RAHMAT SHUKOOR

Compressional and extensional deformation characteristics in K0 consolidated soils

Hirofumi TOYOTA

In recent years, the increased demand for extensive underground space utilization has led to large-scale deep excavations. Excavations involve a process of gradual stress release. Then upward ground movement, which is known as rebound or heaving deformation, occurs. Ground rebound occurs when overburden pressure is removed during deep excavations for high-rise buildings, tunnels, and underground structures. The rebound amount has been estimated using 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 aims to investigate the effective methods to estimate the rebound deformation, which causes swelling deformation in the extension direction. The initial stress state in the ground is reproduced through the K0 consolidation. Bender element tests, local small strain tests, and shear strength tests with compressional and extensional loading are conducted to obtain the deformation coefficient for small and large strains using undisturbed and reconstituted specimens of two different layers 3De-8 (depth: 8.50 to 9.50 m) and 3De-21 (depth: 21.50 to 22.50 m). The results can be used to evaluate the rebound phenomenon in the practical field. The results revealed that the strain dependent trends of the secant shear moduli are equivalent between undisturbed and reconstituted clays under both compressional and extensional loading. Therefore, reconstituted clay can be substitute for undisturbed clay for the small strain test. However, attention is needed in the larger strain because undisturbed clay exhibits higher shear strength and more brittle (less deformation to the failure) compared to reconstituted one. To evaluate the rebound amount in practical field, the use of the secant shear modulus from compressional loading will lead to overestimate rebound amount than the used of extensional results.