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, γ td , and cyclic pore water pressure generation, γ tp , in fully saturated clays, nine multistage cyclic strain-controlled NGI direct simple shear tests are conducted on laboratory-made kaolinite clay (PI ¼ 28) and kaolinite-bentonite clay (PI ¼ 55). Three levels of vertical effective consolidation stress, σ 0 vc (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, γ td varied between 0.012 and 0.014% and γ tp between 0.014 and 0.034%. In two tests on the overconsolidated (OC) kaolinite clay with OCR ¼ 4, γ td was 0.013% and γ tp 0.016 and 0.017%. In two tests on the NC kaolinite-bentonite clay, γ td was 0.013 and 0.016% and γ tp 0.052 and 0.078%. In the test on OC kaolinite-bentonite clay with OCR ¼ 4, γ td was 0.014% and with OCR ¼ 7.8 it was 0.012%. For the same soil γ tp is typically slightly greater than γ td . Clear trends of γ td and γ tp with σ 0 vc , 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 γ td for six different soils ranges between 0.006 and 0.05% and γ tp for eight soils between 0.014 and 0.1%, and that there is a modest trend of γ td and moderate trend of γ tp increasing with PI.

a b s t r a c tCyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, γ c , around the threshold shear strain for cyclic pore water pressure buildup, γ tp E 0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small γ c below γ tp where there is no buildup of cyclic pore water pressure, Δu N , with the number of cycles, N, the cyclic secant shear modulus, G SN , initially increases with N for 10-20% of its initial value G S1 and then levels off or just slightly decreases, (ii) at small γ c between γ tp E0.01% and 0.10-0.15%, Δu N continuously increases with N while the modulus G SN first increases for up to 10% of G S1 and then gradually decreases, and (iii) at γ c larger than approximately 0.15%, relatively large Δu N develops with N while the modulus G SN constantly and significantly decreases. This means that at γ c between γ tp and 0.10-0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of γ c , the pore water pressure Δu N can reach up to 40% of the initial effective confining stress before G SN drops below G S1 . The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening.

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