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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 3A/8 Collapsible Grain Structure in Residual Granite Soils in South­ ern Africa Structure granulaire susceptible de s’effondrer dans des sols de résidu granitique en Afrique du Sud by A .B .A . B rink (Geologist) and B.A. K antey (Civil Engineer) Summary Sommaire Investigation into the cause of the sudden tilting of a water Les recherches de la cause d’un affaissement brusque d’un châ­ tower founded in decomposed granite lead to the discovery that teau d’eau érigé dans un terrain de granité en décomposition, the residual soil exhibited a grain structure which ‘ collapsed ’ donnèrent le résultat suivant : la terre résiduelle avait une struc­ on becoming inundated under load. It was subsequently observed ture qui s’effondrait lorsqu’elle était inondée sous charge. Il fut that the same phenomenon occurs in other places as well, but observé que le même phénomène se produisait aussi dans d’autres appears to be confined to slopes in areas of annual water surplus. endroits, mais semblait être limité aux pentes dans des régions The collapsible grain structure develops as a result of leaching inondées chaque année. L'affaissement de la structure du sol se out of soluble and colloidal matter from the residual soils, and produit en fonction du délavage des matières solubles et colloï­ conditions of advanced decomposition, relatively high annual dales des terres résiduelles. rainfall and good internal drainage are therefore prerequisite Les conditions décomposition avancée, grandes pluies conditions. There are indications that the potential collapse annuelles et bon drainage interne sont donc nécessaires au préa­ decreases with depth in the soil profile which leads to the conclu­ lable. On a remarqué que le potentiel d’affaissement diminue avec sion that the foundation design could be based on permissible la profondeur, ce qui mène à la conclusion que le projet de fonda­ tolerable settlement rather than on permissible bearing capacity. tion doit être basé sur le tassement admissible, plutôt que sur la Pre-collapse of the soil, produced by driven cast-in-situ piles, is charge limite admissible. Le pré-affaissement de la structure du presented as an additional solution. sol produit par des pieux coulés in situ est conseillé comme solu­ tion supplémentaire. D uring 1957, an investigation into the cause of the sudden appears to be confined to slopes where the soils are well tilting of a water tower founded in residual soil formed by drained, and no recorded case has been found on flat plateaux the decomposition of granite [1], lead to the discovery that or in depressions where the internal drainage is impeded. the soil exhibited collapsible grain structure of the type Indeed, in the Cape Town area, dependant on the slopes and internal drainage condition, residual soils from the granites described by J ennings and K night [2]. All previously repor­ may range from expansive to collapsible type, over a com par­ ted cases of collapsible grain structure in soils had been atively short distance. It seems evident, therefore, that the associated with aeolian soils but, after this first recognition phenomenon of collapsible grain structure develops as a of the phenomenon in residual soil at White River, it has result of leaching out of soluble and colloidal matter from been found to occur in decomposed granite in other parts the thoroughly decomposed rock. This results in a high void of Southern Africa, and also as far north as Chingola on the ratio and an unstable arrangement of quartz and felspar Northern Rhodesian Copper Belt. Many cases of buildings grains and mica plates in the leached residuum, giving a which have cracked severely as a result of a sudden collapse grain structure which is very similar to that developed in of grain structure in the granitic soils are to be seen in the weathered aeolian sands, as described by K night [3], northern suburbs of Johannesurg, at Tzaneen in the N or­ Evidence of the opinion that leaching of colloidal matter thern Transvaal and at Mbabane, the capitol of Swaziland. from the residual soils, in sufficiently large quantities to The phenomenon has also been observed in parts of the significantly effect the void ratio and hence grain structure, Cape Peninsula, near Cape Town. is found in the following observation of C.R. van der The distribution of the three main types of granite which M erwe [ 4 ]: occur in South Africa is indicated in Fig. 1, and it will be seen “ Waters, seeping from the granite formations after heavy that the Archaean or Old Granite has by far the greatest rains, contain a marked milky turbidity consisting of a distribution, while the younger Cape Granite is confined to fine colloid suspension which is so fine that a filter paper scattered outcrops in the extreme south and south-west and used for fine precipitates does not separate it from the water... the youngest Bushveld Granite forms a restricted lopolith in The suspended material in the filtrate after having been passed the central Transvaal. The texture of the rock, or of the soil through a very fine filter paper, amounts to 0-892 gm per derived from it, varies greatly within each type, and while litre at 105° C... The molecular silica : alumina ratio is 6-8 the main constituents are always quartz, felspar and mica, indicating... kaolinite with abundant silica of colloidal sizes...” accessory minerals are often abundantly present. All cases Since leaching appears to play so important a part in of collapsible grain structure so far observed have been in the formation of these soils, it follows logically that rainfall either the Old Granite or the Cape Granite. and drainage conditions must be of prime importance. A A marked characteristic of the phenomenon is that it well drained topographic situation, relatively high annual 611 rainfall and a degree of decomposition of the rock affording tions for the development of the collapsible grain structure, high permeability would all appear to be prerequisite condi- Referring once more to Fig. 1, it will be seen that all the a RHODESIA MOCAMBIQUE TRANSVAAL SOUTH WEST AFRICA JOHANNESBURG ORANGE 1ASUTOLAND, DISTRIBUTION OP REO BUSHVELO GRANITE DISTRIBUTION OF YOUNGER CAPE GRANITE CAPE OF GOOD HOPE DISTRIBUTION OF OLD GRANITE AREAS OF ANNUAL WATER SURPLUS Fig. 1 Map of South Africa showing the distribution of granité in relation to areas of annual water surplus. Carte de l’Afrique du Sud montrant la distribution des granités par rapport aux régions ayant un excédent annuel de pluies. places in Southern Africa where the phenomenon has been found to occur, fall within areas of annual water surplus, i.e. areas in which “precipitation exceeds potential évapo­ transpiration” [5], The significance to the soils engineer of these areas of annual water surplus has already been pointed out by Williams [6], and it is now suggested that the phenomenon of collapsible grain structure development may well be confined to well-drained slopes in those granite areas which coincide with the areas of annual water surplus. Weathered aeolian sand deposits which exhibit collapsible grain structure are generally homogeneous in profile by virtue of their method of deposition ; some reduction in compressibility is found with depth, but this is thought to be due to pre-consolidation as a result of increase in weight of overburden. In the decomposed granite soils, however, it would be expected that the degree of development of Fig. 2 Saturated consolidation test results, showing reduction collapsible grain structure would reduce with depth, since in compressibility with depth. the drainage paths would tend to flatten out. Further it Résultats d’essais de consolidation saturée, montrant may be assumed that the higher the rainfall figure, the greater une diminution de la compressibilité avec la pro­ will be the potentiality for leaching to occur, and hence for fondeur. the formation of soils with correspondingly higher void ratios and higher compressibility. Fig. 2 shows saturated consolidation curves from a site in the White River area is illustrated by the increase in the value of the void ratio in where the phenomenon was first recognised. These results relation to the increase in annual rainfall. tend to support the supposition that the compressibility On Fig. 4 values of the Compressibility Index, Cc, at will decrease with depth and similar trends have been found overburden pressure are plotted against depth for samples elsewhere. from various localities. A distinct trend indicating a reduction Typical saturated consolidation curves for soils of this in compressibility with depth, irrespective of locale, is appa­ nature from different areas of South Africa and from approxi­ rent but it seems possible that, when further inform ation is mately the same depth, are plotted on Fig. 3. The influence to hand, this trend may resolve itself into a family of curves of rainfall on the development of collapsible grain structure dictated by rainfall, parent material and possibly topography. 612 investigation of this third method of approach. Cast-in-situ piles with an expanded bulb of the Franki type were driven on a site and undisturbed samples extracted before and after driving to attempt to observe the effect of driving such piles on this type of soil. In the actual event, the particular site proved to be extremely variable and therefore somewhat unsatisfactory for test purposes but Fig.

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