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

New basis for the constitutive modelling of aggregated soils

Natural and compacted soils are usually characterized by aggregation of particles. The mechanical behaviour of these materials depends on soil structure. The oedometric compression tests performed on aggregated samples presented here showed that these materials exhibit a yield limit depending not only on stress history and stress state but also on soil structure. Evidence is provided using the neutron tomography technique. These results revealed that soil structure modification occurs together with plastic deformations. The experimental results are used to propose a new state parameter to quantify the soil structure. Based on pore-scale experimental observations, an evolution law for this parameter is proposed as a function of associated plastic strains. Considering both soil fabric and inter-particle bonding effects, a new yield limit depending on stress state, stress history and soil structure is introduced for the aggregated soils. Accordingly, a new constitutive framework consistent with strain hardening plasticity is proposed to consider soil structure effects in the modelling of aggregated soil

A coupled model of mechanical behaviour and water retention for unsaturated soils with double porosity

Many natural soils and engineering geomaterials, such as aggregated soils and compacted clay pallets, exhibit two levels of porosity corresponding to the inter- and intraaggregate pores within their hierarchical structure. Mechanical behavior of these materials, in particular when unsaturated, is an issue of added complexity which should be described an appropriate constitutive framework. A coupled water retention–mechanical constitutive model for unsaturated soils with double porosity is presented here. Based on the multi-scale experimental results, the model incorporates the inter-particle bonding, fabric and partial saturation effects in a single framework. It is formulated within the framework of hardening elasto-plasticity and is based on the critical state concept. The mechanical model is coupled with the water retention law which itself takes into account the two levels of porosity. The coupling is made through the expression of the effective stress and the evolution of the preconsolidation pressure with suction. On the other hand, the mechanical model at the macro-scale is also coupled with the pore-scale behavior of the materials through an internal variable which accounts for the evolution of the soil structure. The model is used for numerical simulation of the behavior of aggregated and bonded soils. Comparison of numerical simulations and the experimental results show that the model can successfully address the main features in the behavior of unsaturated soils with double porosity

Suction Induced Effects on the Fabric of a Structured Soil

This paper presents the mathematical modelling of the modification of the pore space geometry of a structured soil subjected to suction increase. Structured soil concepts are first introduced considering different fabric units, such as aggregates and fissures. The numerical modelling of the structural evolution is based on experimental test results in which the evolution of the structure of the samples subjected to different suctions is determined using the mercury intrusion porosimetry technique. From this information, the macro and micropore volume evolutions are determined. The results show that drying produces a reduction in the soil total porosity which mainly corresponds to a reduction of the macropore volume. Associated with this phenomenon, an increase in micropore volume is also observed. The proposed model divides pore size distribution into three pore classes (micropores, macropores and non-affected areas). Using the concept of a suction-influenced domain, the proposed model is able to reproduce the main observed fabric evolution between the saturated and dry state

New basis for the constitutive modelling of aggregated soils

Natural and compacted soils are usually characterized by aggregation of particles. The mechanical behaviour of these materials depends on soil structure. The oedometric compression tests performed on aggregated samples presented here showed that these materials exhibit a yield limit depending not only on stress history and stress state but also on soil structure. Evidence is provided using the neutron tomography technique. These results revealed that soil structure modification occurs together with plastic deformations. The experimental results are used to propose a new state parameter to quantify the soil structure. Based on pore-scale experimental observations, an evolution law for this parameter is proposed as a function of associated plastic strains. Considering both soil fabric and inter-particle bonding effects, a new yield limit depending on stress state, stress history and soil structure is introduced for the aggregated soils. Accordingly, a new constitutive framework consistent with strain hardening plasticity is proposed to consider soil structure effects in the modelling of aggregated soils

Structural characterization of unsaturated aggregated soil

Despite the recent experimental studies of soil structure, a comprehensive understanding of the macroscopic response of a soil in relation to its structure has not yet been achieved. This lack of understanding reveals the need for further assessments of soil structure and its evolution under loading. In this work, the structure of an aggregated soil under various conditions of saturation and mechanical loading is studied. We also compare the aggregated soil structure, which shows a double porous fabric, with that of the same soil when reconstituted. The experimental methods selected for this study are a combination of mercury intrusion porosimetry (MIP), environmental scanning electron microscopy (ESEM), and neutron computed tomography (CT). Using MIP and ESEM, we first examine the soil fabric at the intra-aggregate scale. Then, we quantify the structural evolution of the soil using neutron tomography and link it to the macroscopic response of the soil. Based on the experimental evidence, the main features of the soil structure and its evolution are outlined for unsaturated aggregated soil under different loading conditions

Constitutive modeling of unsaturated aggregated soils

A coupled water retention-mechanical constitutive model for unsaturated aggregated soils is presented here. Based on the multi-scale experimental results, the model incorporates the inter-particle bonding, fabric and partial saturation effects in a single framework. It is formulated within the framework of hardening elastoplasticity and is based on the critical state concept. Prior to model validation, we evaluate the model parameters and propose determination procedures for the main new parameters. Finally, the model is examined for its capability in simulating the experimental results of aggregated and bonded soils. Results of these simulations show that the model addresses the most features arising from the combined effects of soil structure and partial saturation