MOHAMMAD MONZUR MORSHED

Deformation characteristics of cohesive soils at various stress conditions

Hirofumi TOYOTA

Large-scale deep excavations have become more common in recent years because of the growing need for significant subsurface space usage. The process of gradual stress release during excavations causes upward ground movement, which is known as rebound or heaving deformation. The rebound amount has been estimated using conventional tests and empirical approaches. In the laboratory, the deformation coefficient, which is used to predict rebound, has been determined through monotonic compression or cyclic loading tests. However, these deformation coefficients are not appropriate for capturing the rebound phenomenon, where the soil swells in the extension direction. This study examined the deformation characteristics of undisturbed and reconstituted samples to evaluate rebound phenomenon for underground constructions.

For this objectives, triaxial extension tests using K0 consolidated specimens were considered as a most reliable test. In practice, triaxial tests with local small strain (LSS) measurement, bender element (BE), and shear strength tests were conducted under K0 and K consolidations. Initial and secant shear modulus (G0, Gsec) were obtained under compression and extension loading. Four kinds of undisturbed samples (two are cohesive soils and the others are sandy soils) are extracted from the different depths of the ground.

The following findings are obtained from the study. G0 is largely different by depths and layers. Therefore, G0 should be measured using in-situ methods. To compare Gsec between undisturbed and reconstituted specimens, Gsec was normalized by each G0, referred to as normalized Gsec. The degradation of normalized Gsec from extension loading of reconstituted samples is approximately consistent with that of undisturbed ones. These findings contribute to accurate estimation of rebound amount using disturbed soil samples, which are easily and inexpensively obtained compared to undisturbed soil samples. As a result, reconstituted soil can be replaced from undisturbed soil in the small strain test for rebound evaluation. The use of Gsec from compressional loadings will lead to overestimate rebound amount than that from extensional loadings.