Mechanical behaviour of unsaturated aggregated soils

Particle aggregation is a commonly observed phenomenon in many types of soils, such as natural clays and agricultural soils. These soils contain porous aggregates, often separated by large, interaggregate pores. Two levels of intra- and interaggregate porosity are, therefore, present in these soils. Depending on the size and strength of the aggregates, aggregation may alter the water retention and mechanical behavior of the soil and make it different from that of a reconstituted soil of the same mineralogy. The present work is aimed at studying the mechanical behavior of unsaturated, aggregated soils with respect to soil structure effects. It involves theoretical developments, a multi-scale experimental study, and constitutive modeling. As a first step, the theory of multiphase mixtures was used to evaluate effective stress and to derive the coupled hydro-mechanical governing equations for a double porous soil. In this way, from the outset, the field variables and the required constitutive equations were identified. In the first experimental part, a new suction-controlled oedometer was developed for investigating the stress-strain response and water retention properties of the soil. The tests were carried out on reconstituted and aggregated samples of silty clays with an average aggregate size of about 2 mm. The results were interpreted in terms of a Bishop's type effective stress, suction, void ratio, and degree of saturation. From the tests carried out on the aggregated samples, an apparent preconsolidation stress was seen which depends not only on stress state and stress history, but also on the soil structure. The results of unsaturated tests revealed that the apparent effective preconsolidation stress increases with suction for both reconstituted and aggregated soils; however, the rate of increase is higher for aggregated soils. The results showed that the virgin compression curve of aggregated soils is on the right side of the normal consolidation line of the corresponding reconstituted soil. The two curves, however, tend to converge at higher values of stress when the aggregated structure is progressively removed by straining. It was observed that the degree of saturation in aggregated samples can increase during mechanical loading under constant suction because of the empty inter-aggregate pores being closed during the compression. In the following experimental part, soil structure and its evolution were tested using a combination of three methods: mercury intrusion porosimetry (MIP), environmental scanning electron microscopy (ESEM), and neutron tomography. Results of the MIP and ESEM tests revealed a homogeneous fabric with a uni-modal pore size distribution for the reconstituted soil, and a bi- or multimodal pore size distribution for the aggregated soil. Comparison of different observations revealed that the larger pores in the aggregated soil disappear as a result of mechanical loading or wetting. The non-destructive method of neutron tomography was used to assess the evolution of the aggregated soil structure during oedometric loading. An important observation was that the change in the volume fraction of macropores is mainly associated with irreversible deformations. Tomography results also suggest similarity of the water retention behavior for single aggregates and the reconstituted soil matrix. Based on the experimental results, a new constitutive framework was proposed for the extension of the elasto-plastic models of reconstituted soils to aggregated soils. Using this framework, a new mechanical constitutive model, called ACMEG-2S, was formulated within the critical state concept and the theory of hardening elasto-plasticity. A parameter called "degree of soil structure" was introduced to quantify the soil structure physically in terms of macroporosity. Evolution of this parameter, as a state parameter, was then linked to the plastic strains. The apparent effective preconsoliodatoin pressure in aggregated soils was introduced as an extension of the effective preconsolidation pressure of the reconstituted soil. The extension is controlled by two multiplicative functions in terms of suction and the degree of soil structure. These functions describe the gain in the apparent preconsolidation pressure due to the current fabric of the soil at the current suction. The model adopts the effective stress and suction as stress variables. It uses non-linear elasticity and two mechanisms of plasticity. In addition to the mechanical model, an improved water retention model was proposed which incorporates the combined effects of suction, volume change, and the evolving double porous fabric. The proposed mechanical model, coupled with the water retention model, unifies the combined effects of partial saturation, inter-particle bonding, and soil fabric. The model was then used to simulate the experiments carried out during the course of this study. Simulations showed that the model could successfully address the main features of the behavior of aggregated soils. Typically, it can reproduce the non-linearity of stress-stress response under virgin compression and the increase of degree of saturation during compression at constant suction. Finally, the model was examined for its capability in reproducing the behavior of structured bonded soils. With for the appropriate set of parameters, the model can reasonably reproduce the mechanical behavior of saturated bonded soils reported in the literature.

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