California Bearing Ratio tests on a lateritic gravel from Kenya

A series of California Bearing Ratio (CBR) tests have been carried out on a lateritic gravel from Kiunyu Quarry in Kenya. Standard CBR tests were carried out at a range of water contents and using three different levels of compaction. These were supported by specialist CBR tests carried out in a CBR apparatus that provided measurement of matric suctions. Tests on the lateritic gravel showed that samples compacted at water contents near Proctor optimum had suctions near to zero, but suctions increased very rapidly in samples compacted dry of Proctor optimum, exceeding 500 kPa at water contents below 75% of Proctor optimum. The patterns of behaviour observed in the standard tests are explained using the concept of “suction stress”. This unsaturated soil approach can explain why the samples show a loss of CBR value at the dry end of the water content spectrum (below 60% of Proctor optimum). Although suction continues to increase as the water content reduces, the degree of saturation is reducing, thereby reducing the “suction stress”. The combined effect results in a loss in CBR value.