Determination of the shear strength of unsaturated soils using the multistage direct shear test

The determination of the shear strength of unsaturated soils is generally more complicated, time consuming and expensive compared to the determination of the shear strength of saturated soils. Although much research has been done on unsaturated soil testing methods, there is still a strong need to translate these studies into practice. Further studies are needed on practical testing methods that can reduce both the cost of and time associated with shear strength testing of unsaturated soils. This paper presents a comprehensive evaluation of the validity of using the multistage direct shear test as a rapid and practical method to determine the shear strength of unsaturated soils. The laboratory tests were performed using a newly-constructed modified direct shear test apparatus that allows independent control of matric suction, referred to as a suction-controlled multistage direct shear test. Unsaturated shear strength was established using multistage loading over a range of net normal stresses and matric suction values. Shear strength parameters obtained from the multistage tests are compared with those from conventional direct shear tests using multiple soil specimens. Recommendations are given on how to carry out multistage direct shear tests to ensure reliable unsaturated shear strength measurements. The tests were performed on undisturbed soil samples obtained from the riverbank of the lower Roanoke River in eastern North Carolina, USA.