Effect of nonplastic silt content on liquefaction behavior of fine sand

Abstract

To identify the effect of non-plastic silt on the cyclic and monotonic behavior of sand-silt mixtures, total 49 stress-controlled cyclic and 13 strain-controlled triaxial tests have been done on sand-silt mixtures. These tests have been done on specimens of size 71 mm diameter and 142 mm height with frequency of 1 Hz for cyclic test and with a rate of 0.05 percent axial strain per minute for monotonic test. All tests were conducted at isotropic (consolidated) effective confining pressure of 100 kPa. Specimens were prepared at a constant relative density, constant gross void ratio and constant density approach by moist tamping method. Concept of Limiting Fines Content (LFC) was verified by these cyclic triaxial tests and undrained monotonic triaxial tests. The rate of generation of excess pore water pressure with respect to cycles of loading was found to increase initially with increase in silt content till the limiting fines content (30%) and thereafter it reverses its trend when the specimens were tested at a constant dry density. The cyclic resistance was observed to be opposite to the pore pressure response. Cyclic Resistance Ratio (CRR) and Secant Shear Modulus decreased till LFC; after that they became constant with increasing silt content. In monotonic test, peak shear strength of sand-silt mixture decreases with increase of silt content up to LFC. Peak shear strength remains almost constant at silt content greater than LFC. At constant gross void ratio approach the undrained peak shear strength decreases with increase in silt content till LFC and for further increment of silt content the peak shear strength increases with increase in silt. The CRR, Secant Shear Modulus and undrained peak shear strength of sand-silt mixtures remain reasonably equal after LFC because of equal inner void among the silt particles, is verified by equal permeability test results (by Falling head method) after LFC till pure silt.