Cyclic threshold shear strain in pore water pressure generation in clay in situ samples

The threshold level of the cyclic shear strain required for pore water pressure generation in clay samples is examined through the results of torsional hollow cylinder cyclic shearing tests according to JGS 0543-2009. The study confirms the previous results, namely, that the threshold cyclic shear strain is dependent on the effective consolidation stress and plasticity index (Ip). The average and standard deviations in the estimated threshold strain levels are 0.038 ± 0.023% (Ip < 30, σ′c ≦ 100 kN/m2), 0.047 ± 0.016% (Ip < 30, σ′c > 100 kN/m2), 0.079 ± 0.028% (30 ≦ Ip < 5 0), and 0.143 ± 0.041% (Ip ≧ 50). As was found in past research, the levels of threshold strain for pore water pressure generation for clay are larger than those for clean sand. An increase in pore water pressure is only observed when the stiffness is reduced to around 80% of its initial value. This delay occurs because there is a difference between the cyclic threshold strain of the pore water pressure generation, γtp, and the cyclic threshold strain of the stiffness degradation, γtd. Since the test procedure of JGS 0543-2009 is a standard scheme in the practical design process, it is expected that more data will become available in the near future which will allow for further discussions on threshold strain.