Thermo-Hydro-Mechanical Effects on the Behaviour of Unsaturated Soil-Structure Interfaces and the Numerical Analysis of Energy Piles
Creators: name: Fu, Zhu
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Unsaturated soils-structure interfaces
Thermo-hydro-mechanical processes
Energy piles
Numerical analysis

description: The shear strength of soil-structure interfaces is relevant to the stability of energy piles. The thermo-hydro-mechanical processes can have a strong effect on the behaviour of interfaces between unsaturated soils and piles. Temperature changes lead to water movement in the soil. The moisture loss or gain in the soil causes drying or wetting. In addition, water movement influences the heat transfer properties of the soil. Temperature and moisture content changes affect the magnitude of soil suction in unsaturated soils. Changes in soil suction alter the strength and deformation characteristics of the soil mass and soil-structure interfaces. Similar to the effects of temperature changes, the mechanical loading and the changes in hydraulic conditions in the ground would cause changes in the void ratio, degree of saturation, suction, strength and deformation characteristics of soil. The interface behaviour under varying thermo-hydro-mechanical (THM) conditions is classified as a coupled problem and this is the subject of the present research. In the present investigation, laboratory studies and numerical analyses are carried out to evaluate the THM effect on the behaviour of interfaces between an energy pile material and an unsaturated soil. A 3D interface apparatus (Fakharian and Evgin 1996) has been modified (Fu et al. 2013) to allow the behaviour of an interface to be studied under thermo-mechanical loading conditions. In the present study, the experiments are conducted on soil samples with low degree of saturation and high degree of saturation. It is found that in interface tests using soil samples with low degree of saturation, the adhesion increased due to a positive effect of suction on strength than the negative effect of increasing temperatures. However, in interface tests on soil samples with high degree of saturation, the adhesion decreased with increasing temperatures while the positive effect of suction was not large enough to overcome the negative effect of increasing temperatures. This is a new finding that has not been reported anywhere in the literature. The friction angle for both soil samples (with different degrees of saturation) changed slightly with temperature change. Coupled finite element analyses conducted in the present study provide the following geotechnical information that would be useful for the design of energy piles: (a) Bearing capacity of the pile with and without the effect of temperature, (b) The effect of degree of saturation (or suction) on the strength and deformation characteristics of both the soil and the soil-structure interface, (c) Temperature effects on the amount of pile head movements (up or down), (d) Temperature induced stresses in the pile, (f) Amount of heat that can be stored or extracted from the ground as a function of time. At the initial stages of this study, THM effects on the behaviour of energy piles under saturated and unsaturated conditions are analyzed by using SIGMA/W and VADOSE/W finite element codes of GeoStudio 2012. Although these codes are not multi-physics FE codes, it is possible to use them sequentially to obtain results that will show the trends in pile behaviour. This numerical approach is used first to analyze the behaviour of an energy pile installed partially in unsaturated soil. The moisture content and temperature distributions around a 10 m long, bored pile are calculated using transient analyses. Changes taking place in the stress state along the pile shaft and the bearing capacity of the pile at different temperatures are calculated. In the second part of the numerical analysis of the present study, THM effects on the behaviour of energy piles under saturated and unsaturated conditions are analyzed by using PLAXIS 2D finite element code. PLAXIS is a fully couples finite element code. In order to enhance present understanding of the behaviour of energy piles and do the analysis correctly, a fully coupled analysis involving thermo-hydro-mechanical processes was carried out. Three simulations (mechanical loading only, thermo-mechanical coupling and thermo-hydro-mechanical coupling) are conducted using case studies that are available in the literature. In addition, the behaviour of a generic energy pile, which is installed in a kaolin-sand mixture, is studied by taking into consideration of thermo–hydro-mechanical processes. The coupled analysis provided the distributions of temperature, degree of saturation, suction and heat flux in the analysis domain. Numerical results of the fully-coupled method are compared with the results of sequential method of analysis.