Numerical Investigation for Slope Stability of Expansive Soils and Large Strain Consolidation of Soft Soils
Creators: name: Qi, Shunchao
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Unsaturated Soils
Slope Stability
Large Strain Consolidation
Expansive Soils
UNSATCON Software
Numerical Model
Soft Soils
Desaturation/Dewatering
Multi-layer Deposition
Tailings

description: Several geotechnical processes can only be reliably interpreted by taking account of the soil-atmosphere interactions. This thesis investigates two geotechnical problems involving soil-atmosphere interactions that drive water flow through the soil skeleton in two opposite directions; Problem 1: slope failure in expansive soils induced by water infiltration, Problem 2: large strain consolidation of soft soils induced by water evaporation. Both problems are of practical interest for safe and economical design of various geotechnical infrastructures. Two major geotechnical activities in the world; namely, the construction of water transfer canal in expansive soil area in China and the deposition of oil sands and hard rock tailings in Canada can be cited as classic examples of Problems 1 and 2, respectively. In such problems, substantial zones of the domain may switch between an unsaturated and saturated condition. Therefore, rational analysis requires simultaneous modelling of both unsaturated and saturated soil behaviour. The first goal of this thesis is to investigate the influence of swelling (the most characteristic behaviour of expansive soils) on slope stability using numerical methods. Swelling of expansive soils contributes to slope instability during rainfall because of two key reasons (i) soil swelling affects the flow process that actually induces swelling, (i.e. a typical coupling phenomenon), and (ii) swelling-induced stress redistribution and displacement development. In this thesis, the first effect is studied by a coupled (mechanical-hydraulic) numerical analysis of the response of a slope to rainfall using commercial software (GeoSlope). The second effect, the swelling-induced stress redistribution and displacement development after wetting, is tracked using a newly developed numerical program. In the program strain softening behaviour is introduced into the elasto-plastic Mohr-Coulomb Model for modelling unsaturated soil. A novel stress (net stress and suction)-dependent model for moduli of elasticity, combined with the predictive model for shear strength based on Soil Water Retention Behaviour are incorporated into the numerical program to achieve a smooth transition between saturated and unsaturated states. The results show that soil swelling can decrease the factor of safety by accelerating the wetting front depth due to hydro-mechanical coupling, while changes of sliding mass geometry has a negligible influence. The change of stress regime associated with soil swelling is significant to induce plastic strain softening (swelling-induced softening) and contribute to the slope failures. The second goal of thesis is to develop a novel computer program for simulation of large strain consolidation of soft soil under both self-weight and evaporation conditions. This program is both theoretically sound and practically applicable. Several basic/advanced constitutive models for unsaturated soils, including State Surface Model (SSM), Barcelona Basic model (BBM), Glasgow Coupled model (GCM) and bounding surface water retention model, are innovatively implemented into a piece-wise linear framework solved using finite difference technique. The developed program is referred to as UNSATCON-(ML), which has been tested using (a) existing analytical/numerical solutions and (b) various laboratory and field studies for single-layer and multiple-layer deposition of hard rock and oil sands tailings. Features of UNSATCON-(ML) that are improvements over existing models typically used to analyze consolidation-desiccation in soft soils include: (i) coupling of soil large deformation with true unsaturated water flow; (ii) correct reproduction of the shrinkage behaviour of soil under evaporation-induced desiccation; (iii) smooth transition between saturated and unsaturated states despite that some selected models are established using two independent stress variables, (iv) ensuring strictly mass conservation of water, and (v) simulation of irrecoverable volume change and hydraulic hysteresis to properly analyze multilayer tailings deposition. A number of hypothetical field case analyses are carried out using UNSATCON-ML, illustrating its applicability to industry.