fabric is imparted by the slight cementing action of soluble salts such as gypsum (Brink:1985).

Another property of the Kalahari Group is the low permeability of the Kalahari sands, and the low rainfall in this region is responsible that the groundwater aquifers are not recharged by rainfall where the sand cover exceeds about 15 m (Brink:1985). The good aquifers found in the lower units of the Kalahari Group are recharged from other, distant sources as for instance by the artesian conditions which can be found in the area of Stampriet. The high concentration of soluble salts, mostly sulphates and chlorides, in these deep-seated aquifers, do render them in some cases unsuitable for use in reinforced concrete and even in the compaction of the selected road pavement layers, although salt was not controlled in subgrade layers and rendered no problems as long it was prevented to penetrate the surfacing of the road as has been proved on main road 61 between Stampriet and Aranos. The same fact has been proved during the construction of trunk road 4/2 from Lüderitzbucht to Haalenberg when seawater has been used for compaction purposes in subgrade layers as long as the soluble salts were prevented to penetrate into the basecourse and the surfacing of the road. The soluble salts also didn't render any problems in the application for reinforced concrete as for instance during the building of bridge 412 over the Auob River at Stampriet.

One remarkable property of aeolian deposits lies in the fact that these deposits could possibly have a collapsible fabric. Such a collapsible fabric is normally associated with an open textured soil with individual grains being separated by a bridging material. In the Namibian parts of the Kalahari this bridging material often consists of calcium or gypsum, like, for instance, in the Gobabis area. Available data have been used in an attempt to obtain a relationship between the collapse potential index and a combination of easily determined properties such as moisture content, dry density and void ratio. To date no meaningful conclusions have emerged due to the small amount of available data and due to the complexity of the mechanism associated with the collapse phenomena of aeolian sands of the Kalahari Group (Brink:1985).

The Kalahari sands have under a partially saturated condition a relatively high shear strength because of the effects of 'apparent cohesion' imparted to the sand by pore water suction. Studies carried out by Mainwaring during the construction of the road from Bulawayo to Victoria Falls in Zimbabwe over Kalahari sands indicated that the composition properties of the aeolian sands in this area are dependent on the plasticity index and the percentage finer than 75 µm sieve [18]. His studies prevailed that when the sand is non-plastic a CBR of the order of 30 is obtained, but with an increase of the PI the strength of the sand was found to decrease to as low as a CBR of five.

Many problems have been encountered with pavements of roads and airfields in most areas of the Kalahari Group. Distress of flexible pavements have been reported by Mainwaring (1968) and Wolmarans and Clifford [19]. This distress is mainly caused by the densification of the aeolian sand subgrade which collapses under compaction by adding a road fill onto the subgrade and under the vibration of the road traffic. This distress is associated with water penetration as well as with the static and vibratory traffic loads. Brink (1985) reports about tests which indicate, that even under light traffic loads, collapse could occur to a depth of 700 mm in an aeolian soil subgrade. Weston recommends that in the case of the presence of collapsible aeolian sands the roadbed should be compacted to achieve 90 % Modified AASHO for a depth of between 0 to 0,5 m and 85 % Modified AASHO for a depth of 0,5 to 1,0 m [20]. Reports of the use of vibratory rollers are known from Weston (1980) and Schwartz et al. [21] The results of these reports indicate that despite of using various techniques like the adding of water, ripping before compaction to break down the clay bonds, it is generally not possible to improve the in situ density below a depth of about 1 m. The results reported by Schwartz et al (1981) indicate that it is possible to achieve densities in excess of 95 % Modified AASHO to a depth of 0,8 m and densities of 90 to 95 % Modified AASHO between 0,5 and 1,0 m.

Experience gained by the Namibian Department of Transport proved, however, that sufficient bearing capacity without collapse of aeolian sand subgrades can be only achieved with compaction to 100 % Modified AASHO. This density can be achieved by means of vibratory or even static rubber wheel compactors as well as the impact roller. The disadvantage of the impact roller is, however, that it needs a platform of gravel on top of the Kalahari sand before it can be used. This has been proved by trial runs of the impact roller on the Gobabis hospital complex during 1975 and during the construction of main road 61 between Stampriet and Aranos during 1976/78. The easiest way to achieve the required 100 % Modified AASHO on collapsible sands is the use of the 50 t vibrating 'super-compactor' which has been used during the construction of the airport Gobabis. 100 % compaction has been achieved to a depth of 1 m and more. After nearly 10 years of service of above two projects no distress due to collapse of Kalahari sands has been encountered.

Most parts of the areas covered by aeolian sands of the Kalahari Group experience a scarcity of natural construction materials of high quality as reported earlier in this thesis. The Department of Transport of Namibia has carried out some studies and constructed some trial sections to determine suitable stabilisation methods to improve the properties of the aeolian soils sufficiently for them to be used as selected layers, subbase or even basecourse in the construction of surfaced roads. These studies indicated that the stabilisation of aeolian sands with bitumen or tar seemed to be the most promising procedure to improve the properties of Kalahari sands to basecourse quality materials. One example is the construction of the test section, the 'holy mile' north of Andoni on trunk road 1/11 from Oshivelo to Ondangwa in Ovamboland during 1965. Different stabilising agents have been used on this test section, like straight bitumen, various cutback bitumens and coal tar. Due to the presence of pedocretic materials in most parts of the Namibian Kalahari it was always possible to use these materials before it was considered to stabilise Kalahari sands which was regarded to be more uneconomical. The only exception is trunk road 8/6 from Kongola to Katima Mulilo in the East Caprivi Strip where a mixture of Kalahari sand and calcrete nodules has been used which was stabilised with bitumen. The subbase on this road has been constructed from sand which has been compacted to 97 % Modified AASHO except one test section where the sand-subbase has been stabilised with 3 % lime. The subgrade layers consisting of unstabilised Kalahari sand has been compacted to 100 % Modified AASHO. In spite of the heavy army traffic on this road no distress, except some edge failures, has been experienced to date.

Particular attention must be also given to erosion protection and the correct design parameters for crossections of roads in Kalahari areas for both fills and excavations. The Namibian Department of Transport is using a side slope of 1:6 and critical sections are covered with a coarse gravel blanket to prevent erosion, as has been shown to be successful on main road 61 from Stampriet to Aranos. Single size Kalahari sands have been even used as fine concrete aggregate as for instance for the construction of bridge 236 over the Nossob River on main road 41 between Leonardville and Zania because no river sands are obtainable in economic distances. Kalahari sand has been used as fine concrete aggregate for drainage structures on main road 75 between Tsumeb and Tsintsabis as well as for various smaller structures in the Tsumeb and Otjitue areas. In both cases this sand had to be blended with crusher-dust to achieve the specified grading (See Codes 49, 58 and 59 on Ann.Table 2 (will be provided in due course)).

 

3.7  THE USE OF PEDOCRETES IN NAMIBIA

 

Pedocrete materials are common deposits in most areas of Namibia and they can be regarded as the natural road building material number one in terms of importance. Brink (1985) is stating that pedocretes are not sedimentary rocks, but materials which have been formed either as weathering residues or by cementation or replacement - sometimes both - of pre-existing soils by various authigenic minerals. In terms of consistency these materials are of two types: indurated (e.g. hardpans, honeycombs, nodules) and non-indurated (soft or powdery forms). Pedocretes can be either cemented or replaced by a rather big variety of pre-existing materials, as for instance by carbonates with the resulting calcrete; dolomites with the resulting dolcrete; iron oxides with the resulting ferricrete, plinthite or laterite; silica with the resulting silcrete; manganese oxides with the existing manganocrete; phosphate with the resulting phoscrete; gypsum with the resulting gypcrete or magnesite with the resulting magnesicrete. Only in cases where more than 50 % of the cementing or replacing materials are existing in the new deposit, they should be referred to with the term pedocretes.

Calcretes, for instance, rarely exceed thicknesses of 1-2 m. In the case of Kalahari limestone, thicknesses of up to 30 m are reported but in excess of above mentioned 1-2 m, they are seldom homogeneous. Silcretes of 5 m thicknesses are reported to have been found in Southern Namibia (Brink:1985), but the younger Pleistocene pedocretes seldom exceed 2 m in thickness. Pedocretes are composed of materials of two origins, namely the host material and the authigenic cement. With the developing pedocrete the authigenic component can increase and ultimately replace almost the whole host material. It is even possible that one pedocrete can replace the other one with resulting mixtures between them. Several horizons of both host material and cementing agent can be encountered in one pedocrete occurrence. Therefore it is difficult to date and correlate pedocretes within the stratigraphic system.

After the formation of pedocretes they will go through a weathering process like any other rock. The resulting boulders, cobbles and gravels may be transported as colluvium or alluvium. They can also be incorporated into another younger pedocrete (Brink:1985).

The Namibian types of calcrete have mainly a pedogenic and not so much a groundwater origin. The main component to form a calcrete, the carbonate, is transported through the soil layers by rainwater. The carbonate itself can be originated from the surrounding soil or it can be transported in the form of dust or by rainwater. Ferruginous pedocretes, however, are formed by a process of accumulation. Brink (1985) reports further that ferricretes, silcretes, gypcretes and phoscretes all appear to be absolute accumulations which are necessary for induration.

Typical constituents of some different pedocretes are summarised in Table 4 (will be provided in the future):

 

TABLE 4  TYPICAL CHEMICAL COMPOSITIONS OF PEDOCRETES (%)


|==================================================================|
|Component |Calcrete|Laterite|Silcrete|Phoscrete|Gypcrete| Form of |
|Fig.: (%) |        |        |        |         |        | Occurr. |
|----------|--------|--------|--------|---------|--------|---------|
| SiO2     | 1-60   | 5-70   | 85-100 | 10-60   |   0-60 |Quartz., |
|          |        |        |        |         |        |felspar, |
|          |        |        |        |         |        |clay,opal|
|          |        |        |        |         |        |chalced. |
|----------|--------|--------|--------|---------|--------|---------|
| Al2O3    | 0-5    | 5-35   | 0-5    | 5-30    | 0-60   |Felspar, |
|          |        |        |        |         |        | clays,  |
|          |        |        |        |         |        |gibbsite |
|----------|--------|--------|--------|---------|--------|---------|
| Fe2O3    | 0-5    | 0-70   | 0-10   | 0-10    | 0-60   |Goethite,|
|          |        |        |        |         |        |haematite|
|----------|--------|--------|--------|---------|--------|---------|
| TiO2     | 0-1    | 0-5    | 0-5    |  -      | 0-60   |Anatase, |
|          |        |        |        |         |        |rutile   |
|----------|--------|--------|--------|---------|--------|---------|
| CaCO3    | 40-100 | 0      | 0-5    | 0-50    | 0-60   |Calcite, |
|          |        |        |        |         |        |dolomite,|
|          |        |        |        |         |        |apatite  |
|----------|--------|--------|--------|---------|--------|---------|
|Ca3(PO4)2 |        | <0,2   | 0-2    | 0-2     | 20-60  |Apatite, |
|          |        |        |        |         |        |colloph.,|
|          |        |        |        |         |        |dahllite |
|----------|--------|--------|--------|---------|--------|---------|
|CaSO42H2O | 0-2    | 0      | 0-2    | 0-2     | 40-90  |Gypsum   |
|----------|--------|--------|--------|---------|--------|---------|
| H2O+     | 0-5    | 5-20   | 0-2    | 0-5     | 10-20  |Clays,   |
|          |        |        |        |         |        |gibbsite,|
|          |        |        |        |         |        |goethite,|
|          |        |        |        |         |        |gypsum   |
|----------|--------|--------|--------|---------|--------|---------|
| Organic  | 0-1    | 0,2-2  | -      | -       | -      |Organic  |
| materials|        |        |        |         |        |matter   |
|----------|--------|--------|--------|---------|--------|---------|
| NaCl     | 0-1    | 0      | -      | -       | 0-4    |Halite,  |
|          |        |        |        |         |        |apatite  |
|==================================================================|

Pedocretes can be found in most areas of Namibia and they must be regarded as an excellent road building material. The semi-arid to arid drainage conditions of the Namibian climate are favouring the forming of calcretes which need seasonal water courses and pans. Well developed calcretes generally only occur where the mean annual rainfall is less than about 550 mm. The N=5 line indicates a reasonable correlation with this limit of calcrete occurrence. Ferricretes, however, need a much more subhumid climate with a N-value of less than 5 for chemical decomposition to release the necessary iron from the host material. Silcretes are limited to the areas of the Kalahari Group. Duripans are mostly occurring in the drier parts of Namibia where the rainfall is less than 300 mm. Gypcretes are mainly restricted to the Namib Desert but the wetter limit of their occurrences seems to be close to that of calcretes (Brink:1985).

Soils classifications for non-pedocrete materials are differentiating between discrete, hard, durable and solid particles which can be classified according to their grain sizes and Atterberg limits. These criteria cannot easily be applied to materials which are weathered, cemented or aggregated because gradings and Atterberg limits are too dependent on the test methods employed and on the methods of excavation and processing the material. Therefore it is advisable to add two extra symbols to the general soils classification in the case of pedocretes, i.e. to show the geological origin and the method of excavation [22]. The sequence in which calcretes are forming has led to a simplified morphogenetic classification of calcretes which also can be applied to other pedocretes, as reported by Weinert (1980) as well as Netterberg and Caiger [23].

Brink (1985) defines the different classes of pedocretes as follows:

1. Calcareous (or ferruginous etc) soil: a soil which exhibits little or no nodular development or massive cementation, but which contains some authigenic mineralisation which is, however, not sufficient to indurate the soil significantly.

2. Calcified (or ferruginised etc) soil: a relatively massive to platy soil which has been indurated by cementation to a firm consistency.

3. Powder pedocrete: a mainly loose silt and fine sand-sized cemented or aggregated soil consisting of nearly pure authigenic minerals.

4. Nodular pedocrete: a naturally occurring mixture of silt to gravel- sized nodules of cemented and aggregated finer particles. The horizon is normally of loose consistency, but the nodules can be quite hard.

5. Honeycomb pedocrete: a partly coalesced nodular pedocrete representing an intergrade between nodular and hardpan stages. It usually still contains loose or soft host soil particles filling the voids between the coalesced nodules. Ripping is normally required in case of excavations.

6. Hardpan-pedocrete: an indurated and strongly cemented, usually quite massive, rock-like horizon cemented to a consistency which varies between a stiff soil and very hard rock. Except for older Tertiary horizons these layers seldom exceed 1 m in thickness and are normally underlain by much softer or looser material.

7. Pedocrete boulders, cobbles and gravels: discrete or partially connected boulder or cobble-sized fragments formed by weathering and breakdown of hardpan pedocretes.

Brink (1985) reports further that the geotechnical properties of pedocretes are depending on mainly three factors: the texture of the host material, the stage of development to a pedocrete and the nature of the cementing or replacing mineral. For example, the properties of calcareous soils are closely akin to those of the host soil, whereas hardpan calcretes essentially behave as limestock rock. During pedocrete development, clay and silt become flocculated and cemented into larger silt to gravel-sized complexes of varying strength and porosity. These particles or aggregations may or may not break down during compaction of a road layer.

One of the most striking properties of some pedocretes is their ability to undergo a process of self-stabilisation. This self-hardening phenomena of, for instance, hardpan calcretes has been known for long periods in Namibia because these self-hardening materials have been extensively used as building blocks. Two of the many more examples are the old police station and the Lutheran church in Grootfontein which have been built from hand-sawn hardpan-calcrete self-hardening building blocks. It is, however, not possible to predict the grade of self-stabilisation of these pedocretes and determine its long-time effect on the bearing ratio of this building material yet. Thus the long time self-stabilising effect on a road layer must be currently regarded as an added and uncertain safety factor which cannot be used to establish the ultimate bearing limit of such a road layer (Brink:1985). But, it must be categorically stated that the self-stabilising phenomena has been proved with the aid of CBR strength tests in the laboratory of the Namibian Department of Transport. The CBR tests have clearly indicated an increase in strength by time-dependent self-stabilisation of the calcretes but the problem still prevails to prove this phenomena in-situ on the road. In-situ test methods like the plate-bearing test are not sufficiently accurate to prove the self-stabilisation effect. An in-situ accurate test procedure in order to prove the self-stabilising effect on the increase of the strength of calcretes must still be found.

Test methods for potentially self-stabilising materials include the soaked CBR test, carried out after allowing the compacted material to dry in the mould or after moist curing, or following wetting and drying cycles [24]. The reasons for the self-stabilising effect, as, for instance, one or other chemical or even organic constituent, are still not clarified as it is still not known which types of calcretes are more prone to self-stabilisation than others.

One of many proved examples for the self-stabilising effects of calcretes is the following: During the prospection for road materials for subbase and basecourse layers for main road 61 between Stampriet and Aranos numerous trial borrow pits have been opened by an obsolete bulldozer which opened the calcrete borrow pits without any effort. Six months later, during the starting phase of the actual construction of the road, it was encountered that some of the bulldozed materials in the borrow pits had become hard as concrete. During this period the natural moist of the calcretes had dried out and a hardening, self-stabilising effect had taken place.

In Namibia pedocretes, especially calcretes, have been used extensively in road construction. Calcretes are mainly used for wearing courses for unpaved gravel roads and for all layers of the road prism of a surfaced road, including basecourses and even surface chippings (Weinert:1980). In Namibia pedocrete surfacing chips have been, however, never used due to their excessive absorption of bituminous binder. In bituminous premix calcretes have been used successfully as crushed premix aggregate, as, for instance, on trunk road 1/10 between Tsumeb and Andoni (Code 64 on Ann.Table 1, Tk on Geological Map, 1980). This hard calcrete premix aggregate behaved well for more than 15 years under extreme high traffic loads before a re-overlay became justified, this time with a premix aggregate of Abenab-dolomite (Code 47 on Ann.Table 1, Na on Geological Map, 1980).

Although Brink (1985) reports that duripan pedocretes have been used in the Namibian south for all road layers up to subbase, this cannot be confirmed. It is doubtful whether duripans have ever been used as road building material in Namibia. It is also not on record that dolcretes from Tsumeb have ever been used. Phoscrete material is not encountered in Namibia. Silcretes which are mainly found in the Kalahari area are used predominantly for concrete aggregates because as material for road layers and wearing courses for gravel roads it is too hard and not sufficiently workable. Silcretes are mainly encountered in the Rundu and Gobabis areas but also in the vicinity of the Omatako Mountains between Okahandja and Otjiwarongo. The road building properties of silcretes are very similar to those of quartzitic materials. For instance, the Nosib quartz from Andara at the Okavango River which has been used as concrete aggregate for the construction of bridge 485 over the Okavango River at Bagani, has been, wrongly, regarded as a silcrete material. (Bridge 485 was the first bridge in Southern Africa which has been constructed by the ingenious German technique of 'Incremental Launching (Taktschiebeverfahren)'). Gypcrete compacted with water nearly saturated with salt makes excellent unpaved 'salt-gypsum-roads' along the Namibian Atlantic coast. Ferricretes have been, during 1964, successfully used as basecourse and subbase layers for trunk road 1/7 between Okahandja and Otjiwarongo in the vicinity of Sukses.

As wearing course for gravel roads calcretes are getting automatically the first choice because it is superior to any known alternative material. Even if calcrete wearing course materials are not in coincidence with the materials specifications, it has been proved that a high success rate still exists in contrast with most other road building materials which would fail under the same conditions.

In general it can be stated that pedocretes as road construction material behaved better than has been originally predicted and could have been expected on grounds of their gradings and Atterberg limits. Brink (1985) reports that failures and ruttings are rare and have generally been confined to areas of poor drainage of the road prism. But, pedocretes can also cause problems in road construction which the designer and builder of roads should be aware of to avoid serviceability limit failures of this otherwise excellent and very economic road construction material. Brink (1985) listed the problem-properties as follows:

1. Difficulty in locating suitable pedocrete materials.

2. Lateral and vertical variability in the borrow pit as well as on the road

3. Sensitivity to drying and manipulation of pedocrete material.

4. High apparent plasticity and poor apparent grading, often accompanied by high CBR values

5. Salt damage to road and drainage structures in gypcretes, calcretes and dorbanks.

6. Sulphate attack on concrete structures in gypcretes.

7. Aggregate degradation and segregation as well as compaction problems.

8. Determination of lime and cement stabilisation requirements and contents.

9 .Stabilisation cracking and failure to stabilise as well as loss of stabilisation.

10. Variable absorption and penetration of bituminous binder and prime coat.

Other potential problems include those of alkali-silica, carbonate and alumina reactions, which are, however, not experienced to date in Namibia, unusual reactions with lime and cement, heave, collapse and small to medium scale karst phenomena. Predicting methods of some of these problems have been determined by Netterberg [25] and are summarised in Table 5:

 

TABLE 5  RECOGNITION OF PROBLEM PEDOCRETES

 

|=====================================================|
| LOW RISK PROPERTIES     | HIGH RISK PROPERTIES      |
|-------------------------|---------------------------|
| Aggregate strength      | Water absorption          |
| Maximum dry density     | Natural moisture content  |
| Reserve of weatherable  | Linear shrinkage          |
| minerals?               | Potential swell           |
| SiO2/R2O3?              | Aggregation index         |
| Fe2O3/Al2O3?            | Optimum moisture content  |
| SiO2/CaCO3              | Salinity                  |
| Calcite/dolomite        | Gypsum content            |
|                         | Mica content              |
|                         | Organic matter content    |
|                         | Amorphus mineral content  |
|                         | Content of hydrated       |
|                         | halloy-site attapulgite,  |
|                         | sepiolite                 |
|                         | glauconite                |
|                         | Variability               |
|                         | Shrinkage limit           |
|=====================================================|

But, it must be kept in mind that not all pedocretes are problem materials. Their performances can range from poor to excellent. Good pedocretes are a very economic road building material and it is of utmost importance to increase the research effort to determine better and more water-proof prospecting methods to establish which pedocretes are potential problem materials and which can be safely and economically used.

The materials specifications for pedocretes in the different road layers underwent a constant change with the increasing experience in the treatment and properties of pedocrete road building materials, gained by the Department of Transport during the last twenty five or so years. It has been found that the traditional materials specifications which have been mainly assembled for non-pedocrete materials have been far too conservative for pedocretes in terms of their strict grading and plasticity requirements. Namibian-adapted materials specifications for pedocretes have not only been the result of extensive research and accumulated experience but also the result of some test sections where for instance sub-standard calcretes with relaxed requirements have been built and monitored. Typical examples are four test sections for the construction of surfaced roads:

1. T.R.3/1: Karasburg-Grünau: Relaxed specifications for calcretes for subbase and basecourse layers.

2. T.R.2/4: Kalkfeld-Omaruru: Relaxed specifications for calcretes for subbase and basecourse layers and lime-stabilisations.

3. T.R.2/3: Omaruru-Karibib: Relaxed specifications and substandards, self-stabilisation tests, low strength specifications (CBR tests), inverted profiles (Subbase was stabilised, but basecourse not).

4. M.R.49: Windhoek-Kupferberg: Low strength specifications.

Various test sections for calcrete wearing courses for gravel roads have been also constructed as for instance a test section on main road 49 between Windhoek and the Gamsberg just south-west of the Gurumanas River bridge. Other gravel road test sections, specifically for the testing of calcretes, shales, schists and quartzites are currently monitored during the 'MDS'-project which will be dealt with in more detail in the next chapter of this thesis.

The result of these test sections are different revised material specifications like those by Von Solms [26]. Other specifications for pedocretes have been suggested by Netterberg [27] and the Botswana Ministry of Works and Communications [28]. A new road building materials specification for the Namibian Department of Transport was released during 1987 and will be discussed in more detail in chapter 4 of this thesis.

The accumulated research effort regarding pedocrete materials for road building purposes in recent years resulted in the criterion that the most important tests for the selection of natural gravels and soils for pavement materials are those for compacted strength at the likely in-service moisture content and not so much those for the determination of grading requirements and Atterberg limits. Brink (1985) further recommends to control the CBR swell and to ensure that one is actually dealing with a pedocrete and not with a different material which only likes like a pedocrete. He suggests that a calcrete should preferably possess at least about 10 % equivalent CaCO3 in the fraction passing 425 µm.

The most important consequence of above test sections, however, is the result that the road building properties of sub-standard calcretes are at least equal to those of the much more expensive crusher-run basecourses. Deflection measurements during 1985 revealed that substandard-calcretes are superior to crusher-run basecourses with the not-foreseen consequence that multi-millions of Rand could have been saved to date if these results would have been known at an earlier stage. One example are the costs for type-A crusher-run and type-C natural gravel basecourses on main road 101 between Okahua and Okakarara. The tender price for type-A crusher-run basecourse was R 16,90/m3 and for type-C natural basecourse R 6,90/m3 (1980 prices). The total price difference for this 79 km long road would have been R 1.587.900,00.

Consequences have been the relaxed specifications regarding plasticity and grading for the upgrading of main road 67 from Pforte to Kamanjab and main road 68 from Pforte to Okaukuejo to surfacing standards. Since 1985 relaxations regarding strength to make full use of the advantageous self-stabilising properties of even weak calcretes are permitted, as for instance on main road 75 between Tsumeb and Tsintsabis where the CBR for basecourse has been relaxed from 80 to 40, the first real low-volume surfaced road in Namibia.

Crushed calcrete boulders and hardpan were used also as a coarse concrete aggregate for the construction of bridge 146 over the Omuramba Ovambo on trunk road 1/10 from Tsumeb to Oshivelo (Code 64 on Ann.Table 1, Tk on Geological Map, 1980). Brink (1985) rightly mentions that so far no problems have been experienced with the usage of calcrete as concrete aggregate but the future use of calcretes and silcretes should consider the possibility of alkali-aggregate reactions.

It must be, however, stated that pedocretes are in many cases difficult to locate. Caiger suggested to use aerial reconnaissance and interpretation of aerial photography for the prospecting of calcretes [29]. For in-situ field prospecting Netterberg and Overby recommended to use vegetation indicators, soil colours, and in the case of calcretes a simple probing device which has been successfully proved [30].

Brink (1985) reports that the oldest, thickest and best developed pedocretes occur as cappings to remnants of the African erosion surface which can normally be readily located. The younger pedocretes under the Kalahari sands, which are also the smallest and most variable of all the pedocrete occurrences, are much more difficult to locate. Examples of how to use the aerial photography and other locating techniques are given by Caiger [31]. Generally it can be stated that pedocrete materials like calcretes are occurring in situations where stereoscopic interpretation of aerial photographs are revealing fairly flat grounds with a collinear drainage pattern and, as indication of a poor drainage, a mottled, grey tone as well as on terraces, associated with seasonal drainage features such as pans, fossil drainage lines and ephemeral drainage lines. The quality of the calcretes generally improves with increasing distance from the centre of the drainage feature (Brink:1985).

In-situ tests in the field can use the colour of the covering soils which reveals a clear indicator for the occurrence of calcretes for any experienced road materials prospector. Other indicators for calcretes are vegetation indicators like the gabbabos (Catophractes alexandri), the salie or bitterbos (Pecheul-Löschea-leubnitziae) and the vaalbos (Tarchonanthus camphoratus). The calcrete probe is a simple but valuable device which enables the prospector to ascertain the presence or absence of calcretes to a depth of up to about 6 m within half a minute (Netterberg:1978). But it must be kept in mind that a technique which has been proved to have been worked well in one area may not necessarily work well in another, even if it likes that this area is similar to the former one. Field calibration of the photographic image of the aerial photograph should always be followed up as quickly as possible by field checking procedures like soils checks, vegetation indicators and field probing (Caiger:1968 and 1980). Difficulties can be especially expected in the zone of the Kalahari Group where the overburden thickness above the sparse calcrete occurrences can be substantial and prospect depths to up to 5 m can be experienced.

In the future pedocrete road materials will continue to play their important role as Namibia's natural road construction material number one. With the increasing provision of roads in so far underprivileged areas, pedocretes will play a special role in the establishing of labour-intensive low-volume roads. New research efforts should be used to prospect for and to use the right natural pedocrete materials at the right locations and especially to make an extra effort to investigate the self-stabilising properties of calcretes to make much more use of even sub-standard calcretes and to save millions of Rands to the Namibian taxpayer. As can be argued, convenient and over-safe 'first-world thinking' on the side of the road designer should be abolished and much more use of economically available natural materials should be made, as has been proved with the example of pedocrete road building materials like calcrete.

 

4.0  CONCLUSION

 

After having investigated and evaluated the environmental influences on road building as well as the properties of road building materials it will be possible to develop cost and quality optimised construction and maintenance models for Namibian roads. The knowledge about the location of road building materials and their properties will serve as the basis to an evaluation of pavements for bitumen surfaced and unpaved roads after having investigated all existing pavements in chapter 4 of this thesis. The interdependence between climate and geophysical influences like runoffs as well as road building materials and traffic loads will be exhibited in this chapter. It will be tried to show the connection between pavement structures, traffic loads and design life of all pavement types with the ultimate objective of developing optimised road models for a future optimal resource allocation for the roads system in an independent Namibia.

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