2.0  OCCURRENCE OF NATURAL ROAD BUILDING MATERIALS

Schall and von Solms [1] have investigated the deposits of naturally occurring building materials in Namibia and reported 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 in most areas in Namibia. The dealt with 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 definite 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 layers in road construction or as coarse concrete aggregate, are scarce. Schall and von Solms (1983) observed that only four occurrences have been 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 known from Hereroland, Bushmanland, Okavango and Caprivi which have not been tested yet but which should be of some interest in the future. These are as follows [2]:

- 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 upstream of Katima Mulilo

- basalt at the confluence of the Chobe and Zambezi Rivers

- basalt at Ngoma at the Chobe River

Sand for the construction of roads and for concrete in structures is usually available in sufficient quantities throughout Namibia. The building sand requires, however, often some procession to meet the construction specifications for the various uses. Sources of sand that are known to be suitable as concrete fine aggregate, are shown in Annexure Table 2.

Building materials for road construction also include such materials which are required for the construction of airport runways, bridges and other concrete structures .All relevant information regarding road building materials has been assembled in form of a suitable size map showing the localities of such sources and a code briefly indicating the description and suitability.

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 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. To avoid confusion no reference has been made in this thesis to the term 'gravel pit'. This means that all the sources of building materials used in the construction of the lower layers of the pavement are excluded. This is not very important because locating such materials is, with a few exceptions, normally not problematic in most of Namibian areas.

The most important part of the pavement is, however, the basecourse and, of course, 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.

Schall and von Solms (1983) reported that the deposits of areas of loose pebbles or rock which often have very definitive economical advantages, are included. 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 the Appendix Map for naturally occurring road building materials to identify the sources thereof [3]:

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 annexure tables 1 and 2: Annexure Table 1 for sources of rock with coded numbers in circles and Annexure Table 2 for sources of sand for concrete with coded numbers in triangles. Following each code number is a brief geological description of the material, the suitability thereof and a quarry reference number. This means, wherever a reference number occurs, the suitability of the material has been established for the specified purpose. The absence of any such reference number, however, indicates that, although no test results for those sources are available yet, such sources of road building materials were adjusted suitable for the utilisation as specified based on empirical experience. The reason for this basis of selection is that some untested but otherwise suitable sources could have been omitted.

Where suitable sources of geologically similar materials occur at reasonably short intervals along a given route, only one such occurrence will be coded with one code number. All other references to materials with similar properties will be coded with this same one number [4].

WB00823_.GIF (134 bytes)

[ Table of Contents ]

forward.GIF (132 bytes)