Threshold Shear Strains for Cyclic Degradation and Cyclic Pore Water Pressure Generation in Two Clays

Abstract

Cyclic threshold shear strains are fundamental cyclic soil properties that have not been fully investigated. To learn more about the threshold shear strains for cyclic degradation, γtd, and cyclic pore water pressure generation, γtp, in fully saturated clays, nine multistage cyclic strain-controlled NGI direct simple shear tests are conducted on laboratory-made kaolinite clay (PI=28) and kaolinite-bentonite clay (PI=55). Three levels of vertical effective consolidation stress, σvc′ (113 kPa, approximately 216 kPa, and approximately 674 kPa); three OCRs (1, 4 and 7.8); and two cyclic loading frequencies, f (0.01 and 0.1 Hz), were applied. In three tests on the normally consolidated (NC) kaolinite clay, γtd varied between 0.012 and 0.014% and γtp between 0.014 and 0.034%. In two tests on the overconsolidated (OC) kaolinite clay with OCR=4, γtd was 0.013% and γtp 0.016 and 0.017%. In two tests on the NC kaolinite-bentonite clay, γtd was 0.013 and 0.016% and γtp 0.052 and 0.078%. In the test on OC kaolinite-bentonite clay with OCR=4, γtd was 0.014% and with OCR=7.8 it was 0.012%. For the same soil γtp is typically slightly greater than γtd. Clear trends of γtd and γtp with σvc′, OCR, and f could not be identified given the relatively small number of tests. The results indicate that if these trends exist they are small. The comparison of the above results with those from the literature shows that γtd for six different soils ranges between 0.006 and 0.05% and γtp for eight soils between 0.014 and 0.1%, and that there is a modest trend of γtd and moderate trend of γtp increasing with PI.