3.3  OCCURRENCE OF NATURAL ROAD BUILDING MATERIALS

 

An influence of climate on the quality of natural road building materials, as well as the general performance of roads, must be, however, realised. This effect is more pronounced in countries with a warm humid climate. A systematic approach on the effect of climate on the performance of road building materials has been established by the ' N-value' which represents the relation between maximum evaporation and the mean annual precipitation.

The 'N-value' [16] can be calculated as follows:

N = 372*Cj(0,58+0,14*W)/(MAP*W0Sj)

.All 'j'-subscripts are meant for the month of January, the warmest month generally in southern Africa, where:

C is the average depression of the wet bulb in (oC)
(difference of dry and wet bulbs (depression of wet bulb) of a pair
of thermometers: degree of saturation (evaporation) of the air)
W is the wind speed in (m/s)
MAP is the " Mean Annual Precipitation" in (mm)
W0S is a correction factor for the latitude, called the " Monthly Field Water Requirement Characteristic Constant" and is 0,75 for 18oS (northern border of Namibia) and 0,82 for 29oS (southern border of Namibia).

The integers of 'N' are significant, and only the following 'N-values' are of importance in road engineering in Namibia: N=2; N=5 and N=10. Where N is more than 5 disintegration, and where N is less than 5 decomposition is taking place. Where N is more than 10, no significant weathering is taking place, and there is only a thin layer, usually less than half a metre thick, of coarse gravel obtained from the disintegrated rock. Weathered rock for road construction for the "N>10-region" can be obtained from "borrow areas", i.e. the required quantity is obtained by ripping large areas to a shallow depth. The gravel contains few fines, because most fines have disappeared due to water and wind erosion. Some binder must therefore be added to this mostly cohesionless material to improve the road building material's properties for layerworks for paved roads and wearing courses for unpaved roads. The "N>10-region" is taking the whole south-west half of Namibia, i.e. south-west of a line stretching from east of Gobabis via Windhoek to the Kaokoveld.

Where N is between 5 and 10, disintegration still predominates, but a deeper weathering profile is usually present and road building materials can be obtained from " borrow pits". Also here it is normally required to add some binder to improve the properties of these gravels. As in the case of "N>10-regions" the physical break-down of the rocks still determines the rock properties of crystalline rocks. This region takes the whole north-eastern part of Namibia with the exception of parts of the Okavango and the Caprivi Strip, where N-values of between 2 and 5 are occurring.

Where the N-value is between 2 and 5, decomposition becomes the predominant form of weathering of crystalline rocks. Weathering from fresh rock to residual soil now develops. It is this region for low 'N-values' where a real negative influence of climatic factors on road building materials can be experienced.

When investigating the deposits of naturally occurring building materials in Namibia [17] (see Appendix Map 3) it can be derived that only crushed stones and gravels for surfacing chips, crusher-run basecourses and concrete aggregates can be dealt with in such a survey. It will not be possible to investigate the numerous borrow pits for the lower road pavement layers which, with some exceptions, are normally available in sufficient quantities. The listed building materials constitute normally outcrops of solid to semi-solid rock where processing such as blasting, crushing or screening will be necessary. Occurrences of loose, alluvial or colluvial gravel are only mentioned in those cases where these have economic advantages.

Outside the sand-covered areas in the eastern parts of Namibia numerous sources of rock and gravel have been investigated and it is normally not very difficult to locate additional deposits if required. However, in the sand-covered north-eastern and eastern regions of the country outcrops of solid rock and deposits of gravel that can be used for basecourse and subbase layers in road construction or as coarse concrete aggregate, are scarce. Only four occurrences have been so far proved to be suitable:

- Quartz gravel from Aranos
- Calcrete/ silcrete from an outcrop on trunk road 8/2 Grootfontein - Rundu, about 90 km from Grootfontein
- Silcrete/ quartzite from the vicinity of Rundu
- Silcrete/quartzite from Andara and Bagani in East Okavango

A number of rock outcrops are also known from Hereroland, Bushmanland, Okavango and Caprivi which have not yet been tested but which will be of some interest in the future. These are as follows (see Appendix Map 3):

- Marble, quartzite, calc-silicate, porphyry, calcrete, silcrete and gravel at various sites in the   Omurambas Epukiro and Eiseb
- Granite, quartzite, dolomite and other rocks in the Aha Mountains at the border between Bushmanland and Botswana
- Sandstone near Katima Mulilo
- Basalt under 9 to 16 m of silcrete at the rapids of the Zambezi River upstream of Katima Mulilo
- Basalt at the confluence of the Chobe and Zambezi Rivers
- Basalt at Ngoma at the Chobe River

As has been outlined earlier it must be stated that all gravel pits for the construction of the lower layers of a road pavement cannot be included in such a survey. There are literally thousands of such borrow pits scattered along all the full lengths of Namibia's roads, in the average one pit for every two to three kilometres of road. The most important parts of the pavement are, however, the basecourse and the bituminous surfacing. Locating sound rock suitable for these two layers can sometimes be of much concern, because of the very stringent specifications for these two applications. Only the sources of solid to semi-solid rock where processing such as blasting, crushing or screening will be necessary in order to produce crushed rock for the basecourse (referred to as crusher-run basecourse), surfacing chips or concrete coarse aggregates are dealt with.

The deposits of areas of loose gravels or rock which often have very definitive economical advantages are included in this survey. Such deposits may be of alluvial origin, when it is crushed mostly for the manufacture of coarse concrete aggregate, or it may be of a colluvial nature which when crushed makes excellent crusher-run basecourses as well as concrete aggregate.

Two distinct different codes are used on Appendix Map 3 for naturally occurring road building materials to identify the sources thereof:

1. The number inside the circle indicates the sources of rock for
the manufacture of surfacing chips, crusher-run basecourses
and concrete aggregates. These sources consist mostly of solid
rock but the worthwhile deposits of loose rocks or pebbles as
previously mentioned are included under this code.

2. The second code, a number inside a triangle, represents the
known sources for use in concrete as fine aggregate.

All sources of building materials with their specific coded numbers are summarised in tables 10 and 11 [17]: Table 10 for sources of rock with coded numbers in circles and table 11 for sources of sand for concrete with coded numbers in triangles.

 

TABLE 10  ROAD BUILDING MATERIALS IN NAMIBIA: ROCKS AND STONES

 

CODE SUITABILITY

DESCRIPTION OF ROCK

REFERENCE

QUANTITY

REMARKS

1
2
3
4
5
6
7
7
8
8
9
10
11
12
12
13
13
14A
14
15
Cx
Cx
Cx
Cx
Cx
CR, Ch, Co
Ch, Co, B
Co, B
CR,Co
Ch
CR, Co
CR, Co
CR, Co
Ch
CR, Co
CR, Co
Ch
CR, Co
CR, Co
Co

Dolerite, Solid Boulders
Dolerite, Solid Boulders
Dolerite, Solid Boulders
Solid Dolerite
Dolerite, Solid Boulders
Surface Quartzite Slab
Solid Dolerite
Baked Shale
Sandstone
Dolerite Boulders
Baked Shale, Tillite
Silcrete Capping
Gneiss Outcrop
Solid Dolerite
Hard Tillite
Granite Outcrop
Dolerite Dyke
Granite/Gneiss
Gneiss
Fish River Alluvial

BP 6415
BP 7495
BP 8555
BP 8825
BP 9530/40/60
        -
commercial source
commercial source
KB 230A
KB 230C
KB 150A
SL 36+600
SL 13+200A
KB 1110A
KB 1112A SL82+200A
SL82+200
Municipal QuarrySL81+800A
KB 1520

unlimited
unlimited
unlimited
unlimited
unlimited
large area
large area
large area
unlimited
unlimited
unlimited
unlimited
unlimited
unlimited
unlimited
unlimited
limited
unlimited
unlimited
-

-
-
-
-
-
no blast
-
-
-
-
check tillite for CR
used by LTA
not used
used by Herma
-
-
used by LTA
-
used by LTA
-

16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
CR, Co
Ch, CR, Co
Ch, CR, Co
Ch, CR, Co
Ch, CR, Co
CR, Co
CR, Co
Cr, Co
CR,Co
CR, Co
Cx
CR, Co
CR, Co
Cx
CR, Co
CR
CR, Co
CR, Co, Ch
CR, Co
CR, Co
CR, Co
CR, Co, Ch
CR, Co, Ch
CR, Co
CR, Co
CR, Co
CR, Co
CR, Co
CR, Ch, Co
Cx
CR, Ch, Co
CR, Ch, Co
CR, Co
CR, Co
CR, Ch, Co
CR, Co
CR, Co
CR, Ch, Co
CR, Ch, Co
Co
CR, Ch, Co
CR, Ch, Co
CR, Ch, Co
CR, Ch, Co
CR, Co
CR, Ch, Co
CR, Ch, Co
CR, Ch, Co
CR, Ch, Co
CR, Co
CR, Co
CR, Co
CR, Co, Ch
Co
Co
Co
Co
Co
Co
Co
CR, Co
CR, Co
Ch
Co
Co
Co

Solid Sandstone
Dolomite Outcrop
Dolomite Outcrop
Alluvial Boulders
Dolomite
Basalt
Sandstone, Tillite
Fish River Alluvial
Quartz Pebbles
Surface Quartz Pebbles
Aris Phonolite
Dolerite Dykes, Boulders
Marble Outcrop
Rössing Quartzite
Indurated Shale
Limestone/Marble
Granite Outcrops
Dolerite Dyke
Crystal Limestone and Dolomite
Crystal Limestone and Dolomite
Dolomite (coarse grained)
Dolomite (Hill)
Dolomite (Hill)
Colluvial Quartz Pebbles
Quartz/Sandstone
Colluvial Quartz Pabbles
Crystalline Limestone and Marble
Crystalline Limetone
Dolomite (Hill)(Elefantenberg)
Dolomite
Dolomite
Dolomite (Berg Aukas)
Calcrete/Silcrete Bank 15 m
Silcrete
Quartzite (Dum Dum)
Silcrete
Silcrete
Dolomite Outcrop
Dolomite
Silcrete/Quartzite
Basalt Screes
Dolomite
Dolomite
Dolomite and Limestone
Chert Pebbles
Quartzite (Ruacana)
Dolomite
Dolomite
Solid Calcrete/Silcrete
Granite/Gneiss
Granite/Gneiss
Quartzite Boulders
Dolomite
Alluvial Conglomerate
Quartzitic Alluvial
Surface Quartzite
Alluvial Conglomerate
Solid Quartzite Formation
Alluvial Rock
Surface Quartzite
Quartzite
Quartzite
Solid Diabase
River Gravel
River Gravel
River Gravel

KB 1920A
KB 3020A
KB 3320A
SB 195A
SB 1A
BP 40.8 + others
several sites
several sites
SA/21/KS and other
BP 35 and others
Commercial Quarry
BP5 and other
BP 72
Commercial Quarry
Karibib
BP 49 and other
BP 12A
BP 5
BP 72 and other
BP 10
BP 22 and other
Commercial Quarry
BDID and other
BP 88,8 and other
Witvley
several sites
BP 7, 15, 28
B1a, B3, B9 et al
B12
Gabus Quarry
B1 - B9
several
BP 33, 44
BP 336
BP 225
Masare
Tkatkawe
BP 8
Henning Crusher
Bagani/Andara
Swartmodder
B2 - B5
B6
BP 45C, 43D, 31D
BP 036, 060, 9B et al
Quarry
Commerc. Quarry
Commerc. Quarry
Clifford & H Quarry
-
several sites
Omatako
Abenab
Holoog River
Several rivers
Rep.26/1 Scott&DW
several rivers
-
Konkiep River
-
SB 2A
-
SB 3A
-
-
-

unlimited
large
-
large
unlimited
unlimited
large
large
limited
large
large
limited
limited
limited
large
unlimited
unlimited
small
unlimited
unlimited
unlimited
-
unlimited
150 000 m³
unlimited
limited
unlimited
unlimited
unlimited
unlimited
unlimited
unlimited
large
large
limited
limited
limited
unlimited
unlimited
unlimited
limited
unlimited
unlimited
unlimited
limited
limited
large
unlimited
limited
large
large
unlimited
unlimited
limited
limited
-
limited
unlimited
-
-
unlimited
unlimited
150 000 m³
limited
limited
limited

-
ACV borderline case
-
-
-
weather. problems
plastic fines
Nat.Conserv.Area
several smaller ar.
no blast
-
-
-
-
variable
-
-
-
-
-
-
-
-
several
-
-
-
-
-
-
-
ex mine
blasting
-
-
-
-
-
ex Tsumeb Mine
-
Monument area
-
-
-
large area
-
-
several
premix suitable
seawater problem
soluble salt problem
no blast
ex mine
-
Commercial Source
-
-
Commercial Source
-
-
along T.R. 14/2
  

NOTA: The suitability is abbreviated as follows:

CR = Crushed Basecourse or Crusher-Run Basecourse
Ch = Surfacing Aggregates or Chips
Co = Coarse Aggregate for Concrete
B = Railway Ballast
Cx = Basecourse, Chips, Concrete, Ballast


Sand for the construction of roads and for concrete in structures is usually available in sufficient quantities throughout Namibia. The building sand requires, however, often some procession to meet the construction specifications for the various uses. Sources of sand that are known to be suitable as concrete fine aggregate, are shown in table 11 [17].

 

TABLE 11  ROAD BUILDING MATERIALS IN NAMIBIA: SAND

 

CODE

DESCRIPTION OF THE SAND

REMARKS

REFERENCE

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
Dune Sand
Dune Sand
Aeolian Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Dune Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Quartzitic Sand
Quartzitic Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Dune Sand
Granitic Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Granitic Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Alluvial Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
In-situ Sand
Kalahari Sand
Kalahari Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
River Sand
Very fine for Co
Very fine for Co
Very fine for Co
Skaap River
Neiams River: fine
Koriseb River
Gurieb River: coarse
Fine to coarse
Konkiep River
Holoog River
Several occurences
Olifants River at 2 m below
-
Several rivers like: Khan, Etiro, Omaruru: Requires screening
Along trunk road 7/1
Otjitazu River
Mooilaagte River
Erundu River (several)
Red Gousob and Naus Rivers
-
Hom River
Several rivers
Löwen River
Konkiep River
-
Hutup River: blend with Fish River sand
Fish River
Small structures
Overburden
Zougab River
Gaub River
Kuiseb River
Most rivers suitable
Most rivers suitable
In-situ below overburden
White Nossob River
White Nossob River
Better than Seeis River
Khan River
Several sources
Most rivers suitable
Ugab River
-
Omuramba Omatako
On coarse side
On coarse side
Swakop River
Very fine. limited quantity
Ugab River
-
Aroab River
Asab River
-
Okavango River
Okavango River
Very fine: has to be blended
Blended with crusher dust
Blended with crusher dust
Several rivers
Omuramba Omatako
-
Otjikaro River
Omuramba Omatako
Omaruru River
Sny River
Otjihavera River
6115
45.OB
SL28 + 880B
KB 929B
KB 1910B
KB 2590B
KB 3072B
SB 482B
SB 2B
-
SA/12/P typical
SA/36/P
BP 75
-
Several sources
Several rivers
S219 + others
1479/CS2
1479/CS
Report 26/1 Sdw
-
-
-
-
Several sites
-
-
Commercial source
Several sites
-
-
-
-
Several rivers
Several rivers
-
-
-
-
-
-
-
-
-
Several rivers
Several rivers
-
-
-
Several rivers
-
Several sites
Several rivers
Andara area
Rundu area
-
-
-
-
-
-
-
-
-
-
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