Rintaro Suzuki

Development of triaxial testing method to evaluate ground rebound during underground construction

Hirofumi Toyota

Deep excavations for underground space development can cause ground rebound (heaving) because the unloading at the excavation base causes soil swelling and expansion. In current practice, rebound prediction and construction design criteria still depend largely on empirical rules. Moreover, deformation moduli used in design are often derived from laboratory compression or cyclic loading tests, which do not directly represent the unloading behavior of the excavated ground.

This study proposes a triaxial testing method that can evaluate rebound of the ground appropriately. Undisturbed specimens extracted from the target ground were first reconsolidated under K0 condition to reproduce the in-situ initial stress state. The small-strain stiffness (10-6–10-3) was then measured using local small-strain (LSS) measurements. Bender element (BE) tests and LSS measurements were performed under both compressional and extensional loadings. In addition, reconstituted specimens were also tested to compare two consolidation paths: K0 consolidation and isotropic consolidation.

Main results are summarized as follows. For undisturbed specimens, the initial shear modulus G0 from the BE test was sometimes larger than that from LSS test, whereas reconstituted specimens showed close agreement between the two methods. For both specimen types, degradation of shear modulus with increasing strain (strain-dependent of shear modulus) largely depended on loading methods: compressional loading produced a greater stiffness reduction than extensional loading. For reconstituted specimens, the strain-dependent of shear modulus showed similar trends in the specimens prepared between K0 and isotropic consolidations.

These findings indicate that the triaxial extension test, which explicitly represents unloading conditions, is more suitable than triaxial compression test to evaluate rebound-related deformation. The reconstituted specimens exhibited comparable strain-dependent of shear modulus trends to the undisturbed specimens under K0 consolidation. Furthermore, the reconstituted specimens presented close strain-dependent of shear modulus trends under both K0 and isotropic consolidations. Therefore, triaxial extension tests with LSS measurements using reconstituted specimens can be practically used to determine the deformation modulus for excavation-rebound ground.