Tetsuya Nishimura

Deformation characteristics in small strain range of undisturbed samples extracted from Kanto diluvial deposits

Hirofumi TOYOTA

Through the recent redevelopment metropolitan areas, deep excavations in close proximity has been increasing because of existing high-rise buildings and subway tunnel. In those underground developments, the ground rebound or heaving occurs when the overburden pressure is removed during the ground excavation. Amount of rebound has been estimated using empirical methods. In laboratory tests, the deformation coefficient to estimate the rebound amount has been measured using the monotonic compression or cyclic loading. However, these deformation coefficients are not suitable for the rebound phenomenon, in which the soil swells in the extension direction.

The purpose of this study is to investigate the effect of the rebound phenomenon, which swells in the extension direction, on the deformation coefficient of the ground. Reconstituted mud stone and undisturbed clay were obtained from the subway tunnel constructed in the Kanto diluvial layers. The stress state in the ground will be reproduced by K0 consolidation. The deformation coefficient of the ground in the small strain region (10-6~10-3) is obtained from bender element (BE) tests and local small strain (LSS) tests with compressional and extensional loading. The results are compared with each other to investigate the influence of rebound phenomenon on the deformation coefficient in detail.

The results obtained are as follows.
1. The secant shear moduli of both reconstituted and undisturbed clays showed a different degradation trend with strain (strain softening), in which the degradation of compressional loading was greater than that of extensional loading. In particular, this difference was more apparent in reconstituted clay than the undisturbed clay.
2. Under the extensional loading, the degradation trends are similar between undisturbed and reconstituted clay. Therefore, reconstituted clay can be substituted for undisturbed clay in extensional loading tests.
3. It is confirmed that when the secant shear modulus obtained from compressional loading is used, the amount of rebound is overestimated comparing to the use of the results from extensional loading.