A bounding surface hysteretic water retention model for deformable soils

Abstract

The paper presents a model that takes into account the influence of void ratio and hydraulic hysteresis on soil water retention. The model is based on the definition of two bounding surfaces, i.e. a main drying surface and a main wetting surface, that delimit the region of admissible soil states in the space of degree of saturation, suction and void ratio. An auxiliary variable, named scaled suction, is introduced to combine the effects of suction and void ratio into a single quantity, so that the main surfaces are recast as curves in the plane of degree of saturation and scaled suction. An increase of scaled suction corresponds to drying of the soil while a decrease of scaled suction corresponds to wetting of the soil; hence the drying/wetting behaviour is governed by changes of both suction and void ratio. The model assumes that the derivative of degree of saturation with respect to scaled suction depends on the distance of the current soil state from the main curves, which ensures a smooth transition of the drying and wetting paths towards their respective main curves. One advantage of the proposed model is that all wetting and drying paths can be integrated in a closed form and are described by means of two explicit equations (one for drying and one for wetting) with different constants of integration. The model requires seven parameters whose values can be obtained from as little as a single drying-wetting test. Predictions are validated against two different data sets published in the literature, showing a good ability of the model to capture the behaviour observed during laboratory tests on fine grained soils

Effect of in situ and laboratory compaction on the retention behaviour of a clayey soil

“…The uniqueness of this relationship between water ratio and suction at high suction (and low saturation) has already been experimentally observed in compacted silts and clays with different degrees of activity [4][5][6]. The above observations are also in agreement with the retention model proposed by Gallipoli (2012) [7] and Gallipoli et al (2015) [8], who suggested that any mechanically-induced change of void ratio will only affect the largest pores while leaving the smaller ones almost unchanged. At high suction (and low saturation), water withdraws in the smallest pores so that any mechanically-induced change of void ratio is unable to cause a variation of water ratio.…”

A coupled hydromechanical bounding surface model predicting the hysteretic behaviour of unsaturated soils

“…The hydraulic law, detailed in Gallipoli et al (2015), describes the hysteretic variation of degree of saturation, caused by changes of suction, (where and are the pore air and water pressures, respectively) and void ratio, . In particular, the variation of the degree of saturation is related to a single independent variable named the "scaled suction" ̅ (Gallipoli et al, 2015), which is equal to the suction scaled by a power function of void ratio :…”

“…The parameter coincides with the asymptotic slope of the two main curves when the scaled suction tends to infinity in the -̅ plane. This slope must be identical for both curves to ensure the continuity of the stress path at the reversal point of drying-wetting cycles as shown by Gallipoli et al (2015). The physical meaning of all model parameters, together with their range of variation, is discussed in Gallipoli et al (2015).…”

“…This paper presents a hydromechanical model for unsaturated soils subjected to isotropic stress states that is formulated within the broad framework of bounding surface plasticity. The model combines the hydraulic law by Gallipoli et al (2015) with the mechanical law by Gallipoli and Bruno (2017) to predict the coupled variation of void ratio and degree of saturation due to changes of stress and suction. The hydraulic law predicts the hysteretic variation of degree of saturation as a function of a single independent variable, named the "scaled suction", which is equal to the suction scaled by a power function of void ratio.…”

Numerical Simulation of the Shear Behavior of Rock Joints Filled with Unsaturated Soil

“…2). In addition, the soil-water retention curve (SWRC) and permeability of the infill will change during shear with its porosity (Mašín 2010;Gallipoli et al 2015;Hashem 2016;Carrier and Beckman 1984). All these factors may cause large variation of suction and saturation values in the shear process under hydraulic-mechanical coupling.…”