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

However, recently published results of cyclic strain–controlled triaxial and simple shear tests reveal that, at cyclic shear strain amplitudes, 𝛾𝑐, 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 𝐺𝑆1 and then decreases, while the cyclic pore water pressure, Δ𝑢_𝑁, monotonically increases, and that Δ𝑢_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 𝛾𝑐 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 Δ𝑢_𝑁 reached up to 50% of the initial effective vertical stress before 𝐺_𝑆𝑁