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, 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.

Formulation and implementation of a practical multi-surface soil plasticity model

“…Hyde and Ward, 1986;Yasuhara et al, 2003;Thian and Lee, 2017). Additionally, Vucetic (1994) and Mortezaie and Vucetic (2016) showed that below a certain shear strain value, termed the 'cyclic threshold shear strain', cyclic degradation does not occur to any significant degree. The magnitude of the cyclic threshold strain value is typically considered to be unaffected by the previous cyclic loading history.…”

Dynamic characterization of fine-grained soils in Central Italy by laboratory testing

“…In turn, a pronounced pore water pressure build-up can be identified from the first c levels. Looking at the main characteristics of the outliers (Table 2), it is evident that, first of all, tp values (ranging from 0.0005 to 0.007%) are totally out of the expected range of values (for example Mortezaie and Vucetic 2016 identified tp values which range from 0.014 to 0.034% for two reconstituted clays). These low tp values may be partially due to excessive sample disturbance effects that destroyed the soils structure, given the low V S,lab ∕V S,field observed, less than 0.6 for 4 outliers.…”

“…Above tv , at medium to large strains, soils exhibit a gradual degradation behaviour, experiencing permanent changes in the microstructure and, in drained conditions, volume variation. Vucetic (1994) and more recently Tabata and Vucetic (2010) and Mortezaie and Vucetic (2016) distinguished the amplitude below which there is practically no degradation, defined as cyclic degradation threshold shear strain ( td ), and the amplitude below which there is practically no permanent cyclic pore water pressure build-up, known as cyclic pore-water pressure generation threshold shear strain ( tp ). td is typically smaller than tv and the ratio tp ∕ td varies between 1.2 and 1.6.…”

Effect of Variable Confining Pressure on Cyclic Triaxial Behavior of K0-consolidated Soft Marine Clay

“…When fully saturated soils are subjected to cyclic loading in undrained conditions involving moderate and large cyclic stress levels, their structure is permanently altered, the generation of excess porewater pressure and thus to a deterioration of bearing capacity and soil stiffness [1][2][3]. In undrained conditions, cyclic degradation of stiffness and cyclic pore water pressure changes are among the most important phenomena in soil dynamics [4]. The degradation of soils will then significantly influence the development of the accumulation of plastic strains and resilient stiffness.…”