title: Modelling the hydraulic behaviour of unsaturated soils and application to the numerical and experimental study of capillary barrier systems
Creators: name: Scarfone, Riccardo
Publisher: name: University of Glasgow
Year: 2020

description: Nowadays, considerable research effort is addressed towards the mitigation of the effects of climate change. The development and application of low-carbon solutions in geotechnical engineering practice is essential for the mitigation of the effects of climate change. Under unsaturated conditions, suction has a beneficial effect on the shear strength of soils but it may easily vanish after intense rainfall. If suction can be maintained in the ground in the long term, its effect can be taken into account in geotechnical design as a natural soil reinforcement, and this can lead to low-carbon designs. Capillary barrier systems can be used to prevent or limit the infiltration of water into the ground, thereby maintaining suction in the long term. Capillary barrier systems are soil covers made of a finer-grained layer overlying a coarser-grained layer. Under unsaturated conditions, the coarser layer is typically at very low degree of saturation and the corresponding unsaturated hydraulic conductivity is so low that it can be considered as impermeable. In these conditions, rainwater is stored in the finer layer and subsequently removed by evapotranspiration or lateral drainage. Accurate modelling of the hydraulic behaviour of unsaturated soils is crucial for the interpretation of the behaviour of capillary barrier systems. The first part of this thesis is focused on the interpretation and modelling of the hydraulic behaviour of unsaturated soils. A critical review of the interpretation of the hydraulic behaviour of unsaturated soils leads to the identification of inaccuracies and inconsistencies in existing conventional hydraulic constitutive models for the soil water retention curve (SWRC) and the soil hydraulic conductivity curve (SHCC). These inaccuracies and inconsistencies relate particularly to the very high degree of saturation range and the very low degree of saturation range. At very high values of degree of saturation, the apparent SWRC measured in a wetting test in the laboratory may differ from the true SWRC, because of the occurrence of air trapping. Analytical and numerical modelling of the phenomenon of gas trapping during wetting shows that, once air trapping occurs, the apparent SWRC depends upon many aspects of the wetting test conditions and is not a fundamental representation of the soil behaviour. The only correct way to represent the occurrence and influence of air trapping during wetting within numerical modelling of boundary value problems is to use the true SWRC in combination with a gas conductivity expression that goes to zero when the gas phase becomes discontinuous. At low values of degree of saturation, conventional models for the SHCC are typically inaccurate. A new predictive hydraulic conductivity model, accurate for the full range of degree of saturation is developed. The hydraulic conductivity is divided into two components: a bulk water component and a liquid film component; each of which varies with degree of saturation or suction. This model is coupled with SWRC models improved at low degree of saturation. Hydraulic hysteresis is subsequently introduced in the SWRC and SHCC by using an original bounding surface approach. The new hydraulic models were validated against experimental data. This set of hydraulic models forms a complete framework of hydraulic constitutive models for unsaturated soil, which was implemented in the numerical finite element software Code_Bright. Finally, these new models are applied to the numerical study of the hydraulic behaviour of capillary barrier systems (CBSs). The new hydraulic conductivity model is able to predict the behaviour of CBSs better than conventional models and the numerical modelling highlights the role of liquid film flow, which is often neglected. Water retention hysteresis is shown to have a significant impact on: i) movement and redistribution of water within the finer layer of a CBS; ii) the phenomenon of water breakthrough across the interface between the finer and coarser layers of a CBS and the subsequent restoration of the CBS after infiltration at the ground surface ceases; iii) the prediction of evaporation from a CBS into the atmosphere. In the second part of this thesis, an original concept of multi-layered capillary barrier systems is presented and analysed. The use of multi-layered CBSs may lead to a substantial increase of the water storage capacity of CBSs, and hence their effectiveness. A simplified method of analysis of multi-layered CBSs is developed and validated against results from numerical finite element analyses and laboratory physical tests. Parametric analyses show the impact of number of layers, materials thickness of the CBS and infiltration rate on the water storage capacity of multi-layered CBS. Laboratory infiltration tests on different multi-layered CBSs are performed demonstrating the efficiency of multi-layered CBSs and clarifying their hydraulic behaviour. In the third part of the thesis, advanced numerical thermo-hydraulic finite element analyses and limit analyses are performed to assess the application of CBSs for suction control and slope stability purposes in the long term. It is demonstrated that sloping CBSs are effective at maintaining suction in the ground and preventing rainfall-induced slope instability for different climatic conditions. In addition, the role of different parameters such as materials, thickness of the CBS and slope height are assessed. In particular, it is shown that CBSs with the finer layer made of a relatively fine material, such as silty sand, are more effective in dry and warm climates due to their ability of storing water, which can subsequently be removed by evaporation, whereas CBSs with a finer layer made of a slightly coarser material, such as fine sand, are more effective in wet and cool climates due to their ability of diverting water laterally down the slope. The effectiveness of solutions aimed to extend the application of CBS to slope of any height, such as the use of multi-layered CBSs and the use of multiple drains, is finally demonstrated.