CRS compression tests of an unsaturated kaolin with pore-water pressure measurement

Oedometer test is commonly used to investigate the compression behaviour of saturated soils. However, it requires long test duration (approximately two weeks). The Constant Rate of Strain (CRS) test was developed to overcome this problem. The duration for CRS test is governed by the porewater pressure ratio which is suggested to be between 3+ACU- And 15+ACU-. When dealing with unsaturated soils, the CRS test procedure becomes more complicated as there is no available guidance on strain rate. The CRS test can be conducted in two ways, first by controlling the matric suction of the soils so that it is constant throughout the test (constant matric suction test), and second is by allowing the matric suction to change (constant water content test). Constant matric suction test is usually preferred as the transducer that is used to measure the matric suction of the soil specimen is susceptible to cavitation under high matric suction. However, constant matric suction test requires the matric suction to be controlled throughout the test and requires modification to the CRS apparatus. Constant water content test is simpler but the matric suction change needs to be measured. In this study, a high suction tensiometer is used. The objective of this study is to evaluate the response of the pore-water pressure for unsaturated kaolin tested under CRS condition and that obtained from the test under oedometer condition. Several strain rates were used in the CRS tests. During loading, the pore-water pressure increases and can even become positive. When the pore-water pressure becomes positive, the behaviour follows that of the fully saturated soil test. Some observations on the appropriate strain rate to be used in the CRS test for unsaturated soils are made.