Change of Cyclic and Postcyclic Shear Behavior of Low-Plasticity Silt Due to Addition of Clay

Abstract

This paper investigates change of cyclic and postcyclic shear behavior of low-plasticity silt due to addition of clay. Bentonite was added into low-plasticity Mississippi River Valley (MRV) silt with a plasticity index (PI) of 5.8, and a series of triaxial tests were conducted in the laboratory. With a higher PI, the excess pore pressure developed more slowly with cyclic loading. Opposite to MRV silt, the silt-bentonite mixture (PI = 6.2) showed cyclic softening rather than initial liquefaction. It showed higher cyclic shear strength rather than MRV silt. After cyclic loading, the reconsolidation behaved more like a compression process for both MRV silt and its mixture with bentonite. With reconsolidation, the shear strength of the postliquefaction specimen was increased largely compared to that without previous cyclic loading. Without reconsolidation, there was a more obvious drop of shear strength for MRV silt than for MRV silt-bentonite mixture. This paper suggests that the behavior of MRV silt transforms from intermediate material to clay-like material at a PI of 6.0. However, for engineering practice, the PI seems to be not a good factor to identify the transformation of soil behavior, because small changes in PI produces significant transformation of soil behavior and is difficult to capture in engineering practice and even in the laboratory.