Cyclic behaviour of unsaturated silt in suction-controlled simple shear apparatus

In the tropics, surficial soils in subgrade for highway or supporting shallow foundations are unsaturated in nature. Conventionally, the properties of unsaturated soils are obtained under unknown matric suction condition in laboratory tests although it is well known that matric suction affects the properties of unsaturated soils. In this paper, a suction-controlled cyclic simple shear apparatus was developed to study the properties of unsaturated soils under cyclic loading. A series of constant water content tests at a known initial matric suction was conducted to investigate the cyclic behaviour of a silt. Such test results are applicable to traffic loading on highways and shallow foundations for vibrating machinery. The silt was tested at cyclic shear strain levels from about +/-0.002% to +/-2% each for 10 loading cycles at a frequency of 0.5 Hz. Results indicated significant effects of matric suction on the cyclic behaviour of the unsaturated silt. The volumetric strains resulting from the cyclic loading were also examined in this study. The silt experienced a permanent volume change once the volumetric threshold shear strain was exceeded.