3.3  OCCURRENCE OF NATURAL ROAD BUILDING MATERIALS

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

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

N = 372*C_j(0,58+0,14*W)/(MAP*W0S_j)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

TABLE 10  ROAD BUILDING MATERIALS IN NAMIBIA: ROCKS AND STONES

TABLE 11  ROAD BUILDING MATERIALS IN NAMIBIA: SAND