The most common cause of the cracking of buildings in the southern portion of the African continent is differential heaving rather than differential settlement. The present stage of knowledge regarding the recognition of soils likely to cause heaving is given together with general observations of the time: heaving movements experienced. Constructional procedures at present satisfactorily employed under these conditions are mentioned.
Potentially expansive soils are classified according to their geneses, and typical examples of each type are given. Residual soils which develop expansive characteristics are those derived from basic igneous rocks and from argillaceous Karroo sediments: Quaternary clay deposits comprise the transported expansive soils which are extensively distributed throughout the union.
With the discovery of gold in the Odendaalsrus, Welkom and Virginia areas, a considerable building programme became necessary. The sub-strata in these areas were found to consist of expansive clay formations, which tended to heave upwards under the centre of newly constructed buildings with resultant cracking of both the foundation beams and the superstructure. As a result of subsequent research and experience, it was found that several possible methods of construction gave a reasonable measure of safety. This paper is confined to a discussion on the use of concrete piles with greatly enlarged bases to act as anchors against the upward movement.
In brief outline, the methods of foundation design currently in use are presented. The importance of the soil survey as a preliminary to design is stressed as well as the identification and classification of soils. The C.B.R. test on soaked samples provides the principal design criteria and is so arranged as to enable an estimate to be made of the ultimate in situ C.B.R. of the constructed foundations. The wheel load design Curves used for design are based upon American practice, but test results are applied using judgment and experience of local factors. Unreliable design results have led to checking of designs using the resistance value and the group index methods of design. Special tests are used for decomposed materials and shales and for cement and lime stabilized soils for use in base courses. Further research is essential to develop locally applicable design data and to make the best use of local sources of foundation materials.
Certain aspects of the C.B.R. test procedure and design standards are discussed and certain modifications are proposed with the object of simulating more closely anticipated service conditions, improving the accuracy of the results and reducing the amount of work involved in carrying out the test. The paper also descriptionbes an approximate correlation established between the C.B.R. and indicator test results, from which design charts may be prepared for estimating cover requirements when only the indicator test results are available. The measure of agreement between the C.B.R. method, the group index method and the interpolated design or cover charts is analysed statistically. The Author does not claim that the analysis is complete.
In this paper the Author presents a general panorama of the main tests carried out on road soils, with particular emphasis on experience in Mozambique. Some aspects which require perfection or investigation are introduced, and account is given of the different developments lately registered, as well as of important studies in progress.
In the limited space available, the paper, discusses some unusual soil conditions which exist in East Africa and which require special consideration in road design. The effect of the very low in situ densities encountered and the effect of compaction on design are considered. Areas exist where there are craters filled with rock and soil in various stages of weathering. The modulus of subgrade reaction is given, and also the method of construction over them. The lack of high water table and the high voids ratio of the soils is mentioned. Road design is based on standard C.B.R. modified by the group index method. Stone quarrying presents considerable difficulty due to the heterogeneous mixture of soft and hard rock in the same quarry. The danger of using soft stone bases is stressed. Mechanical stabilization cannot be used due to lack of sands and gravels. Other methods of soil stabilization for subbases are discussed, but economy prevents their general use.
This paper deals with the construction of slimes dams for the gold mining industry. The paper descriptionbes firstly the methods of construction and then states the problems encountered and possible means of overcoming these difficulties.
When the economic and social evolution of an undeveloped country requires important Public Works quickly, penetrometers are most useful for rapid design and control of .the foundations for the Civil Engineering constructions which are planned. Undertaken with care and correctly interpreted, penetrometer tests rapidly provide fair ideas about the qualities of soil and will lead to economical and adequate foundation work. Their main advantage lies in the relatively light equipment which is also sturdy and cheap, and at the same time so simple to operate that even non-skilled native personnel may be quickly trained to perform efficient work in the field.
The present state of knowledge on the shear strength of partially saturated soils is presented in this paper. The importance of considering effective stresses is emphasised and methods of predicting the development of pore pressures are reviewed. Some experimental results are given, although they form only the beginning of a comprehensive research programme.
Geophysical methods of site exploration should be used whenever a saving of time and cost can be accomplished. Depth of solid bed-rock can readily be obtained by an electrical resistivity survey; igneous dyke and sill intrusions are located by magnetic survey; the gravitational method is applied for determining safe and dangerous zones for sink-holes in dolomite; and fault-zones are traced by the electromagnetic method. Sink-holes may occur in dolomite where excess water causes loose material at the surface to slump into near-surface cavities. If the dolomite over an extensive area is covered by younger formation such an area is considered safe. It has been found possible to delineate both safe and dangerous zones in the dolomite area south of Pretoria by extensive geophysical surveys.