Cyclic threshold strains in clays versus sands and the change of secant shear modulus and pore water pressure at small cyclic strains.

摘要

When fully saturated soils are subjected to cyclic loading in undrained conditions involving moderate and large cyclic shear strain amplitudes, gamma c, their stiffness and strength decrease and the pore water pressure changes permanently with the number of cycles, N. Such cyclic degradation of stiffness and pore water pressure change are among the most fundamental and important phenomena in soil dynamics.;When fully saturated soils are subjected to very small cyclic shear strains, gamma c, soil's structure practically does not change and, consequently, neither cyclic degradation nor permanent cyclic pore pressure change occur. The amplitude gamma c below which there is no cyclic degradation and above which the degradation occurs is known as the threshold shear strain for cyclic degradation , gamma td. The amplitude gammac below which there is no permanent pore water pressure change with N and above which the permanent pore pressure is recorded after every cycle is known as the threshold shear strain for cyclic pore water pressure change, gammatp.;The thesis describes laboratory investigation focused on gamma td in a clean sand and gammatp in two laboratory-made normally consolidated (NC) and overconsolidated (OC) clays, kaolinite clay having PI=28 and kaolinite-betonite clay having PI=55, and on the comparison and connection between different thresholds. The Norwegian Geotechnical Institute (NGI) type of direct simple shear device (DSS) was employed for the constant-volume equivalent-undrained cyclic testing. Two types of tests were conducted, single-stage cyclic strain-controlled test with constant gammac throughout the test, and the multi-stage cyclic strain-controlled test in which gammac was constant in each stage but larger in every subsequent stage. The magnitude of gamma c covered the range from 0.003% to 2%.;In the context of investigating gammatd in sand and gamma tp in clays, the following tasks were also performed: (1) the NGI-DSS device was adapted for small-strain cyclic testing and a procedure for eliminating false loads and deformations from test records was developed, (2) the effect of the vertical consolidation stress, sigmavc', and frequency of cyclic straining, ƒ, on cyclic degradation and pore water pressure change in clays was tested, and (3) cyclic stress-strain behavior and the change in secant shear modulus, GSN, with N in sands at very small cyclic strains was investigated.;The following conclusions are derived and results obtained: (1) small-strain cyclic testing at gammac as small as 0.003% can be conducted in the standard NGI-DSS device and the results can be used in soil dynamics analyses if the device is properly modified and the false loads are eliminated from the test records, (2) cyclic degradation in clays is affected moderately to significantly by sigmavc' and frequency, ƒ, (3) in sands modulus GSN increases with N at very small gammac below gammatp (sand is stiffening) while the cyclic pore water pressure does not develop; (4) in sands at small to moderate gamma c above gammatp modulus GSN first increases and then decreases with N while the cyclic pore water pressure continuously increases, (5) because of such behavior in which soil stiffness starts degrading after certain number of cycles, gammatd in sand cannot be defined like gammatd in clays, (6) in both clays tested gamma td is not visibly affected by overconsolidation ratio, OCR, (7) in kaolinite clay gammatp is not affected by OCR, (8) in kaolinite-bentonite clay with PI=55 and overconsolidated to OCR=4 and 7.8 the pore water pressures between gammac=0.003% and 0.3% did not change in a consistent manner and gammatp could not be evaluated, (9) the thresholds gamma tp tested show increase with PI just like in the previous studies, but their values are at or below the lower bound of published gammatp-PI trends, and (10) the tested gammatd thresholds in clays do not follow the trend of increase with PI like in the previous studies and they are smaller than those published earlier for similar soils. (Abstract shortened by UMI.)