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

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, ${\gamma }_{\mathrm{td}}$, and cyclic pore water pressure generation, ${\gamma }_{\mathrm{tp}}$, in fully saturated clays, nine multistage cyclic strain-controlled NGI direct simple shear tests are conducted on laboratory-made kaolinite clay ($\mathrm{PI}=28$) and kaolinite-bentonite clay ($\mathrm{PI}=55$). Three levels of vertical effective consolidation stress, ${\sigma }_{\mathrm{vc}}^{\prime }$ (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, ${\gamma }_{\mathrm{td}}$ varied between 0.012 and 0.014% and ${\gamma }_{\mathrm{tp}}$ between 0.014 and 0.034%. In two tests on the overconsolidated (OC) kaolinite clay with $\mathrm{OCR}=4$, ${\gamma }_{\mathrm{td}}$ was 0.013% and ${\gamma }_{\mathrm{tp}}$ 0.016 and 0.017%. In two tests on the NC kaolinite-bentonite clay, ${\gamma }_{\mathrm{td}}$ was 0.013 and 0.016% and ${\gamma }_{\mathrm{tp}}$ 0.052 and 0.078%. In the test on OC kaolinite-bentonite clay with $\mathrm{OCR}=4$, ${\gamma }_{\mathrm{td}}$ was 0.014% and with $\mathrm{OCR}=7.8$ it was 0.012%. For the same soil ${\gamma }_{\mathrm{tp}}$ is typically slightly greater than ${\gamma }_{\mathrm{td}}$. Clear trends of ${\gamma }_{\mathrm{td}}$ and ${\gamma }_{\mathrm{tp}}$ with ${\sigma }_{\mathrm{vc}}^{\prime }$, 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 ${\gamma }_{\mathrm{td}}$ for six different soils ranges between 0.006 and 0.05% and ${\gamma }_{\mathrm{tp}}$ for eight soils between 0.014 and 0.1%, and that there is a modest trend of ${\gamma }_{\mathrm{td}}$ and moderate trend of ${\gamma }_{\mathrm{tp}}$ increasing with PI.