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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.

Abstract. 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 states.Key words: structured soil, fabric, suction, pore size distribution. List of notation for mathematical equations RThe entrance pore radius T s Surface tension of the liquid α Contact angle of fluid interface to solidThe drained pore radius at that value of suction ψ Total moving volume fraction from macropores to micropores C 2 , C 3 Proportionality coefficients

Aggregated soils are structured systems susceptible to non-uniform flow. The hydraulic properties depend on the aggregate fabric and the way the aggregates are assembled. We examined the hydraulic behavior of an aggregate packing. We focused on conditions when water mostly flows through the aggregates, leaving the inter-aggregate pore space air-filled. The aggregates were packed in 3 mm thick slabs forming a quasi two-dimensional bedding. The larger aggregates were wetted with water and embedded in smaller aggregates equilibrated at a lower water content. The water exchange between wet and drier aggregates was monitored by neutron radiography. The three-dimensional arrangement of the aggregates was reconstructed by neutron tomography. The water flow turned out to be controlled by the contacts between aggregates, bottle-necks that slow down the flow. The bottle-neck effect is due to the narrow flow cross section of the contacts. The water exchange was simulated by considering the contact area between aggregates as the key parameter. In order to match the observed water flow, the contact area must be reduced by one to two orders of magnitude relative to that obtained from image analysis. The narrowness of the contacts is due to air-filled voids within the contacts. A. Carminati et al.

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.