Calculation of Tangent Modulus of Soils under Different Stress Paths

Abstract

During excavation of foundation pit, soils of different sites may undergo different unloading paths. This study focuses on the tangent modulus of soil under different stress paths and provides theoretical basis for the deformation calculation of soil. In this paper, conventional triaxial compression test and 0 consolidation unloading test were conducted with mucky soil and silty clay to obtain the deformation characteristics of soils under different stress paths. Experiment results show that the soil samples exhibit distinct stress-strain characteristics under different stress paths, but they all show nonlinearity. The initial tangent modulus increases as the consolidation confining pressure intensifies. Then, based on the test data, the power function relationship between initial tangent modulus and confining pressure under unloading was verified. Simultaneously, a hyperbola function to express stress-strain relation of soils under the 0 consolidation unloading condition was proposed and proved. Finally, the formulas of the tangent modulus under 0 consolidation unloading were established referring to the derivation of that in Duncan-Chang model. The theoretical calculation results agree well with the test results. It can expand the use of the Duncan-Chang model and improve its application to engineering practice.

The impact of stress disturbance on undrained cyclic behaviour of a kaolin clay and settlement of tunnels under cyclic loading

“…According to Huang et al [10] and Verruijt [27], the construction of a pit or tunnel will change the stress states in soils along different stress paths, i.e. the static deviatoric stress would decrease (for the soil directly above or below excavation), increase (for soil next to excavation) or remain unchanged (for soil in some other influenced zone or far away from excavation).…”

“…10b, as investigated by many researchers [8,11,17,37,38]. For example, within zone 1, as suggested by Huang et al [10], the deviatoric stress in the soil would increase, and mean principal stress would decrease. If the existing tunnel is in this zone, then the traffic loading in the tunnel could potentially cause more settlement due to the increase of deviatoric stress in the soil.…”

“…As discussed earlier, the cyclic deformation behaviour of clay in the subsequent bundle of loading cycles is governed by the stress disturbance-induced static deviatoric stress, and the construction of a pit or tunnel will change stress states in soil at different locations along different stress paths [10,27]. This means that the effects of such construction activities on traffic loading-induced settlement of existing tunnels varies from locations, if those existing structures are within the influenced zones.…”

“…If the existing tunnel is in zone 3, where the deviatoric stress decreases after the excavation, then the effect of traffic loading-induced settlement of the tunnel could be ignored, as the decrease of deviatoric stress would cause little additional deformation under subsequent cyclic loading as discussed above. For zone 2, due to complexity of the variation in deviatoric stress as suggested by Huang et al [10], it is harder to tell whether the excavation will affect the traffic loading-induced long-term settlement of the existing tunnel. Therefore, if the existing tunnel is within zone 2, then To analyse the effect of tunnelling on the stress disturbance in ground, Verruijt' solution [27] is used to obtain the variation of lateral and vertical stresses around the new tunnel as revealed in Fig.…”

“…As suggested by Shen et al [25] and Ou et al [18], in addition to cyclic loading, the disturbance from nearby construction is also a possible reason for such unexpected greater settlement of metro tunnels or railways. This is because the construction activities change the stress states in soils under different stress paths [10,17,[26][27][28]36] and affect the cyclic deformation characteristics of soils within the influenced zones. This means that the effects of including various stress disturbances between loading cycles should be considered in analysing the soil's deformation behaviour under subsequent cyclic loadings.…”