Cyclic Secant Shear Modulus and Pore Water Pressure Change in Sands at Small Cyclic Strains

When fully saturated sandy soil is subjected to cyclic straining in undrained conditions, pore water pressure increases and effective stress decreases. Therefore, it has been assumed that in such a case sand stiffness always degrades. However, recently published results of cyclic strain–controlled triaxial and simple shear tests reveal that, at cyclic shear strain amplitudes, ${\gamma }_c$, between 0.01% and 0.10%–0.15%, the secant shear modulus at cycle $N$, $G_{SN}$, increases with $N$ up to 10% of the initial $G_{S1}$ and then decreases, while the cyclic pore water pressure, $\mathrm{\Delta }{u}_N$, monotonically increases, and that $\mathrm{\Delta }{u}_N$ can actually reach up to 40% of the initial effective vertical stress before $G_{SN}$ drops below $G_{S1}$ and sands start to truly degrade. To investigate if such a behavior is universal and occurs under various loading conditions not tested before, new cyclic simple shear tests were conducted and analyzed. They included three sands, two different sand structures, cyclic strain–controlled single- and multistage tests with different sequences of ${\gamma }_c$ between 0.005% and 0.16%, cyclic stress–controlled tests, tests with different consolidation stresses, and several tests with a large number of cycles. The new test results revealed the same trends and confirmed that such behavior is truly universal. However, in the new tests $G_{SN}$ increased up to 15% of the initial $G_{S1}$ and $\mathrm{\Delta }{u}_N$ reached up to 50% of the initial effective vertical stress before $G_{SN}$ dropped below $G_{S1}$.