| 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 [70]: "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
8.3.1.1 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)) [71][72]. 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. 8.3.1.1.1 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. |
| 8.3.1.1.2 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.
8.3.1.2 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.
8.3.1.3 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.
8.3.2.1 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.
8.3.2.1.1 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 'BMZ'-standards). - 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 (No 'BMZ'-standards). - Side slope: 1:2 maximum 1:4 normal 1:6 in sand (Kalahari) (No 'BMZ'-standards). - Cross fall: 1:40 (No 'BMZ'-standards).
8.3.2.1.2 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.
8.3.2.2 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.
8.3.2.3 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 [73] 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.
8.3.3.1 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:
8.3.3.1.1 GEOMETRICAL DESIGN PARAMETERS
8.3.3.1.2 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.
8.3.3.2 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.
8.3.3.2.1 BITUMINOUS BINDERS
FOR PRIMING: (i) MSP 1 (inverted
emulsion)(inverted: less viscous)(MSP: medium setting prime) FOR SEAL WORK
for slurry: (i): "stable" grade or (iv) plus +/- 1,5% diesel or paraffin; (vi)-(viii) for rejuvenating old seals.
8.3.3.2.2 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).
8.3.3.2.3 SEAL TYPES
8.3.3.3 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" [74]. 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)). 8.3.4.1 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):
8.3.4.1.1 GEOMETRICAL DESIGN PARAMETERS
- Up to 100 vehicles per day. 8.3.4.1.2 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:
|
| 8.3.4.2 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:
8.3.4.2.1 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.
8.3.4.2.2 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.
8.3.4.2.3 ESTABLISHMENT
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.
8.3.4.2.4 LABOUR
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.
8.3.4.2.5 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.
8.3.4.2.6 TRANSPORT
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.
8.3.4.2.7 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: In-Situ Alternative
Imported Alternative
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)) [75][89]. 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.
8.3.4.3 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
8.3.5.1 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 [75]. 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:
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| 8.3.5.2 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
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TABLE 42 SPOORBAAN EXPERIMENTAL SECTION: OSHANA REGION
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8.3.5.3 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 |