Effects of repeated hydraulic loads on microstructure and hydraulic behaviour of a compacted clayey silt

Abstract

Soils used in earth construction projects are mostly unsaturated, and they undergo frequent drying–wetting cycles (repeated hydraulic loads) due to changes in climatic conditions or variations of the ground water level, particularly at shallow depths. After compaction, changes in water content can significantly influence the hydromechanical response of the construction material, which therefore must be assessed for repeated hydraulic loads. This research investigates the effect of such loads on the microstructure and hydraulic behaviour of a silty soil, typically used in the construction of embankments and dykes, with the aim of providing a better understanding of the consequences of drying–wetting cycles on the response of the material over time. Experimental tests were performed to study the impact of drying–wetting cycles on the water retention, hydraulic conductivity, and fabric of compacted specimens. Fabric changes are documented to take place even without significant volumetric strains, promoting an irreversible increase in the hydraulic conductivity and a reduction in the capacity to retain water compared to the as-compacted soil. The fabric changes are interpreted and quantified by means of a hydromechanical model, which accounts for the evolving pore-size distribution at different structural levels. The proposed model reproduces quite well the microstructural observations, together with the evolution of the water retention behaviour and hydraulic conductivity.

A microstructure-based elastoplastic model to describe the behaviour of a compacted clayey silt in isotropic and triaxial compression

“…A larger macro-porosity emerges, justifying the increase in permeability and the decrease in water storage capacity. This evolution of the soil fabric and of the hydraulic behaviour during drying-wetting cycles can be reproduced adequately with double porosity hydromechanical volumetric models such as the one proposed by Azizi et al (2019). However, to the authors' knowledge, limited attention has been paid to the influence of cyclic wetting and drying on the triaxial compression behaviour of unsaturated compacted soils, although it seems reasonable to infer that the fabric changes responsible for the changes in the hydraulic response will impact also on the mechanical behaviour.…”

“…Full details on sample preparation and on the cyclic hydraulic history simulating drying-wetting in the field are given in Azizi et al (2019). In summary, drying was imposed by exposing the samples to the laboratory environment having a controlled temperature of 21 and relative humidity of 38.5%, corresponding to a suction s = 128.8 MPa, which ℃ brought the water content to a minimum of w  0.4%.…”

“…The evolution of the microstructure of a soil used in the construction of river embankments is analysed experimentally over different hydromechanical paths, both immediately after compaction and after exposure to drying-wetting cycles. Stemming from the premises on the water retention behaviour in Azizi et al (2019), the comparison between experimental data on ascompacted and dried-wetted samples is exploited to describe the mechanical response of the soil, within a doubleporosity elastic-plastic framework. The proposed model is used to simulate the results of drying, isotropic compression and triaxial compression (shear) at constant water content tests on both as-compacted and dried-wetted samples, with the purpose of showing and start quantifying the influence of natural drying-wetting cycles on the lifetime response of embankments made of similar compacted clayey silts.…”

“…According to Blight (1964), the pore pressure distribution within a sample sheared under unsaturated conditions is uniform when the time to failure t f is equal or greater than the time for consolidation t 100 . On the basis of the measured hydraulic conductivity (Azizi et al 2019) and sample compressibility, t 100 is expected to be of the order of a few hours or less. Assuming that failure occurs when  a = 20%, the axial strain rate imposed during triaxial compression was 0.25% / hour, which implies t f = 80 hours.…”

Water retention and shrinkage curves of weathered pyroclastic soil

“…). This observation can be explained by the fact that the soil void ratio tends to decrease with increasing suction as a consequence of the ssociated effective stress increase in the soil [9,10,12,16,17]. However, this curve is incomplete because the void ratio decreases up to a threshold correspondent to the shrinkage limit characteristic of each soil, which cannot be observed in Figure 2 there is coherency among data sets (SC and the initial conditions of the specimens tested represented in Figure 2, align with the he specimens used to obtain WRC presented a higher water content and porosity subjected to a lower matric suction, hence a…”