Undrained cyclic behavior of nonplastic silt.

摘要

Silts often are considered on the borderline between liquefiable and nonliquefiable soils. However, silty soils are encountered commonly, and their importance cannot be underestimated. This research was performed to characterize the undrained cyclic behavior of a nonplastic silt. The influence of cyclic shear strain amplitude, {dollar}gammasb{lcub}rm cy{rcub}{dollar}, number of loading cycles, N, overconsolidation ratio, OCR, and consolidation stress ratio, K{dollar}sb{lcub}rm c{rcub}{dollar} = {dollar}barsigmasb{lcub}rm ci{rcub}/barsigmasb{lcub}rm vi{rcub}{dollar}, on the potential for pore water pressure generation, low-amplitude shear modulus, shear modulus degradation, and damping ratio were investigated.; An extensive experimental program on reconstituted specimens of a nonplastic silt was carried out using a hybrid resonant column/quasi-static torsional simple shear apparatus. Specimens were prepared at 50% relative density and consolidated at stress ratios of K{dollar}sb{lcub}rm c{rcub}{dollar} = 0.6, 1.0, and 1.5 with overconsolidation ratios of OCR = 1.0, 2.0, and 4.0. Resonant column tests were conducted at various stages along the consolidation stress paths. After consolidation to the final effective confining pressure, undrained resonant column testing was conducted, followed by undrained strain-controlled cyclic torsional shear testing.; Test results showed that the dynamic response of nonplastic silt was similar to that of sands. The significant agreement between the pore water pressure characteristics of nonplastic silt and sand implies that a loose nonplastic silt deposit may have similar liquefaction potential to that of a loose sand deposit. The ranges of modulus reduction and damping curves for silt were found to lie between the ranges for sand and the ranges for clay. The results also indicated that the threshold strain is not a constant characteristic of each type of soil, since it depends on various parameters such as number of loading cycles and overconsolidation ratio. It was concluded that the stiffness deterioration in silts is due to microstructural mechanisms and pore water pressure buildup. It was also shown that the classic Hardin's equation to estimate the low-amplitude shear modulus, G{dollar}sb{lcub}rm max{rcub}{dollar}, is valid only for isotropic stress conditions, since G{dollar}sb{lcub}rm max{rcub}{dollar} was found to be dependent on single stress components and stress ratio. Comparisons of the test results with modulus reduction curves recommended for seismic site response evaluation suggest that modulus reduction and damping curves should be developed from direct experimental measurements on undisturbed specimens.