Physical properties and hydro-mechanical behaviour of bentonite-soil mixtures with temperature effects

Abstract

Temperature is involved in many geotechnical engineering practices, such as the disposal of high-level radioactive waste, energy piles, submarine pipelines, heat energy storage, and heating and vacuum preloading treatment in land reclamation. Understanding the temperature effects on the physical properties and hydro-mechanical behaviours of soils has been of great concern. In the past decades, some attempts have been made to the temperature effects on the properties and behaviour of various types of soils from experimental and theoretical approaches. However, the temperature effects on the basic physical properties of soils are still not clear. Moreover, most existing studies mainly focus on the thermal effects on the behaviour of the natural soils. For binary bentonite mixtures exhibiting distinct behaviours, there is still a lack of consensus on the thermal effects on binary mixtures. In order to improve the consolidation of slurry in ground improvement projects and dredged slurry treatment, heating is considered to be an alternative way to accelerate the consolidation process. The effects of temperature on the efficiency of vacuum preloading consolidation of soft soils have not been well studied, especially for improvement with prefabricated horizontal drains (PHDs). This study presents systematic experimental studies and fully coupled consolidation modelling on temperature effects on physical properties and hydro-mechanical behaviour of bentonite soils, including bentonite-sand mixtures with different bentonite content and a local waste bentonite slurry. A temperature- and humidity-controlled chamber and a temperature-controlled oedometer were developed. A series of cone penetrometer tests, rheometer tests, flask volumetric method tests, microscopic tests, and oedometer tests were conducted on bentonite-sand mixtures and PHD vacuum preloading physical model tests on bentonite slurry. Firstly, the experimental results revealed that the elevating temperature could increase the liquid limit and yield stress in rheological property, whereas reduce the bound water content of bentonite-sand mixtures. The evolution of microstructure and micropores with temperature was also presented and provided an evidence of the mechanism of temperature effects. Secondly, the results of oedometer tests disclosed that increasing temperature causes a reduction in the viscosity of pore water and an increase in the permeability and efficient of consolidation. The compression index and creep coefficient were found to increase, and the swelling index decreased as temperature increased. An interesting finding that differs from the previous studies is that there is an overshooting of normal compression lines for the binary mixture containing bentonite greater than 25% at different constant temperatures, which may be attributed to the thermal-induced consolidation, expansive characteristic of bentonite clay, and generation of thermal structure with zero pre-consolidation pressure. Thirdly, physical model tests illustrated that heating could induce the rise of pore water pressure and that the thermal-induced pore water pressure dissipated very quickly. The elevating temperature will also significantly boost the vacuum transfer to higher soil layers, causing greater effective stress on the soil slurry. It is observed that the water content distribution and void ratio profile of soil after treatment at high temperatures are more uniform, meaning that heating could reduce the non-uniform consolidation of slurry by PHD-vacuum preloading in the vertical direction. The undrained shear stress increases with increasing treated temperature. In addition, the compressibility extremely decreases for the soils improved by PHD-vacuum preloading at high temperatures. Based on the hypothesis B method, which considers the creep occurs during both primary and secondary consolidation, a simple calculation method incorporating the temperature effects and particle migration is proposed. The calculation results are very close to the measured settlement at different temperatures. Finally, a one-dimensional fully coupled finite strain consolidation analysis for soft clay treated by thermal prefabricated horizontal drain under vacuum and heat preloading with the consideration of thermal elastic viscoplastic (TEVP) behaviour of soft clay. A modified finite difference method has been employed to solve the model. The model was verified with a benchmark case and used to compare with the results of the PHD-vacuum preloading physical model tests at different temperatures. It can be observed a good model performance in the comparison with three physical experiments. The calculation results demonstrate that the combined method of using PHD with vacuum and heat preloading is an effective method to improve consolidation efficiency.