Suction-controlled multistage triaxial testing on clayey silty soil

Multistage triaxial testing offers a potentially reliable and time-saving alternative method of determining the shear strength of soils under suction-controlled conditions, while addressing the issues related to soil variability and lack of replicates. However, evidence of its suitability for testing geomaterials in unsaturated conditions is rather limited. In this work, a series of suction-controlled multistage triaxial tests were performed on compacted specimens of clayey silty soil, wherein each test was performed on the same soil specimen at a constant suction under drained condition, but at varying net confining pressures. The point of termination of the shearing stage was defined through a selection process involving either the stiffness or the volume change in monotonically sheared specimen, based on whichever prevents premature failure of the specimen. This was the first attempt where all the stages of a multistage triaxial test were compared against that from single-stage triaxial tests for validation of the approach. Moreover, the influence of errors introduced by multistage triaxial testing on the shear strength predictions of unsaturated soil was considered by constructing a three-dimensional failure envelope on p-s-tau space using a well-known relation and comparing them against those obtained from single-stage triaxial tests. It was also noted that for determining the shear strength parameters of soil, the multistage testing, reduced the testing times to half of that required by single-stage triaxial testing, which would translate into significant reduction of cost of testing, thereby making unsaturated soil testing much more appealing to the practitioners.