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 neutro