title: Capillary barriers on slopes subjected to rainfall: numerical modelling and development of simplified methods of analysis
Creators: name: Mudcharoen, Paverean
Publisher: name: University of Glasgow
Year: 2023

description: Climate change is leading to increasing rainfall (particularly increasing frequency and severity of extreme rainfall events). As a consequence, problems of rainfallinduced slope instabilities are increasing and there is increasing need for methods of enhancing slope stability and preventing rainfall-induced instabilities. Capillary barrier systems (CBSs) are an attractive option for enhancing slope stability, because they use only naturally occurring geomaterials (sands and gravels) and therefore have low carbon footprint, as long as they can be locally sourced. CBSs are soil cover systems, consisting of a finer layer of soil overlying a coarser layer of soil, that are intended to limit infiltration of water into the underlying soil. The function of a CBS is based on the contrast between the hydraulic properties (i.e. soil water retention curve and soil hydraulic conductivity curve) of the finer and coarser layers. The performance of a CBS on a slope depends upon both the water storage capacity of the CBS and the water transfer capacity down the slope. The aim of this PhD project was to use hydraulic and thermo hydraulic numerical modelling to develop and validate simplified methods of analysis for sloping CBSs for enhancing slope stability that are suitable for use by practising geotechnical engineers in industry and that can account for the climate and meteorological conditions of an individual site. The original research presented in the thesis can be divided into four major parts. The first part investigated horizontal CBSs subjected to continuous rainfall of constant intensity. This involved one-dimensional hydraulic FE numerical analyses and numerical validation of existing simplified methods (due to Stormont and Morris (1998) and Scarfone (2020)) for calculating water storage capacity at steady state of conventional horizontal CBSs and multi layered horizontal CBSs respectively. This represented a more comprehensive numerical validation of these existing simplified methods than previously reported in the literature. The second major component of original research investigated sloping CBSs subjected to continuous rainfall of constant intensity and involved twodimensional hydraulic FE numerical analyses and development and numerical validation of a new simplified method for calculating water storage capacity and water transfer capacity (and hence diversion length). In all cases analysed, the proposed new simplified method provided an excellent match to the results from the FE simulations, in contrast to an existing simplified method (proposed by Parent and Cabral (2006)), which assumed an unrealistic approximate final steady state suction profile (more appropriate for a horizontal CBS). The new simplified method was also successfully extended to multi-layered sloping CBSs. The third part of the work used thermo-hydraulic FE analyses and hydraulic FE analyses to investigate the behaviour of sloping CBSs subjected to various simple patterns of intermittent rainfall of varying intensity. This led to improved understanding of the behaviour of sloping CBSs under more realistic rainfall conditions, including the effects of evaporation from the ground surface and hysteresis in the hydraulic behaviour of the CBS materials. The final major original part of the research described in the thesis involved the development and successful numerical validation of a new simplified method of analysis for sloping CBSs subjected to extreme rainfall events. The new simplified method uses a method of slices to predict the variation with time of interface flow velocity (across the interface between finer and coarser layers) at any value of horizontal coordinate within the slope for any specified extreme rainfall event (i.e. any specified variation of rainfall intensity with time). This simplified method of analysis for extreme rainfall events could form a central part of a practical design methodology for sloping CBSs, suitable for use by practising engineers in industry.