Numerical study of large deformation retrogressive landslides in sensitive clay triggered by toe erosion and earthquake

Wang, Chen (2020) Numerical study of large deformation retrogressive landslides in sensitive clay triggered by toe erosion and earthquake. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Landslides in sensitive clays represent a severe geohazard in eastern Canada and Scandinavia. Triggered by various factors, such as toe erosion, earthquake, and human activities, a sensitive clay landslide can affect a large area and cause damage to infrastructure. The evaluation of risk associated with sensitive clay landslides is an important but challenging task because the failure mechanisms are not well understood. Different types of landslide (e.g. flowslide, monolithic slide, and spread) occur through significantly different failure processes that affect both retrogression and run-out. Full-scale modeling of such large-scale landslides is not practically feasible. On the other hand, real-time monitoring of the failure in the field is not possible. Therefore, the characteristics of the landslides are generally evaluated by comparing post-slide field investigations with available information on the site before the landslide. Numerical modeling could be an alternative tool to obtain further insights into the failure mechanisms. The failure occurs by progressive formation of shear bands where extremely large plastic shear strain generates, and the failed soil displaces over a large distance. Consequently, the methods commonly used for slope stability analysis, such as limit equilibrium (LE) methods and Lagrangian-based finite element (FE) methods, cannot be used to model the whole process of a sensitive clay landslide. The main objective of the present study is to analyze the factors affecting the failure pattern and extent of sensitive clay landslides triggered by toe erosion and seismic loading. A large deformation finite element (LDFE) method based on Eulerian approach is used to simulate the triggering of the landslide, subsequent failure of soil blocks and run-out of the debris. The landslide generally occurs rapidly in a matter of few minutes; therefore, the simulation is performed for the undrained condition. The strain-softening behavior of sensitive clay is defined as a function of plastic shear displacement that reduces the undrained shear strength to a very low value at a large strain. A strain-rate dependent undrained shear strength model is used, which can model the behavior of soil and remolded clay that flows at a high speed as a fluid-like material. The formation of a slope generally occurs due to the removal of the materials in drained condition. Moreover, groundwater seepage might dominate the failure of a slope. Numerical simulation techniques for the Eulerian based LDFE method are developed to simulate in-situ effective stresses, which can be used for the cases of widely varying earth pressure coefficient at rest, even greater than unity. Based on the thermal-hydraulic analogy, a numerical modeling technique is developed for seepage analysis. The above-mentioned methods can successfully simulate the initial stress condition in the soil that affects the failure mechanisms significantly. Many failures of sensitive clay slope are initiated by toe erosion. Conducting LDFE simulations, the potential conditions required for a flowslide and a spread are identified. The type and extent (retrogression and run-out) of a landslide depend on a combination of several factors related to geometry and soil properties. A single parameter, such as stability number, remolded shear strength, liquidity index or remolded energy, may not always be suitable to categorize failure type. Increasing lateral earth pressure coefficient at-rest shows a trend of occurring spreads, while a low remolded shear strength and favorable conditions for rapid displacement of debris result in flowslides. The comparison of LDFE simulations and post-slide investigations of the 2010 Saint-Jude landslide show that the present numerical simulations can explain several features of the landslide, including the effects of seepage and an opposite riverbank on progressive failure. Finally, pseudostatic and dynamic analyses are performed using the developed LDFE method to study the progressive formation of failure planes in clay slopes subjected to earthquake loading. The LDFE modeling in Eulerian approach can simulate the large displacement of the failed soil blocks, considering the reduction of shear strength due to strain-softening.

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