The occurrence and extent of collapse settlement in residual granite in the Stellenbosch area

[摘要] ENGLISH ABSTRACT: Large areas of the earth's surface are covered by soils that are susceptible to largedecreases in bulk volume when they become saturated. These soils are termedcollapsing soils and are very common in parts of the USA, Asia, South America andSouthern Africa. This study is concerned with the occurrence of these collapsible soilsin the residual granites of the Stellenbosch area. The study was undertaken asrelatively little is known about the collapse phenomenon in the problematic weatheredgranites of the Western Cape. The majority of research thus far has been carried outon the deep residual soils formed on basement-granite in the Transvaal areas, whereaslittle attention has been paid to the Cape granites.The aim of the study was achieved through the experimental work which includeddouble oedometer testing, indicator analyses and shear strength testing. Doubleoedometer tests were performed to quantify the potential collapse settlement of thesoils from the demarcated study area. To provide a better understanding of thecollapse behaviour of the soils, indicator analysis, which included Atterberg limitsand particle size distributions, were performed. Direct shear tests were further carriedout on saturated and natural moisture content specimens to establish the effect ofcollapsibility on shear strength and whether substantial additional settlement of thesaturated soils would occur during shear.It was found that collapsible soils are prevalent in the demarcated study area as themajority of soils showed a potential collapse settlement of 1% or more. Collapseexceeding 5% were calculated in a few instances proving some soils to be highlycollapsible. The double oedometer and indicator analysis results were used in anattempt to obtain a relationship between collapse settlement and a combination ofeasily determined properties such as dry density (void ratio), moisture content andgrading, but no meaningful conclusions have emerged. The shear strength testsindicated that a clear correlation does not exist between collapsibility and shearstrength. It was further established that a relationship between collapse settlementdetermined during the double oedometer testing and the volume change during shear strength testing cannot be assumed. It can thus be concluded that soils can be veryunpredictable and further research on the collapse phenomenon is indicated.

[发布机构] Stellenbosch University