Study on the temperature and time dependent stress-strain behaviour and consolidation of a soft marine clayey soil

Abstract

Conventional geotechnical engineering has not paid much attention to the thermal factor. In recent engineering practices, such as thermal-boosted vacuum preloading, submarine pipelines, disposal of radioactive waste, utilization of geothermal resources, heat energy storage and frozen soil foundation, the effects of temperature on the hydro-mechanical behaviour of soft soils are non-neglectable. In the past decades, research efforts have been devoted to both experimental and theoretical studies on the temperature-dependent hydro-mechanical behaviour of various types of soft soils. However, there is still a lack of consensus on a few key aspects, especially the temperature effect on the time-dependent stress-strain behaviour of soft soils. The complete mechanism for thermal improvement of vacuum consolidation is not clear to researchers. There is also room of research on developing new thermal elastic visco-plastic (TEVP) constitutive models in both one-dimension (1-D) and three-dimensional (3-D) conditions with distinct physical meaning and easily calibrated parameters. This study presents systematic experimental studies and constitutive modelling on the temperature and time dependent hydro-mechanical behaviour of Hong Kong marine deposits (HKMD), which is a typical clayey soil in Hong Kong. A series of temperature-controlled oedometer tests, constant-rate-of-strain (CRS) tests, triaxial tests and vacuum consolidation model tests were carried out to study the temperature and time dependent compression and shear behaviour of HKMD under complicated temperature conditions. It was revealed that increasing temperature will cause reduction of apparent pre-consolidation pressure, degradation of creep coefficient and enhancement of hydraulic conductivity, while compression indices and critical state line are not sensitive to temperatures. Heating under an undrained condition was found to cause reduction of shear strength due to loss in effective stress. Temperature-induced elastic and visco-plastic volumetric strain under non-isothermal conditions was innovatively interpreted with the concept of virgin heating line proposed in this study. The microstructure evolution of HKMD under different temperatures was investigated through SEM and MIP tests to reveal the origin of thermal compression and thermal creep. Physical modelling studies on the vacuum-heat preloading technology for HKMD slurry were conducted and the mechanism of temperature effects was discussed through the back analysis. Based on the element test results, a new 1-D TEVP model was proposed in which the temperature was considered as a state variable in the constitutive equations for any thermal and stress paths of soft clayey soils. The virgin heating line and cooling / reheating line were defined, and the equivalent time initially proposed by Yin and Graham (1989, 1994) was extended, to describe the effects of temperature on the elastic and visco-plastic behaviour. The model was then expanded into a 3-D model with the overstress theory, the modified Cam-Clay model and the anisotropic S-CLAY1 model. The proposed models were verified by s series of laboratory tests under complicated temperature conditions through both simple calculation method and numerical implementation with satisfactory accuracy.