Journal of Geotechnical and Geoenvironmental Engineering

Conventional conceptual mechanisms for the hysteresis of soil-water retention are the ink-bottle pore neck and the solid–liquid–air-contact angle. However, these mechanisms fail to explain hydraulic hysteresis for matric suction greater than 10 MPa. A conceptual model, based on hydration-water retention, is provided in this paper. Two hydration mechanisms, namely, particle-surface hydration and crystalline cation hydration, are distinguished to explain hydraulic hysteresis. The former is mainly involved in water retention by anions of oxygen and/or hydroxyls on particle surface, leading to reversible water adsorption and desorption. By contrast, cation hydration is controlled by both exchangeable cations and the intermolecular forces such as Coulomb attraction and London dispersion, leading to hysteretic water-retention behavior. Based on this hysteresis model, the highest total suction for any soil can be identified. From the isotherms of various soils at 25°C, it is found that the highest total suction varies from 475 to 1,180 MPa. This value depends on soil types and can be uniquely related to the BET adsorption constant, which represents the energy needed to change soil water from gas phase to liquid phase.

Mechanical consequences of suffusion on the noncohesive soils with various initial fine contents under different initial effective confining pressures are presented in this paper. By using the modified triaxial permeameter, seepage tests and successive drained monotonic compression tests are performed. It is found that soil-drained strength decreases after suffusion and a temporary drop in stiffness at the initial stage of shearing with respect to the axial strain ranging from 0 to 1% is observed. The tests suggest that suffusion might create a distinct packing of soil grains, which might result from possible accumulation of fine grains at the spots where the constriction size—representing the size of pore channels in a soil—is smaller than that of fines. Those surviving fines after suffusion may function as reinforcement or jamming at the subsequent compression, resulting in a larger initial stiffness of the suffusional soils.

This study presents the results from physical modeling experiments on centrifuge-scale energy foundations in dry sand and unsaturated silt layers. These experiments were performed to characterize end restraint effects on soil-structure interaction for energy foundations in different soils and include tests on foundations with semifloating and end-bearing toe boundary conditions and free-expansion and restrained-expansion head boundary conditions. Two scale-model energy foundations having different lengths were constructed from reinforced concrete to simulate end-bearing and semifloating conditions in soil layers having the same thickness. The foundations include embedded thermocouples and strain gauges, which were calibrated under applied mechanical loads and nonisothermal conditions before testing. The variables measured during the experiments include axial strain and temperature distributions in the foundation, temperature, and volumetric water content measurements in the soil, vertical displacements of the foundation head and soil surface, and axial stress at the foundation head. These variables were used to calculate the distributions in thermal axial stress and thermal axial displacement, which are useful in evaluating soil-structure interaction mechanisms. The results confirm observations from full-scale energy foundations in the field for end-bearing foundations and provide new insight into the behavior of semifloating foundations. Heating of the semifloating foundations in compacted silt led to a clear increase in ultimate capacity, potentially due to changes in radial normal stress and thermally induced water flow, while heating of the semifloating foundations in dry sand led to a negligible change in ultimate capacity.

Consolidated-undrained tests on saturated compacted clays are used to provide strengths for design scenarios such as rapid drawdown, rapid flood, and earthquake loading of dams and levees. Geotechnical engineers have long recognized the influence of stress path, particularly principal stress reorientation from consolidation to failure, on the undrained strength of clay. While stress reorientation effects have been explored for undisturbed soils, their influence on the strength of compacted clay has not been examined in detail. The results of triaxial tests with stress reorientation and direct simple shear tests on a compacted clay are presented and compared to results from isotropically consolidated triaxial compression (ICU-TC) tests. Triaxial extension (TE) tests are shown to impose a stress system very dissimilar to the plane strain field conditions. TE tests also have problems with necking. The undrained strengths from TE tests were 25–55% lower than those measured in ICU-TC. In contrast, triaxial compression tests that incorporate a 90° stress reorientation resulted in only a slight reduction in strength from ICU-TC. Compacted clays were found to be difficult to test in conventional direct simple shear equipment because of their stiffness and initially unsaturated condition. Further improvement of DSS testing techniques is needed before these tests can be used for compacted clays.

Confining pressure significantly influences the mechanical behavior of soil. Understanding geotechnical problems related to low confining pressures requires precise modeling of soil behavior at low confining stress. Yet laboratory tests, including element tests under low confining pressure, are so far quite limited. In this note, a series of element tests under low confining pressure (5–20 kPa) were carried out to investigate the effect of low confining pressure on the mechanical behavior of loose and medium dense Toyoura sand. Element tests include undrained cyclic loading tests, as well as drained and undrained strain-controlled monotonic loading tests. The results show that loose sand under low confining pressure behaves much like dense sand under higher pressure. Model tests of slope failure flow under microgravity, carried out to supplement the element test results, also confirmed the previously mentioned mechanical behavior. The results obtained in this paper provide a more fundamental understanding of geotechnical problems related to low confining stress.