From saturated to unsaturated conditions and vice versa
Representing transitions between saturated and unsaturated conditions, during drying, wetting and loading paths, is a necessary step for a consistent unification between saturated and unsaturated soil mechanics. Transitions from saturated to unsaturated conditions during drying will occur at a nonzero air-entry value of suction, whereas transitions from unsaturated to saturated conditions during wetting or loading will occur at a lower nonzero air-exclusion value of suction. Air-entry and air-exclusion values of suction for a given soil will differ (representing hysteresis in the retention behaviour) and both are affected by changes in the dry density of the soil or by the occurrence of plastic volumetric strains. The paper demonstrates, through model simulations and comparison with experimental data from the literature (covering drying, wetting and loading tests), that the Glasgow Coupled Model (GCM), a coupled elasto-plastic constitutive model covering both mechanical and retention behaviour, represents transitions between unsaturated and saturated behaviour in a consistent fashion. Key aspects of the GCM are the use of Bishop’s stress tensor for mechanical behaviour, the additional influence of degree of saturation on mechanical yielding, inclusion of hysteresis in the retention behaviour, and the role of plastic volumetric strains (and not total volumetric strains) in the description of the water retention response. The success of the GCM in representing consistently transitions between saturated and unsaturated conditions, together with subsequent mechanical and retention responses, demonstrates the potential of this coupled constitutive model for numerical modelling of boundary value problems involving saturated and unsaturated conditions.
Hydromechanical behavior of unsaturated soil with different initial densities over a wide suction range
Many civil engineering projects are related to hydromechanical behavior of unsaturated soils over a wide suction range, which was investigated by imposing suctions on clayey silt specimens using the axis translation technique and the vapor equilibrium technique with saturated salt solution in this paper. Firstly, the effect of void ratio or density on the water retention behavior was discussed based on the measured data. The test data indicate that shear tests under constant water content can be considered as those under constant suction in high suction range. Secondly, the mercury intrusion porosimetry tests were also performed to observe the pore-size distribution of compacted specimens with different void ratios. The experimental results show that the inter-aggregate pores of compacted specimens decrease with increasing the initial void ratio, but the intra-aggregate pores remain unaffected, which can explain the water retention behavior of compacted specimens with different initial void ratios over a wide suction range. Finally, for triaxial shear tests, the shear strength increases with increasing the suction until a maximum strength is reached at a specific suction value. When the suction is higher than the specific suction value, the tendency of strength change depends on the density in high suction range.
Dynamic Effect in the Capillary Pressure–Saturation Relationship and its Impacts on Unsaturated Flow
Capillary pressure plays a central role in the description of water flow in unsaturated soils. While capillarity is ubiquitous in unsaturated analyses, the theoretical basis and practical implications of capillarity in soils remain poorly understood. In most traditional treatments of capillary pressure, it is defined as the difference between pressures of phases, in this case air and water, and is assumed to be a function of saturation. Recent theories have indicated that capillary pressure should be given a more general thermodynamic definition, and its functional dependence should be generalized to include dynamic effects. Experimental evidence has slowly accumulated in the past decades to support a more general description of capillary pressure that includes dynamic effects. A review of these experiments shows that the coefficient arising in the theoretical analysis can be estimated from the reported data. The calculated values range from 104 to 107 kg (m s)−1 In addition, recently developed pore‐scale models that simulate interface dynamics within a network of pores can also be used to estimate the appropriate dynamic coefficients. Analyses of experiments reported in the literature, and of simulations based on pore‐scale models, indicate a range of dynamic coefficients that spans about three orders of magnitude. To examine whether these coefficients have any practical effects on larger‐scale problems, continuum‐scale simulators may be constructed in which the dynamic effects are included. These simulators may then be run to determine the range of coefficients for which discernable effects occur. Results from such simulations indicate that measured values of dynamic coefficients are within one order of magnitude of those values that produce significant effects in field simulations. This indicates that dynamic effects may be important for some field situations, and numerical simulators for unsaturated flow should generally include the additional term(s) associated with dynamic capillary pressure.