Time-Dependent Increase in Shear Modulus of Clay

Two time domains for modulus increase have been identified, primary and secondary. Primary is the same as for primary consolidation and secondary is the open interval after primary in which modulus increases continuously with time. It was found that all cohesive soils exhibit secondary modulus increase, i.e., (1) The modulus increase for clays ranges from 3% to 20% per logarithmic cycle of time; (2) confining pressure dependence can be removed by normalizing modulus based on the 1,000-min modulus; and (3) secondary increases in modulus occur at all shear strain amplitudes. The rate of secondary increase is related to the undrained shearing strength and the initial void ratio of the soil. An empirical equation is presented to show this relationship. Thixotropy best explains secondary modulus increase.