Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains

摘要

Cyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, gamma(c), around the threshold shear strain for cyclic pore water pressure buildup, gamma(tp) approximate to 0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small gamma(c) below gamma(tp) where there is no buildup of cyclic pore water pressure, Delta u(N), with the number of cycles, N, the cyclic secant shear modulus, G(SN), initially increases with N for 10-20% of its initial value G(S1) and then levels off or just slightly decreases, (ii) at small gamma(c) between gamma(tp) approximate to 0.01% and 0.10-0.15%, Delta u(N) continuously increases with N while the modulus G(SN) first increases for up to 10% of G(S1) and then gradually decreases, and (iii) at gamma(c) larger than approximately 0.15%, relatively large Delta u(N) develops with N while the modulus G(SN) constantly and significantly decreases. This means that at gamma(c) between gamma(tp) and 0.10-0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of gamma(c), the pore water pressure Delta u(N) can reach up to 40% of the initial effective confining stress before G(SN) drops below G(S1). The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening. (C) 2014 Elsevier Ltd. All rights reserved.