Abstract
This paper details development of a numerical modelling approach that has been employed to forecast the long-term performance of a cut slope formed in high plasticity clay. It links hydrological and mechanical behaviour in a coupled saturated and unsaturated model. This is used to investigate the influence of combined dissipation of excavation-generated excess pore water pressures and seasonal weather-driven near-surface cyclic pore water pressures. Deterioration of slope performance is defined in terms of both slope deformations (i.e. service) and factor of safety against shear failure (i.e. safety). Uniquely, the modelling approach has been validated using 16 years of measured pore water pressure data from multiple locations in a London Clay cut slope. Slope deterioration was shown to be a function of both construction-induced pore water pressure dissipation and seasonal weather-driven pore water pressure cycles. These lead to both transient and permanent changes in factor of safety due to effective stress variation and mobilisation of post-peak strength reduction over time, respectively, ultimately causing shallow first-time progressive failure. It is demonstrated that this long-term (90 year) deterioration in slope performance is governed by the hydrological processes in the weathered near surface soil zone that forms following slope excavation.
Highlights
Uncontrolled deformations of cut slopes along transport infrastruc ture routes cause reduced service performance (Briggs et al, 2019) leading to delays and disruption to the movement of people and goods, impacting the economy (Power and Abbott, 2019)
The following section presents the results of the numerical analysis of Newbury cutting using the methodology, material properties and boundary conditions described
The paper details a numerical modelling approach that allows fore cast of long-term deterioration in the performance of cut slopes formed in high plasticity clays subject to seasonal weather sequences
Summary
Uncontrolled deformations of cut slopes along transport infrastruc ture routes cause reduced service performance (Briggs et al, 2019) leading to delays and disruption to the movement of people and goods, impacting the economy (Power and Abbott, 2019). Shear failures within high-plasticity clay cut slopes are primarily driven by two inter-related processes: dissipation of suppressed postexcavation pore water pressures with associated stress relief (Vaughan and Walbancke, 1973), and annual wetting and drying cycles. These cycles, due to the high plasticity of the material, cause shrink-swell cycles and result in the development of accumulated annual displace ments. This process is known as seasonal ratcheting (Take, 2003; Take and Bolton, 2004, 2011) Both of these mechanisms can generate stress changes that induce strain-softening behaviour, localised mobilisation of post-peak strength and stress re-distribution within high-plasticity clay cut slopes. This study uses numerical modelling to show how deterioration of slope performance, as defined using both slope deformations and factor of safety against shear failure respectively, is driven by a combination of post-excavation pore water
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