|8 TECHNICAL ASPECTS OF
APPROPRIATE LOW-VOLUME ROADS
8.1 GENERAL APPROACH
As had been stated above, the roads system in the "modern" sector of Namibia has been well established over the last twenty five years. These roads have been planned, constructed and maintained to a high degree of technological sophistication and provide a more than adequate service to the central and southern parts, the so-called modern sector of Namibia. Almost 80% of all these roads in the "modern" sector can be classified as low-volume roads which in many cases are designed and constructed to exaggerated high standards (only 3,9% of all proclaimed roads (except farm roads) in Namibia carry more than 600 vehicles per day: see nota to table 5). In the lesser developed areas in Namibia with, partly, the highest population densities in the country, a comparable roads system does not exist.
Careful planning and new ideas must be provided to ensure the most economical roads service. A new approach regarding appropriate, low-volume and low-cost-roads must be followed and complete new design concepts at variance with those of high standard-thinking will be required. Before this can be done it is a prerequisite to know the environmental influences on road building in Namibia as well as the occurrences and properties of road building materials. This knowledge is the basis to develop adequate low-volume road concepts for Namibia.
There is need to appreciate Namibia's geographical and ecological situation. The dry climate means less or even no seasonality problems like in other African countries and allows road transport around the year, even on unpaved roads. With many Namibian roads to be nearly always all-weather roads, transport costs are small relative to other IDC's ( Independent Development Countries). This in turn means less expenditure and easier transport distribution activities. Unconventional transport modes should be incorporated in any low-volume-roads design parameter development. Bicycles, motor cycles, motor scooters and other light motor vehicles can, because of the relatively dry and flat terrain in many parts of Namibia, serve the remotest corners of the country.
Technologically, the Namibian engineering community can do what is necessary to help meet the challenges facing the independent Republic of Namibia, but it sometimes forgets that Namibia is part of the "Third World" and that the application of inappropriate technologies has to be avoided. Any new design method must take " Namibia-Adapted Traffic Patterns" into consideration and " First-World-Thinking" has to be abolished. The definition of such a "low cost roads system" has been established by the 10th International Road Congress :
"A low cost road is one, which having regard to considerations of climate and traffic, has been located and built to geometrical standards commensurate with future requirements, but has been constructed with bases and surface to meet the present traffic requirements. It is, however, one which should be so designed, constructed, and maintained that it allows for stage construction when traffic requires it and improvement in economic conditions permit".
Should the replacement of, for instance, existing road infrastructures be considered in the future, the prescribed standards should be reviewed. It may be possible that a cost-benefit analysis could show that it would be to the advantage to replace a paved road with a low-volume-road like a gravel or low-volume paved road (Class B, C or D road) for instance, rather than to rebuild the paved road to the same standard.
The following aspects require attention for this new policy:
The indirect influence of poor "other" roads or even the non-existence of such "other" roads outside the presently well established "modern" sector of Namibia has a very direct bearing on transportation costs and the whole economy of these underdeveloped areas where a sound infrastructure must save existing complexes and newly to be established agro- and other industries to create labour possibilities in these densely populated Namibian areas.
Any appropriate low-volume road concept has to be judged under the following three basic essentials:
(i) The concept has to be sound in an engineering sense. Where are the technical boundary conditions?
(ii) The concept must be cost-optimised and economically
and financially feasible.
(iii) The concept must solve social problems like
unemployment problems and others under the premise that als
8.2 PERFORMANCE OF LOW-VOLUME AND LOW-COST ROADS
The demand to construct optimised technically and economically appropriate roads must lead to a complete new philosophy of low-volume-roads.
On a random basis road authorities in South Africa, Zimbabwe, Zambia, Botswana and also in Namibia have gained experience and actual "know-how" mainly through failures, trial and error and sometimes successes in the performance of low-volume and low-cost roads. Unfortunately this wealth of experience is usually not recorded in a systematic and coordinated manner. It is also seldomly shared between countries and road authorities with similar interests.
8.2.1 DESIGN STANDARDS FOR ROAD PAVEMENTS
A comparable fraction of roads in the "modern" sector of Namibia has been designed and built to a higher standard than could be economically justified, as outlined in former chapters of this thesis. It was shown that most roads in these areas are in fact low-volume roads with a distinct low-traffic-volume character. In respect of the pavement design for the future expanded roads system in structural under-developed areas in Namibia, there will be much room for innovations and re-thinking in the philosophy of pavement designs.
With above input parameters in mind new design concepts like single lane paved roads or spoorbaan roads (as explained later) could be proved to be a success. It is proposed that these low-volume-roads must be divided into the following classes i.e.:
A. Graded and improved earth roads (in-situ
materials): CLASS A
The annual average daily traffic ' AADT' is used as basic design parameter for all four classes of low-volume roads. The design 'AADT' is the 'AADT' at the end of the design period. The 'AADT' is expressed in the number of light vehicles where one heavy vehicle has a light vehicle equivalence factor of:
- over flat terrain 2,0
These equivalence factors are valid for all four classes of low-volume roads.
8.3 DISCUSSION OF A PROPOSED SYSTEM OF LOW-VOLUME ROADS
8.3.1 CLASS A LOW-VOLUME ROADS: EARTH ROADS
220.127.116.11 DESIGN CONSIDERATIONS
The need for cheap, but at the same time efficient low-volume-roads in thinly populated areas in Namibia is resulting in the development of earth roads whereby use is made of local natural materials. This type of road is the lowest possible standard road and is the first step from the non-built and randomly performed "veld track" or "other" road to a technically controlled and built-up road for very low traffic. Sometimes the earth road is regarded as the first phase in the construction of a road of higher quality by employing special treatment to the base and surface to enable it to carry heavier traffic. Chapter 9 revealed (figure 15: Optimal Point of Surfacing Unpaved Roads) that it will be cost and quality optimised to improve this lowest class of a low-volume road to the next class for a traffic load of 40 vehicles per day. The construction standards for such roads will be compared with those proposed by the German " Federal Ministry of Economic Co-operation" ( Bundesministerium für Wirtschaftliche Zusammenarbeit: BMZ)(tertiary roads in Africa: type III/3 (access roads) and III/2 (feeder roads)) .
An earth road is subject to erosion, potholing, corrugation and dusting. Some basic properties of such roads must be considered and are proposed below, based on personal experience.
18.104.22.168.1 GEOMETRICAL DESIGN PARAMETERS
- Some basic alignment without geometrical and vertical design and staking should be envisaged. It should follow the shortest possible line - i.e. the most effective and economical alignment with the avoidance of min. radii at the beginning and end of straight sections of more than 5 km length and for isolated curves.
22.214.171.124.2 STRUCTURAL DESIGN PARAMETERS
300 mm over heavy sand and highly plastic material
For more details for a proposed appropriate materials specification for class-A low-volume roads see chapter 7.
126.96.36.199 CONSTRUCTION CONSIDERATIONS
The construction of an earth road comprises the preparation of the subgrade or fill followed in some isolated cases -if warranted- by the construction of a base or gravel layer and the laying of a wearing course, but it is in most cases not required for reasons of economy. This type of minimum standard road with a traffic load of between one and 30 vehicles per day is representing approximately up to 35% (nota to table 5: 32,3%) of all existing and required roads in Namibia. The construction to above mentioned minimum standards could be done especially labour-intensive by small local private contractors, village-communication-projects, small departmental betterment units and should not be undertaken by modern "first world type" private contractors. Construction is straight forward and can be done with minimum training provisions. The equipment required can be anything from a mule-drawn-grader or an ox-cart to the latest modern equipment available. Construction costs can, depending on circumstances like required drainage/structures and equipment used, vary from a few hundred US $ per km to about US $ 12.000 per km.
188.8.131.52 MAINTENANCE CONSIDERATIONS
Maintenance will be dependent on the volume and type of traffic, the existing in-situ road material and climatic factors like rainfall intensities etc. Shaping with a motor grader or with unconventional equipment like a large tree branch will be possible and economic. In sandy areas the innovation of the Namibian Department of Transport, the unique " sandspoor grader", drawn by a tractor in Kalahari areas, which also can be drawn by oxen or mules, can be used economically. Grading will normally be required every three to six months. With current average motor grader costs of US $ 6,00 per blade kilometre (December 1989) the maintenance costs will be between US $ 50 to US $ 100 per km and year.
8.3.2 CLASS B LOW-VOLUME-ROADS: GRAVEL ROADS
Roads in this category amount to approximately 55% (nota to table 5: 57,1%) of the existing road network, mainly in the "modern" sector of Namibia. These engineered gravel roads are providing an important service in linking smaller towns and centres with the major arterial roads. The existing roads in this category are mainly main and some more important district roads. The design and construction standards of these roads in the "modern" sector are in many cases too high for the existing and envisaged traffic loads and low-cost applications will have to apply for this category of roads much more than in the past. ('BMZ' tertiary roads in Africa: type III/1 (connecting roads) and II (secondary (regional) roads)). Chapter 9 revealed (figure 15) that it will be cost and quality optimised to improve Class B of a low-volume road to a Class C (appropriate, low-volume paved road) for a traffic load of 125 vehicles per day and to a Class D (appropriate spoorbaan-type road)("Spoorbaan" road has been derived from the original German "Spurbahn" concept for low-traffic-roads in Germany) for a traffic load of approximately 100 vehicles per day.
184.108.40.206 DESIGN CONSIDERATIONS
This type of road with a calculated traffic load of 30 to more than 100 'AADT' ( BMZ: <60 'AADT' for type III/1 and 60-120 'AADT' for type II road) for the prognosis year is representing an intermediate road-class between the normal earth road (Class A) and a paved low-volume road (Class C or Class D). The following basic design requirements should apply.
220.127.116.11.1 GEOMETRICAL DESIGN PARAMETERS
- Final alignment by proper geometric design - horizontally and vertically.
- Design speed: 100 km/h for flat terrain
80 km/h for rolling terrain
60 km/h for mountainous terrain
BMZ: < 60 km/h for type III/1 road
60-80 km/h for type II road.
- Min.radii: 500 m for flat terrain
150 m for rolling terrain
50 m for mountainous terrain.
Avoid min. radii at the beginning and end of all straight sections with a length of more than 5 km
and or isolated curves. Curves with larger radii must be used where terrain permits. (No
- Width of road: 7 m including shoulders: 'AADT' < 50
Gravel Wearing Course: 5,00 m
Shoulder width: 1,00 m minimum (fill material)
BMZ: 6 m + 2*0,75 m shoulders: 'AADT' < 60
9 m including shoulders: 'AADT' < 125
Gravel Wearing Course: 7,00 m
Shoulder width: 1,00 m minimum (fill material)
BMZ: 6,00 m + 2*1,00 m shoulders: 'AADT' < 120
- Max.slope: 10% over rolling terrain
12% over mountainous terrain - 300 m max.length
- Side slope: 1:2 maximum
1:6 in sand (Kalahari)
- Cross fall: 1:40 (No 'BMZ'-standards).
18.104.22.168.2 STRUCTURAL DESIGN PARAMETERS
- For a traffic load of more than 50 'AADT' per day the road should be constructed to all-weather standard, with minor structures for return periods of between 5 and 15 years and with major structures (bridges) for a return period of 25 years (currently 50 years). For a traffic load of less than 125 ' AADT' per day major rivers should be crossed by low-level bridges or concrete causeways.
For more details for a proposed appropriate materials specification for class-B low-volume roads see chapter 7.
22.214.171.124 CONSTRUCTION CONSIDERATIONS
This type of higher standard gravel road should be constructed generally by departmental construction forces or local informal contractors. Modern "first world type" private contract construction should be rather the exception.
Labour-based techniques can be applied in a restricted manner only, for instance shoulder and slope trimming. But, even here innovative new ideas for labour-based activities have to be developed, tested and used. Under normal circumstances the road prism itself will be constructed by normal equipment-based techniques.
Fill material for this type of road is obtained from sources as near as possible to the road or even from within the proclaimed road reserve (normally 60 m). The last method also facilitates, if properly applied, the drainage of the road which is contrary to current policies of construction. It is also economical to allow the traffic to compact the fill for a while, prior to placing the gravel wearing course.
It has been found that the so-called "private haul-contractors" for placing gravel wearing course material were an economical proposition for gravelling or regravelling roads for many years. Production by gravel units of the Department of Transport can reach up to 10 km of re-gravelled road per month. The construction costs are influenced by the standard of drainage structures, topography, rainfall and run-off, road building materials and the accepted design standards as established for the extrapolated traffic loads for the prognosis year.
Construction costs vary between US $ 12.000 per km for a minimum standard gravel road without larger drainage structures and more than US $ 50.000 per km for a full all-weather gravel road in difficult terrain whereby the latter can reach construction costs which are similar to those of an appropriate paved road.
126.96.36.199 MAINTENANCE CONSIDERATIONS
Depending on the type and volume of traffic the life of the gravel wearing course (normally under Namibian conditions between 6 to 15 years: see chapter 9 and figure 12: average annual gravel loss for 100 vehicles per day: 8-10 mm) is greatly influenced by the regularity of grader maintenance as well as occasional patching with gravel in cases where the wearing course is busy to deteriorate. An important factor is replenishing the fines in the wearing course from the shoulders or sides of the gravel road where a good grader operator will always leave a wind-row for this purpose. For instance, in Kenya between 25 and 33 t of material per kilometre can be lost annually in the form of dust  i.e. a layer 1,8 to 2,5 mm thick, thus creating an environmental hazard next to such roads and potential traffic hazards from dust on these roads. This rate is given by the following formula:
GLA = f(TA2/(TA2+50))*(4,2+0,092TA+3,50RL2+1,88VC) (mm)
where: GLA is the annual gravel loss (mm)
TA is the annual traffic
volume in both directions,
This dust problem is creating a definite limit to the performance of a gravel road and to the volume of traffic handled by it. In many parts of Namibia the in-situ soil material forming an earth road has quite favourable properties and is not creating excessive dust problems, but has therefore the tendency of slipperiness in the rainy seasons. In these cases sound engineering judgement has to be used in regard to the gravelling such an earth road.
The life expectancy of a Class-B low-volume-road is running between 6 and 15 years, depending on the specific traffic volumes and patterns. Frequency of grading should be approximately every three to four weeks (17 times per year) with a traffic volume of more than 100 vehicles per day. The first gravel wearing course also has a shorter life owing to consolidation and compaction under traffic, and it was found that a second gravel wearing course after 5 to 12 years had a much longer life.
The maintenance costs with aid of a motor grader will be US $ 200 to US $ 900 per km annually; re-gravelling costs are varying from US $ 3.000 to US $ 18.000 per km (Maintenance costs: Department.
Under Namibian conditions, with a traffic volume of between 100 to 150 vehicles per day, the problems with dust forming, safety, environmental hazards, road maintenance and vehicle operating costs on gravel roads can become unbearable and surfacing of these roads must be envisaged. Even special roads like spoorbaan roads which can be economically comparable to a full scale gravel road, can be more advantageous and much better to use than a Class B gravel road. Another problem arising is the growing scarcity of proper natural gravel wearing course material sources, which leads invariably to longer hauls and also increasing maintenance costs. Under these circumstances the surfacing and the development of these roads to a Class C or Class D type low-volume-road must be considered.
8.3.3 CLASS C: LOW-COST ROADS WITH BITUMEN PAVING
Low-cost-roads with special bituminous surface dressings for light traffic volumes were not built in Namibia until 1989. All paved roads have been constructed to considerable high construction standards using conventional design concepts. Some sections of the older paved trunk roads built in the fifties and sixties are not capable any more to carry the current traffic loads safely and had or have to be re-surfaced, rehabilitated or re-built. The reason for the relatively poor performance of these roads is not that they have been planned and designed originally as low-cost or low-volume roads but that forward planning of correct future traffic loads for a realistic prognosis year was not done so far in accordance with scientific principles and that the current traffic loads have not been envisaged. These road sections were also not under-designed but execution mistakes during the construction phase occurred, for instance defective materials not according to specifications or dimensional mistakes like layer thicknesses not to specification etc. Low-cost-roads may be proposed only in accordance with a proper traffic load prognosis and for traffic loads between 100 and more than 500 vehicles per day for the assumed prognosis year (see chapter 9 and figure 15: Optimal Point of Surfacing Unpaved Roads).
Except in the case of some important link roads in the Windhoek and Swakopmund/ Walvis Bay/ Rössing vicinities as well as the central parts of Owamboland and some of the strategically important arterial roads between Namibia's southern and northern borders and to the Atlantic coast, all presently paved roads would fall into this low-volume-road category.
188.8.131.52 DESIGN CONSIDERATIONS
The main principle for the design of low-volume bituminous surfaced roads should be " stage construction" and special lower design criteria according to specific Namibian conditions. For low-volume-roads with very low traffic and where, for a variety of reasons, a paved road will be desirable, "one-lane-roads" (4 m wide) with wider shoulders (at least 3 m wide at each side) or even "Zimbabwe-type-surfaced bituminous strips" on a natural gravel base from approved borrow pits as "first stage" should be considered. No specifications for this type of road exists in the ' BMZ'-standards but a basic comparison will be made between the design considerations of a Class-C road and the 'BMZ' type I (primary (national) roads) category.
The following basic design requirements should be complied with:
184.108.40.206.1 GEOMETRICAL DESIGN PARAMETERS
220.127.116.11.2 STRUCTURAL DESIGN PARAMETERS
For more details for a proposed appropriate materials specification for class-C low-volume roads see chapter 6 (section 6.8) and chapter 7.
18.104.22.168 CONSTRUCTION CONSIDERATIONS
The same construction methods can be applied to these low-volume paved roads (stage construction: - surfaced strips; -one lane surfacing; -two lane surfacing) as for Class B gravel roads, although tighter control on compaction, finishing, tolerances etc. must be applied. Proper drainage of the road prism requires special attention. Any ponding of rain water at structures and/or drainage banks or drains must be prevented for this special type of road.
Priming and surfacing are to be applied to normal conventional accepted standards for surfacing works on the current paved road network in Namibia, under the proviso that a maximum of labour-based techniques must be applied.
In respect of the first stages (surfaced strips or surfaced single lane construction for traffic loads less than 80 respective 120 vehicles per day) it seems to be more favourable to use departmental construction forces to build these roads. A thorough investigation regarding the effectiveness of these state-run departmental construction units will, however, have to be lodged. A fair and complete comparison regarding all relevant costs between the running of state-owned construction units and private contracting construction units will be necessary before deciding upon who is the most cost-efficient to construct these low-volume paved roads.
A cost analysis for this type of road is difficult to make at this stage. The costs for a double lane low-volume paved road could be 25%-40% cheaper than a conventional type of paved road (currently between US $ 100.000 and US $ 190.000 per km)(December 1989 prices). Stage construction with strip-surfacing or single lane-surfacing will achieve 15%-25% more savings against above two lane low-volume paved road.
Following are some specifications for bituminous binders and aggregates for paved roads which were used in Namibia with advantageous results.
22.214.171.124.1 BITUMINOUS BINDERS
(i) MSP 1 (inverted
emulsion)(inverted: less viscous)(MSP: medium setting prime)
FOR SEAL WORK
(i)-(iv) for surface dressings; (v) for asphaltic concrete
for slurry: (i): "stable" grade or (iv) plus +/- 1,5% diesel
or paraffin; (vi)-(viii) for rejuvenating old seals.
126.96.36.199.2 TYPE OF AGGREGATES
The nominal sizes of the aggregates used mostly are 19 mm, 13 mm and 9 mm or 7 mm stone chips. For normal slurry, coarse slurry and blotting of binder normal grading crusher dust (-4,75 mm) or coarse graded crusher dust (-6,70 mm) are used.
Geologically speaking the mostly used aggregates for surface dressings are: Dolerite (Karoo Sequence)(ex Keetmanshoop), Dolomite (Damara Sequence)(ex Outjo, Tsumeb, Grootfontein), Gneiss (Damara Sequence)(ex Rössing) and Phonolite (Tertiary)(ex Aris). Quartzite (Sinclair Sequence)(ex Witvley), however, was also used as surfacing aggregate as well as a type of gneiss/ sandstone (Damara Sequence)(ex Bagani). Quartzite, quarts and even a very sound calcrete as well as dolomite and phonolite were used in premixes on roads throughout Namibia. For all these aggregates no polishing problems were encountered except in isolated cases for phonolite (mainly in municipal areas on intersections with heavy traffic but if the stone is getting smooth it is time for a reseal in any case). (see chapter 6: The Location and Properties of Road Building Materials).
188.8.131.52.3 SEAL TYPES
184.108.40.206 MAINTENANCE CONSIDERATIONS
Low-volume paved roads of this type will naturally require more light maintenance in the form of patching, slurry-sealing, sand-sealing or even full re-sealing in its early life than a conventional high-standard paved road. Maintenance costs per km per year are in the region of US $ 365/km/year against US $ 250/km/year for a conventional bitumen-surfaced road. Maintenance can be, however, organised effectively by labour-based means -a great advantage- because these types of roads are required mainly just in the most densely and so far infrastructurally underprivileged areas in Namibia with high unemployment rates.
8.3.4 CLASS D: LOW-COST ROADS WITH CONCRETE PAVINGS
With the ever-increasing costs of bituminous materials refined from crude oil as well as the fact that such resources are in any case not originated from Namibia yet, it may be well worthwhile to investigate the application of cement-concrete as a wearing course in place of bitumen products - even on low-volume roads. All ingredients for making cement concrete can or could easily and cheaply be obtained locally in Namibia. The idea to build unique spoorbaan concrete roads could assist to promote the Namibian cement industry making Namibia more independent economically and stimulating employment for Namibians and general economic growth.
It is argued that besides the local content argument cement concrete pavements have other advantages against bituminous surfacings. In an economic respect, with the continuously increasing costs of bituminous materials and the drain on foreign currencies, cement concrete pavements can well prove to be competitive against bituminous surfacings. These pavements can be built by labour-based and " Namibia-Adapted-Techniques". This type of road can be built almost completely by manual means, because the demands regarding the quality for subgrade, subbase and basecourse are lower than for flexible, bituminous pavements ( surface dressings). The latter needs much more equipment-based activities. Even the concrete pavement itself and not only the subgrade/subbase layers can be built manually as has been proven on a current concrete roads contract in the Eastern Cape in South Africa. "Construction Week" reported on 27 March 1986 that 50 black men are building a section of a concrete road manually in the Eastern Cape, South Africa to provide work to the unemployed at US $ 2,40 (for December 1989 consumer price level) per day.
One very economic application of cement concrete pavements can be the development of cement-concrete-strip-roads in spoorbaan technique as a real "Namibia-type" low-volume-road. The optimal application of such spoorbaan concrete strips lies in Owamboland with its dense population concentrations, where paved low-volume-roads are most seriously needed, but can also be used in areas like Okavango, Caprivi and Hereroland. In these areas there is a general shortage of appropriate natural road building materials which restricts the construction of Class A earth or Class B gravel roads. This policy would embrace the use of alternative materials and construction methods enabling not only cost savings to be effected and creating employment possibilities for the impoverished local inhabitants, but at the same time providing a higher standard of comfort, convenience and safety to the travelling public, especially during the rainy season. Chapter 7 revealed that it will be cost and quality optimised to improve a Class A of a low-volume road to a Class D type spoorbaan road for a traffic load of 70 vehicles per day.
These concrete-strip-roads can be built by using precast concrete blocks of a particular design, the unique spoorbaan road. The principle of a concrete stone road is the laying of prefabricated high-quality concrete stones made from local materials on a prepared sandbed "by hand" . It will be demonstrated that the costs of such a road will be comparable to that of a more conventional design, that of the built and gravelled Class B low-cost-road for between 30 to 125 vehicles per day.
Although a Class B road is an improvement over a "veld track" and also over a Class A earth road it has all the disadvantages of a gravel road, especially the relatively high maintenance costs, the lack of comfort, dust hazards, corrugations, the non-trafficability during the rainy season and the effects of wear and tear to the user's vehicle. These disadvantages can largely be negated by the strip road concept and within this framework the use of the spoorbaan system will be shown with its own unique set of benefits.The original method of concrete stone roads was developed in Germany but was not widely used in road construction, because this method is too labour-intensive for European conditions (The interlocking concrete stone concept was used during the Second World War in Germany to repair and construct quickly and efficiently war damaged roads).
All the materials for concrete strip roads are locally available. The strip road can be manufactured from sand concrete in areas where no concrete aggregates are available. As subgrade/subbase for these strips the locally available sub-standard calcretes could serve. The strips could be constructed quickly and effectively by labour-based means. These all-weather type and dust-proof strip roads can carry any traffic load up to approximately 100 vehicles per day safely and economically. This requirement is fulfilled by the majority of feeder roads in the more densely populated areas in Owamboland, Okavango and Caprivi.
In cases of exceptional floods and partial washaways individual blocks can be re-laid thereby making the repair of such spoorbaan roads as simple as that for gravel roads. The blocks can be cast at central locations where suitable sand for sand concrete is available, on a repetitive basis made by a mobile machine or by hand. Blocks can then easily be replaced if they are damaged and have a high salvage value if the road is abandoned or relocated. Although this simple concept was - according to the author's knowledge- never used for big scale roads projects, it is suitable for heavy wheel loads. The principle characteristics of interlocking concrete block paving which are relevant to road engineering, include:
- No reliance on imported material.
The blocks are manufactured on a mobile vibratory block making machine thus ensuring tight control on the quality of the final product. (Appendix Sketch 1 (Appendix 7) shows a sketch of the spoorbaan road crossection (only in the original thesis)).
220.127.116.11 DESIGN CONSIDERATIONS
Principle for the design of spoorbaan concrete pavements for low-cost roads should also be, as in the case of bituminous surfacings, stage construction. The first stage for an extrapolated prognosis-year-traffic of up to 100 vehicles per day could be spoorbaan cement-concrete strips on a gravel subbase and for higher traffic loads a higher standard road like a full width paved road (Class C road) could be envisaged. The following design standards are proposed (No comparable ' BMZ'-specifications exist):
18.104.22.168.1 GEOMETRICAL DESIGN PARAMETERS
- Up to 100 vehicles per day.
22.214.171.124.2 STRUCTURAL DESIGN PARAMETERS
- Drainage structures: see (Class C road).
- Micro drainage: a crossfall of 1:40 to facilitate the drainage of the formation. Between the strips a crossfall
of 1:20 to drain the median to minimise the erosion of the wearing course at the edges of the blocks is required.
- Materials: fill: in-situ material cut from road reserve or from borrow pits
subbase underneath wearing course: high plasticity indices will be allowed (PI=15);
CBR as low as 10 to 15 can be allowed. Compaction controlled to 93% MOD.AASHO.
wearing course underneath and around pavement blocks: PI to 10; CBR minimum 30.
Compaction controlled to 95% MOD.AASHO.
In order to determine the right design concept a soils investigation must be instituted.
Depending on the variability of the geology of the concerned area a sufficient number of test holes
should be dug to determine the most suitable design of the road over specific lengths.
- Concrete pavement: compression strength: 25 MPa.
Interlocking concrete blocks are very effective in distributing vertical and transverse horizontal forces from one block to the next by their interlocking key as proved by the first experimental section constructed during July and August 1990 which will be dealt with below. A concrete block pavement can undergo severe surface deformation without reaching the ultimate (collapse) limit state. It is the interlocking mechanism which provides the strength in a concrete block pavement.
- Formation Design: Two alternatives are discussed:
The following additional surveys have also to be conducted:
126.96.36.199 CONSTRUCTION CONSIDERATIONS
For this type of road small labour-based departmental construction units with a central spoorbaan interlocking stone manufacturing point could do the job most effectively. Alternatively small local building contractors who can be stimulated to create a local construction industry in structurally underdeveloped areas in Namibia or " village-development-project-units" can be used.
Special attention has to be given to improve the drainage to eliminate saturation of the road foundation layers. In areas where volume changes owing to plastic soils can be expected, pre-wetting, stage construction, as mentioned before and even traffic compaction of the subgrade prior to placing of the interlocking blocks has to be considered.
In order to keep costs down aggregate sources should not be in excess of 40 km of the project. The use of sand concrete where concrete aggregates are not easily available must be investigated and has been proved feasible in areas like Owamboland. Costs must be, however, kept low in order to justify spoorbaan cement concrete pavements. The lowest possible average annual cost over the pavement life requires the balancing of the following costs (figure 15):
- initial construction;
The blocks have an interlocking key to transmit vertical and transverse horizontal forces from one block to the next. Vertical forces are distributed by the blocks onto the subbase. Longitudinal horizontal forces are taken up by the friction between the line of blocks and the subbase. Transverse horizontal forces are resisted by the gravel wearing course layer in the middle and shoulder strip and adjacent blocks. The blocks are laid in two parallel strips with a 1,00 m spacing between their inside edges, which results in an outside edge width of the strips of 2,20 m and which are centrally located on the 8,00 m wide total traffic surface.
Below some details regarding production, material and labour resources and costs will be given:
188.8.131.52.1 CONSTRUCTION POINTS
-Scarify and re-compact in-situ roadbed;
- Import, spread, level and compact to 93% AASHO subbase layer;
- Import, spread, level and compact to 95% AASHO wearing course layer;
- Hand excavate for strip spoorbaan block strips;
- Laying of spoorbaan blocks into excavated strips;
- Backfill adjacent strip footing and compact.
184.108.40.206.2 MANUFACTURE OF "SPOORBAAN" BLOCKS
Spoorbaan concrete blocks are manufactured on block making machines. It is envisaged that for the use in Namibia a mobile block making plant will be the most economic solution, thus reducing the costs of transporting the spoorbaan blocks and solving other logistical problems.
Establishment of block making plant:
The following factors will influence the decision where best to locate the plant:
1. Total length of road to be constructed.
For each kilometre of road 2 * 1000 * 3,3 = 6.600 blocks are required. A section of 100 km will therefore require 660.000 blocks. 5.600 blocks can be manufactured per working day i.e. 118 working days for 100 km, this means a period of five months is anticipated between each machine establishment, or a move every six months including setting-up.
The making of blocks, even though the actual operation is done by machine, essentially is a labour-based activity, requiring operators for the machine and labourers to produce stack, cure and expedite shipments to site.
220.127.116.11.5 RAW MATERIALS
Cement will have to be obtained from the nearest railhead ( Tsumeb resp. Grootfontein) and transported in bulk by road to the machine site. Depending upon the results of the soils investigation the other ingredients like water, sand and concrete aggregates, if economically available, must be obtained from a source nearest to the plant site. It is envisaged that in most cases where no concrete aggregates are economically available, sand concrete blocks of 25 MPa compression strength could be investigated to be used.
The average haul distance of the blocks should not be more than 25 km. To transport a day production of 5.600 blocks or 224 t in mass will require 18 trips of 13 t trucks.
It is estimated, however, that the blocks can be laid at a rate of 480 m per day or 3.168 blocks per day or 127 t or 10 trips: three 13 t trucks will be required to meet this construction rate.
18.104.22.168.7 CONSTRUCTION OF THE SPOORBAAN ROAD
The two alternative laying specifications which follow assume that the in-situ or respectively imported subbase/wearing course has been properly cleared, levelled and compacted, allowing for the thickness of the sand and spoorbaan blocks. The construction comprises the provision of a suitable subbase, drainage and kerbs, followed by the laying of the concrete blocks. These comprise the spreading and screeding, usually from the kerbs, of the in-situ or imported material which must then be compacted. After the laying of the spoorbaan blocks on top of the prepared and compacted in-situ or imported material, adequate restraint to the sides of the blocks, i.e. kerbing or compacted material and the vibrating of the block surface to achieve interlock and the compaction of the bedding sand is required. Thereafter fine sand must be washed or brushed into the joints and care must be taken to ensure complete filling of all joints.
For the above mentioned two alternatives described the following construction sequences are suggested:
A cost comparison had been made for the two alternatives which revealed US $ 21.000/km for alternative 1 "In-Situ" and US $ 25.000/km for alternative 2 "Imported" (re-calculated for December 1989 prices for equipment and labour (US $ 1,00 per hour for unskilled labour): based on a detailed, priced schedule of quantities for a spoorbaan section of 50 km length in Owamboland, based on July 1991 prices (US $ 30.000/km). The labour-intensive Alternative 'B' of this spoorbaan project is used for this estimate (40% labour, 23% materials, 37% plant)) . The cost estimates did not include road signs etc. and covered purely the road construction costs. These estimates for a spoorbaan concrete strip road is regarded realistic for conditions in Owamboland and indicates that the proposed construction method with spoorbaan blocks will result in roads which can be constructed at very economic rates and at the same time offer substantial advantages over other types of low-volume-roads. A comparable 8,00 m wide conventional bituminous surfaced road will cost at least US $ 138.000/km and a low-volume bituminous surfaced road US $ 82.500/km, as established in chapter 9 of this thesis.
In addition the use of the spoorbaan concept being essentially a labour intensive construction method making use of locally manufactured blocks will provide job opportunities for Namibians in so far densely populated but underdeveloped areas and will develop new skills as well as the nucleus for a local block-making industry.
The interlock of the spoorbaan block coupled with the generally acknowledged advantages of a flexible paving design makes this method preferable to a rigid continuous concrete strip road where the joints could easily be damaged or where intermediate cracking could occur. Quality control in a mobile plant will be stricter than that for in-situ construction, thus ensuring a better quality product.
22.214.171.124 MAINTENANCE CONSIDERATIONS
The maintenance of especially spoorbaan concrete block roads is also highly labour intensive and very cost-effective. Experience on the first experimental spoorbaan section, which will be discussed in the next section, has shown that virtually all the deformation takes place shortly after the application of the traffic loading. Experiences so far gained show that the deformations will not grow very much further and the life of interlocking stone pavements compares very favourably with conventional flexible or rigid pavement designs.
8.3.5 FIRST EXPERIENCES WITH SPOORBAAN TEST
126.96.36.199 OBJECTIVE OF THE TEST SECTION
During July and August 1990 the Ministry of Works, Transport and Communication of the Republic of Namibia constructed a 3 km experimental section of spoorbaan road between Oshakati and Okahao on District Road 3613 in order to establish whether this concept could be considered an appropriate technology for the construction of labour-intensive feeder roads and to:
The construction details were entirely based on the specifications developed in above sections of the thesis and realised by means of appropriate pilot projects by the Ministry of Works, Transport and Communication after independence . Strip roads with spoorbaan blocks were not built before. To assess the suitability of this system and establish a set of construction parameters the following principal factors had to be established and this was practically only possible by full scale trials under actual user conditions:
Furthermore the test section had to be built under the auspices of a prescribed set of design parameters which were established as follows:
|188.8.131.52 IMPLEMENTATION OF THE TEST SECTION
In May 1990, approval was given by the Cabinet of the Republic of Namibia for the implementation of the trial section. At an early stage of the construction of the road formation in accordance with above parameters, it became evident that the properties of materials being supplied for the wearing course substantially exceeded the recommended design standards. As the borrow pit of the imported gravel or silcrete was some 15 km distant from the road ( Elim borrow pit), the cost of transporting would have a considerable influence on the ultimate economic evaluation of the system.
To assess the various factors above the experimental road was built in four trial sections. The material in the sections two to four would be of a lower standard than originally designed for, but was to be made up of material freely available adjacent to the site. The four sections are summarised in table 41: Trial sections 1 to 3: "Imported Alternative" and trial section 4: "In-situ Alternative". The actual material parameters of the four trial sections are summarised in table 42:
TABLE 41 SPOORBAAN EXPERIMENTAL SECTION: BASIC PARAMETERS
TABLE 42 SPOORBAAN EXPERIMENTAL SECTION: OSHANA REGION
184.108.40.206 EVALUATION OF PERFORMANCE
After 17 months of use (January 1992) some 200.000 vehicles (ADT = approximately between 300 and 500 with 7,0% heavy; average speed: 65,43 km/h for light and 62,7 km/h for heavy vehicles) had passed over the four trial sections. This was much more than originally envisaged (between 60 to 100 light duty vehicles per day). The riding quality is more or less equal on all trial sections. Measurements were taken by means of the 'LDI'-instrument (see chapter 7 for a description of the ' LDI') (See figure 16). After applying the calibration equation and other conversion factors the result came out as follows: Average IRI=4,30 for the whole experimental section but it was also revealed that the roughness on the section 0,8 to 1,2 (trial section No.1b) is substantially less than the adjacent sections (average: IRI=3,50), and it can probably be used as a sample section to set standards for the minimum evenness to which the spoorbaan blocks must be laid. For the cost/quality optimised investigations in this thesis the IRI=4,30 was used throughout. An IRI=4,30 is on the rough side which is underscored by the fact that one can actually hear the roughness by listening to the relatively older local vehicles travelling over the spoorbaan. The reason for the relatively high 'IRI' is probably because the changes in the longitudinal profile of the riding surface are not gradual, but rather abrupt.
FIGURE 16 RIDING QUALITY RESULTS: SPOORBAAN TRIAL: D.R. 3613
No problems in driving off the block strips onto the gravel wearing course with one or two wheels and back again were encountered. Speeds of ± 80 km/h were safely maintained. However, at speeds of more than 100 km/h, the vehicles tended to drift on and off the strips and driver control became uncomfortable. A fair degree of vibration was experienced in the vehicles, particularly at higher speeds, due to the width of the interlocking joint between each block and the relatively low riding quality. Understandably, at the early stage of the completion of the trial (August/September 1990), there was a hesitancy on the part of the drivers on the proper use of the strip road, particularly with regard to the passing mode. However, it was established that the stopping distance on the blocks with skidding (blocked) wheels is about one half to one third of the braking length needed on a gravel wearing course.
Out of the 19.000 blocks laid only about 20 showed edge damages and 384 were found with complete transverse cracks in the longitudinal direction (type A blocks: table 42). A probable reason for this behaviour is that the support underneath the inner and outer edges of the block is better than in the centre, and hence the crack occurs under heavy loading. A further reason is also that the strengths of some of the concrete blocks were found to be not up to specification because an unconfined compressive strength test on some blocks (age > 2 weeks) revealed values between 15 and 20 MPa (specified 25 MPa). Compression tests which resulted in above average concrete compressions of the spoorbaan blocks (see table 42) did not reveal a connection between broken (type A blocks) and not broken blocks (type B blocks). But, it was also shown that virtually all the cracks developed shortly after the application of the traffic loading (3 months) and less than 5% new cracks have been formed since then. The cracking in no way impaired the serviceability of the road.
During and after the exceptionally good rainy season 1990/91 in Owamboland, the trial section had been subjected to extremely wet conditions. The precast concrete blocks were intact and secure for the full length of the trial section, although no preventive maintenance had taken place between August 1990 and March 1991. Consolidation and abrasion due to traffic of the wearing course material had taken place adjacent to the concrete strips both along the inside of the median as well as the outside section running along the shoulder. A sensible maintenance method has still to be established. It has, for instance to be investigated how practical it is to add thin layers of material to a relatively well compacted surface. The difference in levels from the top block to wearing course layer for the various sections were as follows:
Section 1: 5 to 8 mm;
A very successful result is the effective elimination of corrugations by this building method. Section 9.2 has revealed that corrugations are the main contributor to the road user rejection of unpaved roads with undesirable to unacceptable riding qualities.
An assessment has been made of the performance of each section of the road, tested in order to evaluate the practicality of applying the different parameters to a full scale low-volume road in Owamboland .
220.127.116.11.1 EXPERIENCES WITH SECTION 1
18.104.22.168 CRITICAL ASSESSMENT OF THE TEST
Due to the high volume of traffic (up to ADT of between 300 and 500 instead of the envisaged 60 to 100) the trial section was actually subjected to a more intensive test than had originally been envisaged.
The above mentioned characteristics and indicators for the four trial sections have to be critically looked at. They are probably not sufficient to explain the differences in performance of the four sections, and other factors like "resistance and abrasion by traffic", erodability by water, position and number of low points (longitudinally), slipperiness during wet weather etc. will have to be taken into account when finally deciding on which material is good enough .
Trial sections 2 and 3 (Imported Alternative) appear to be the most successful because they perform equally well as section 1 but as they are constructed with inferior quality local road building material (local clayey sand for subbase and calcrete resp. natural sand clay for wearing course), they are substantially cheaper. Both sections suffered little distress despite the increased traffic volume and wet conditions. But, the acceptable distress mode has to be defined. For roads with a traffic load of less than a 100 vehicles a day, and with proper maintenance, the standards set in these two sections should prove to be adequate. The serviceability would be improved if increased densities could be achieved in both the subbase layer and the wearing course. For trial section No. 4 (In-situ Alternative) the blocks were not laid into excavated slots in the wearing course, but backfilled with material levelled and compacted manually and with small compaction plant. This method is distinctly inferior with respect to the bedding down of the blocks and the quality of the wearing course itself. The blocks are not firmly held and they are rocking on their support.
Although certain sections of the road suffered damage from the wet conditions they remained trafficable at all times. The top of the wearing course will have to be constructed and maintained at a level slightly higher than the top of the blocks to ensure continuous run-off from the median. Labour-based methods could be used to undertake this maintenance. It can be mentioned that it seems that the spoorbaan concept is sensitive to timely and continuous maintenance. How successful maintenance of this technology can be applied by labour-based means has to be confirmed by further testing. Certain set standards before maintenance is due (so-called trigger values) will have to be established .
The trenches for the blocks will have to be excavated with as little over break as possible. The resulting void between the block and the side of the slots will have to be filled with a dry sand-cement mortar. It is further recommended that the bedding sand layer be increased to 35 mm in order to cater for any inaccuracies that might arise along the bottom of the trench.
For large trucks whose rear wheels overhang the outside edges of the strips, the block width could be increased from 600 to 800 mm resulting in an overall width of 2,60 m instead of 2,20 m of the "spoor". This may be deemed unnecessary, since vehicles of such dimensions will seldomly use low-volume roads for which the system is primarily intended. Therefore, for practical and economic reasons it is not recommended to increase the width of the blocks to more than 600 mm. However, it is recommended that for low volume feeder roads, the centre to centre dimension of the strips be reduced from 1,6 m to 1,5 m. This would then ensure that approximately 95% of the vehicles using the road would run with their wheels along the strips. The small percentage of remaining vehicles may have to override the strips either on one side or the other .
The width of the blocks and the shape of the interlock could be modified in order to reduce the gap between the blocks. This would then have the effect of improving the riding quality and also reducing the flow of water through the joints.
The blocks used to date are specified to be made of high strength concrete with high quality aggregates normally not found in Owamboland and which have to be imported from outside Owamboland. Blocks made with lower quality concrete with aggregates more readily available (sand), but with an increased thickness also has to be tested.
A further alternative solution, which uses four in place of the presently used two strips might also be considered, for traffic loads of more than 100 vehicles per day. With each traffic direction having its own two strips, the necessity to drive off the strips arises only during overtaking and this just for a short distance, until the approaching traffic strips are reached. The amount of unavoidable traffic on the gravel wearing course is thereby substantially reduced, probably to 5% or less. Thus a wearing course construction with real inferior quality material, which in the rainy season becomes otherwise unacceptable, could still be used. The doubling of the cost of the block strips can thus be offset, at least partially, against the possible saving when using a locally available weaker wearing course material. Stage construction becomes also a possibility, by adding to an existing two strip road two more strips on their outsides. Substantial increases in traffic volume can thus be accommodated.
To date, with the experiences of much higher traffic loadings than expected and the extraordinary heavy rainfalls of the rainy season 1990/91, it is possible to make first conclusive statements on the suitability of the system. It can be concluded that the structural integrity of the system behaved as expected with some minor modifications as result of the spoorbaan trial. The technology is feasible and acceptable within certain set parameters. There is further no doubt that with equal costs there exists no other method of low-volume road which is more labour intensive and more appropriate for the particular conditions in Owamboland and elsewhere, especially in areas deficient of natural good quality road building materials. Further conclusions could be drawn so far:
The main advantages of spoorbaan concrete strip roads can thus be summarised as follows:
The main disadvantages of spoorbaan concrete strip roads can be summarised as follows:
22.214.171.124 RECOMMENDATIONS FOR A SPECIFICATION
As previously stated, varying materials were used to construct both the subbase layer and wearing course along the trial section. The four individual foundation designs are summarised in tables 41 and 42. Obviously the design life and success of any pavement is directly related to the quality of the materials used for layerwork construction. In line with the philosophy to built a low-cost and low-volume spoorbaan road, the use of quality gravel was found to be uneconomic and impractical. Thus a decision was taken to use rather the locally available sands and clays from nearby Oshana (flat dry river beds) deposits. The natural sandy clay is by no means the ideal road building material but will be an economic solution to built future spoorbaan roads.
As was outlined above, it appeared that section 2, from Chainage 1,400 to 2,000 km offers the best riding surface, both on and off the concrete blocks. The design parameters and as-built results are summarised in table 43:
TABLE 43 DESIGN PARAMETERS: SECTION 2: SPOORBAAN EXPERIMENT
In addition to the layerworks performance, further consideration has to be given to the following criteria:
The proposed specification for the spoorbaan block is as follows:
Height: 100 mm tolerance: ± 3 mm;
Concrete Mix Design - Spoorbaan @ 25 MPa Wet Crushing Strength
Crusher Sand - 420 kg;
See Appendix Sketch 2
Water cured at least three times per day for a minimum period of seven days.
126.96.36.199 FURTHER CLASS D ROAD CONSIDERATIONS
188.8.131.52.1 SUB-SURFACE MEMBRANES AND MATRICES
Another appropriate low-volume road concept is the idea to move heavy containers over unstable sand using wheeled vehicles .
Investigations in regard to cheap and efficient road making in desert and semi-desert environments were undertaken  . These studies dealt mainly with the usage of sub-surface membranes and matrices. The American method of using slotted aluminium grid work was not included in these testings due to the prohibitive costs involved under southern African conditions. The "paper cell" method was tested positively and found to be effective under dry conditions, but lost it's positive properties when wet. A further solution was investigated to use waterproof material such as plastics. Further investigations resulted in a new concept "Sandfix" which was tested at several sites in southern Africa. It has been found that sand-grid base layers create a good base to traffic loads on sands.
Glued PVC plastic grid sections of a "honeycomb" type matrix seems to give the most promising results to date. A Namibian test section doesn't give final results regarding performance and costs yet. Further testing is required. A " Sandfix" test section using the new concept of sand-grid-base layers has been conducted at the site of the departmental construction unit 12 at Windhoek. The performance is not very satisfactory yet and further investigations are required. Due to the absence of a cost effective manufacturing process a suitable plastic grid matrix for a reasonable price is not available yet. But, it seems that this low-volume concept can be a valuable contribution in constructing cost efficient low-cost roads in sandy areas like Owamboland, Okavango, Caprivi and alike.
184.108.40.206.2 STABILISATION WITH FOAMED BITUMEN
Another example of new innovative low-volume road concepts could be those of stabilising poor quality aggregates with a binder such as foamed bitumen. Poor materials which are very common in areas like Owamboland can be considerably improved and will give satisfactory performance with above mentioned bitumen foamed binder .
Foamed bitumen is produced by combining, under well controlled conditions, a small quantity of water with a hot penetrating grade bituminous binder. The foamed bitumen is then used as a binder in the stabilising process. The unique properties of the foamed bitumen allow intimate mixing with cold moist aggregates. Mixing of the material can take place in-situ or in a central mixing plant.
Many economic benefits may result from the use of cheap locally available materials and also from the possible reduction in the total thickness of the pavement structure. But, the main advantage lies in the fact that so far sub-standard "other roads" or sub-standard classified roads as, for instance, many district roads in densely populated and under-developed areas like Owamboland can be upgraded by in-situ stabilisation with foamed bitumen of the existing base.
The mix is clean and easily handled and can be readily compacted with all types of compactors. This process does not require the evaporation of solvent or water prior to compaction, nor the control of temperature. At the construction stage a foamed bitumen mix can be handled as though it is an unbound material. This means that foamed bitumen mixes can be reworked or stockpiled if required.
Foamed bitumen is employed to add cohesion to a mix and to coat any plastic fines that may be present. A well designed foam stabilised material will therefore have adequate stability in its dry and saturated states.
For materials that do not possess sufficient shear strength from inter particle friction, foamed bitumen is added to impart cohesion to the mix. Materials in this category will include materials like natural sand and granular gravels that may be poorly graded or have a rounded particle shape.
Natural gravels and quarry waste are often excluded as a good base quality material, because of a high PI. Foamed bitumen is added to such material to coat the plastic fines present and hence waterproof the mix. Coarse graded materials with a PI up to 12 have been successfully stabilised with foamed bitumen.
The structural design of the pavement must be, as in all other cases of pavement design for all above dealt classes of roads, in accordance with approved procedures . The design is dependent on the class of road, the predicted traffic load and environmental conditions. The layer thickness required is a function of the relative strengths of the available pavement materials. A typical example for traffic loads during the design life of the road is the following:
- 0,1 - 0,2 * 106 E 80 standard axle
Further tests must be recorded under Namibian conditions before the real properties of foamed bitumen can be evaluated. The Namibian Department of Transport has developed two short test sections in Owamboland which are still under observation with no final test results available yet: district roads 3620 and 3622. Both roads have a surface treatment of sand seal which has not performed satisfactorily due to poor construction. In South Africa a foamed bitumen stabilised road has been built from "Cape Flat Sands" which performs very satisfactorily after 8 years of usage. This low-volume method is surely an innovative method which will work very well in poor-material-areas of especially northern Namibia, after having gained the necessary experience.
220.127.116.11.3 PERFORMANCE OF SALT-GRAVEL ROADS
One unique " Namibia-Adapted Roads Technology" is represented by the technology of the "West-Coast-Salt-Gravel-Roads". The standard maintenance method for these road surfaces consists of watering the wearing course with brine and of cutting the loose fine material from the sides . It must be observed that this material will not be contaminated by cohesionless east-wind-blown sands with a resulting "biscuiting" effect where the low plasticity high salt content crust may not bind adequately with the underlying in-situ layer. A smooth, dense crust of about 20 mm in thickness is then established by means of a pneumatic roller. This is sufficient as long as the right relationship between the most beneficial ingredients like soluble salts, plasticity and gypsum are retained. The roller wheel tracks are removed by re-shaping the riding surface with a grader. The surface is, before this re-shaping operation, never ripped because the resulting ruts cannot be removed any more. For the same reason grid or tamper rollers are never used for compacting the road surface. This kind of maintenance is normally done on an annual basis.
Special maintenance may be required after the seldomly occurring rains when the beneficial soluble salts have been leached out of the wearing course resulting in a surface with poor riding quality, or in the case of salt-gravel surfaces with high PIs (see: 7.4.2), the plastic fines are picked up in form of mud by the traffic with the consequence of a coarse, open-texture surface.
In case of new salt-gravel-road construction the standard procedure would be to do some roadbed preparation by shaping the existing material or cutting it from the sides or bringing in suitable material from approved borrow-pits. Any oversize material must be reduced by means of grid and tamper rollers which are used to compact the road layers with seawater, which normally is more easily available than brine. This road layerwork is followed by the final salt-gravel wearing course with a normal thickness of 100 mm. Gypsum blocks are reduced by compacting techniques, before water-rolling of the wearing course with brine, which has to be near a saturated condition with NaCl will commence. The final salt-gravel surface will be achieved by rolling with a pneumatic roller as discussed above.
8.3.6 A CASE FOR LABOUR-BASED METHODS
The development of labour-based methods in Namibian road construction has to be based on systematic and scientific arguments and not dealt with verbally. It will be attempted to develop a basic model taking into account all factors influencing the choice of labour-based construction methods against equipment-based ones. Such a model will establish the limits of labour-based activities. The point where the engineer has to say "no" to such activities in favour of equipment-based ones has to be clearly marked. But, it must also be stated that the limits where labour-based activities are economically feasible can only be established for Namibian conditions after more field data are obtainable which as yet have not been established.
Chronic economic problems caused by the political unstable transition process in Namibia before independence associated with huge pools of unemployed, unskilled manpower who would be prepared to work for minimum wages are making labour-based road construction a sensible option for Namibia. But, it has been experienced in Africa that a reproach of " neo-colonialism" has to be taken into consideration. Similar experiences could be expected in Namibia, but are not easily quantifiable. Although this has been discussed in length in the past by many bodies  , the idea of labour-based construction methods was not enthusiastically supported by the "first world minded" Department of Transport of Namibia of the past administration and the Namibian and South African construction industries. These capital-intensive road construction methods have the consequence that labour-based road maintenance is more difficult to apply. The restricted experience in using labour-based techniques in road construction and maintenance which has been assembled so far, was not evaluated in a systematical manner by the Namibian Department of Transport before independence in March 1990.
Although since 1981 a clause had been included in the departmental " Conditions of Contract for Road Construction Works" in order to subsidise labour-based activities and especially appoint additional unskilled labourers, it seems that the "first world orientated" Namibian and South African contract industries approached this idea in a rather negative sense. In any case, no systematic studies and evaluations of any accumulated experiences in this regard have been collected to date. It also seems that some private contractors used the subsidy scheme as additional source of profit-making in place of creating additional employment.
It appears that for earth works and other road construction activities well organised manual labour can be cheaper or at least break-even with machine labour in various ' IDCs'. As a first step into this direction a departmental labour-based road construction project was envisaged during 1986 for the construction of the 3rd phase of the Windhoek Western By-Pass. For this project it was proposed to do all earth work movements with manual labour by means of pick and shovel as well as with wheel barrows from borrow to fill. The objective was to build the road fills manually after bulldozers have loosened the fill material in adjacent excavations. Provisional calculations revealed, however, that manual labour for these earth works resulted in 4 times higher unit costs than equipment-intensive construction. These calculations have been based on the assumption of an average of 200 m haul distance as well as 1,5m3/day productivity rate per labourer. This productivity figure seems to be rather low and indicates very inefficient labour values and organisation.
If the same labour-based task will be organised in an optimal efficient way this unfavourable labour/equipment ratio can be decreased considerably and it could be proved that this task can be tackled as economically by labour-based means. Much wasted effort can often be observed in labour-based quarrying and construction material handling operations, as for instance:
But, to prove an economically viable labour/equipment ratio requires careful study of both methods. To date such a study has not been carried out in Namibia (The first study on a labour-based constructed road commences on district road 3619 to Onaanda in Owamboland during January 1992, based on task work for US $ 4/day). The current thinking in many ' IDCs' is that if the wage is about US $ 4 or less per day, than there exists a good chance of carrying out a lot of the engineering tasks more economically by manual labour. In some cases this minimum wage can go up to US $ 6 and more. If it is below US $ 2 than any project can virtually be tackled cheaper by manual labour. Table 44 shows a list of tasks which can be done economically using labour in the road construction industry on the basis of daily unskilled labour wages.
In order to tackle the Namibian unemployment problem it has to be considered to do those road construction activities which can compete with machine labour, labour-intensively and thus simultaneously improve the dignity and life-style of impoverished Namibians. Such activities could be, for instance, the earlier discussed " low-volume road" principles which in themselves are based on labour-based foundations. The spoorbaan roads could be an excellent example to provide feeder roads in the sandy areas of northern Namibia where labour-based technologies in construction and maintenance are economical propositions, bringing self-contained farming communities within reach of larger markets. An additional bonus could be the creation of an informal indigenous Namibian construction sector and the development of new skills. Such labour-based activities could counter-balance any reproach of " neo-colonialism" in using such construction methods.
It can also be stated that labour-based technologies should not be implied if it is much more economical to use machine labour, but road projects have to be planned and designed accordingly to make them effective for labour-based methods. Further it has to be observed that labour-based road programmes by their nature will be real asset creating programmes on long term. Such roads have to be modestly engineered in keeping with the aim of appropriate low-volume roads that can be negotiated with reduced design speeds to avoid massive earth works (design speeds of 100 km/h and more, as applied currently in Namibia, are an unwarranted luxury). Mechanical equipment is limited to that equipment which has to move material over longer hauls. Excavations have to be minimised by appropriate design, but when required they have to be strictly tackled by pick and shovel and other special hand tools. Most earth work movements should not involve high cartage since the road profile is developed by throwing soil to the centre line from the ditches, or to one side where the ground has a crossfall.
Drainage structures have to be, like spoorbaan interlocking pavement blocks, manufactured locally. Hand mixed concrete can be recommended if some quality control is assured. This method is in any case not interrupted by mechanical breakdowns and doesn't require the use of imported fuel. The old Namibian technique of using natural stone as building material can be applied in the labour-based provision of drainage structures. Many other labour-intensive applications are thinkable. The experiences regarding self-help construction schemes in other African countries have to be evaluated and adopted to Namibian circumstances.
TABLE 44 LOWER COST ACTIVITIES: LABOUR AGAINST EQUIPMENT
To make any labour-based scheme a success a pilot project has to be initiated. A pilot project will have a lot of training element in it plus a lot of data collection potential in order to compare rates and prices. Such pilot project has to involve a considerable amount of experimentation in order to modify labour-based techniques to suit local conditions. The spoorbaan project will be well acquainted to serve as such a labour-based pilot project when everything is done by manual labour except possibly the compaction of the selected layers. This pilot scheme could eventually develop into a full programme, using thousands of labourers in Owamboland and building hundreds of kilometres of urgently required rural feeder roads in spoorbaan technique all over the densely populated areas in Owamboland. Self-help road construction schemes involving labour-based technologies have to be vigorously promoted in Namibia.
Taking these arguments into account, the following basic model for labour-based activities in Namibian road construction is developed:
FIGURE 17 APPROPRIATENESS OF LABOUR BASED ACTIVITIES
NOTA: L.B.A. = Labour Based Activity
according to table 44