Journal of Geotechnical and Geoenvironmental Engineering

Thermoactive geostructures (such as energy piles) are used for heating and cooling of buildings, which generate daily temperature changes and cycles in the geostructure and surrounding soil due to the intermittent operation of ground-source heat pumps. For energy piles, daily temperature changes and cycles result in cyclic displacement (expansion and contraction) in both the axial and radial directions of the pile and alter soil properties. In this study, a fully automated modified thermal borehole shear test (modified-TBST) device was utilized to perform tests in normally consolidated clayey soil to investigate the effects of temperature cycles (TC) and radial expansion/contraction displacement cycles (RDC) on the soil-energy pile interaction. In addition to directly measuring the shear stress–vertical displacement curves (t-z curves), the soil temperature at different locations and pore pressure were monitored. The fully automated modified-TBST device uses two concrete plates to simulate the pile surface with temperature and expansion/contraction controls. The tests were conducted with temperature changes ($\mathrm{\Delta }T$) at the soil–concrete interface of $-18$, 0, and $+20°\mathrm{C}$. The radial expansion and contraction displacements ($\mathrm{\Delta }D$) were $+120$ and $-120\mathrm{\mu m}$, respectively. Tests were conducted at different interface horizontal normal stresses and numbers of cycles. This paper focuses on summarizing the results of 16 tests: 6 conducted with temperature changes and cycles, 8 with radial displacement change and cycles only, and 4 with combined temperature and displacement cycles. When the soil–concrete interface was subjected to combined radial displacement and temperature cycles, the interface shear strength experienced significant changes. Therefore, it was concluded that the intermittent operation of heat pumps connected to energy piles installed in normally consolidated clayey soils has a significant effect on shaft resistance.

This paper presents a numerical investigation of the deformation and failure behavior of geosynthetic reinforced soil (GRS) bridge abutments. The backfill soil was characterized using a nonlinear elastoplastic constitutive model that incorporates a hyperbolic stress–strain relationship with strain-softening behavior and the Mohr–Coulomb failure criterion. The geogrid reinforcement was characterized using a hyperbolic load–strain–time model. The abutments were numerically constructed in stages, including soil compaction effects, and then monotonically loaded in stages to failure. Simulation results indicate that a nonlinear reinforcement model is needed to characterize deformation behavior for high applied stress conditions. A parametric study was conducted to investigate the effects of reinforcement, backfill soil, and abutment geometry on abutment deformation and failure behavior. Results indicate that reinforcement vertical spacing, reinforcement stiffness, backfill soil friction angle, and lower GRS wall height are the most significant parameters. The shape of the failure surface is controlled primarily by abutment geometry and can be approximated as bilinear.

Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in soils surrounding geothermal heat exchangers may play important roles in both the heat injection and retention processes that are not considered in established design models for these systems. To better understand these roles, this study incorporates recently-developed coupled thermohydraulic constitutive relationships for unsaturated soils into a coupled heat transfer and water flow model that considers time-dependent, nonequilibrium water phase change and enhanced vapor diffusion. After calibration of key parameters using a tank-scale heating test on compacted silt, the subsurface response during 90 days of heat injection from a geothermal heat exchanger followed by 90 days of ambient cooling was investigated. Significant decreases in degree of saturation and thermal conductivity of the ground surrounding the heat exchanger were observed during the heat injection period that were not recovered during the cooling period. This effect can lead to a greater amount of heat retained in the ground beyond that estimated in conduction-based design models.

Large-scale triaxial compression and creep experiments were conducted with a saturated limestone rockfill material. The relationships between dilatancy ratio and stress ratio during loading and creeping were analyzed. It was found that the dilatancy ratio during both loading and creeping decreased nonlinearly when the stress ratio increased. In addition, the positive dilatancy ratio was considerably higher in creep than in vertical loading under the same stress state. Experimental data also showed that creep strains evolve linearly with the logarithm of time after a certain period, and both the final amounts of creep strains at the end of experiments and their rates are influenced by the stress states. A higher confining pressure and higher deviatoric stress lead to larger creep strains and higher strain rates. Empirical equations relating the final amounts of creep strains and their rates to the stress states are suggested.

A method to estimate the equivalent diffusion coefficient and partition coefficient of a coextruded ethylene-vinyl alcohol (EVOH) geomembrane (GM) is presented. The method is based on the weighted harmonic mean of diffusion coefficients of each layer and the partition coefficient of the outer layers. The method was evaluated using experimental data obtained with five volatile organic compounds (VOCs) typically found in landfill leachate [methylene chloride (MC), methyl tertiary butyl ether (MTBE), trichloroethylene (TCE), toluene (TOL), and chlorobenzene (CB)] and a coextruded GM composed of outer layers of low-density polyethylene (LLDPE) or high-density polyethylene (HDPE), an adhesive tie layer (maleic anhydride), and a core of EVOH. Equivalent diffusion coefficients computed with the recommended equation were found to be no different statistically from equivalent diffusion coefficients measured using a modified double-compartment apparatus. VOC transport through a composite liner using a coextruded EVOH GM was predicted using a finite-difference model with the equivalent transport parameters as input. Good agreement was obtained between predictions from the model and measured concentrations from a column test simulating a composite liner.

The potential for enhanced strength or stiffness arising from the microstructure of natural silty soils compared with their reconstituted state is examined using the framework for clays, to assess the extent to which it can be used for silts. A detailed study performed on two clayey silts of low and high plasticity, respectively, in their intact and reconstituted state, is used to illustrate the different response of the silts compared with clays in terms of compressibility and strength, e.g., multiplicity and shape of the normal compression lines and strong dilative tendencies during undrained and drained shearing. This paper shows that the response of the reconstituted silts can be used as a reference to identify the influence of the microstructure on the compressibility and strength of the intact silts. The results are found to give clear signs of structure in both compression and shearing only for the low-plasticity silt. Stiffness measurements corroborate these findings.

Geotechnical engineering preparation is essential for successful lunar exploration due to the distinct differences in the environment between the Earth and the Moon. In particular, the bearing capacity of the lunar surface is an important index related to the landing of lunar probes. This study suggests two experimental methods using a centrifuge and lunar simulant for simulating the reduced-gravity environment of the Moon and investigates the influence of low gravity on the bearing capacity of shallow footings. First, the effects of footing size and gravity on the bearing capacity are investigated through a series of loading tests performed under various conditions of footing size and centrifugal acceleration. Then a predictive equation for the bearing capacity of shallow footings on the lunar surface is suggested. Finally, centrifuge model tests for directly simulating lunar prototype footings are performed for comparison with the suggested bearing capacity equations of shallow footings on the lunar surface, and the reliability of simulating the gravity of the Moon is verified.

Direct use of naturally occurring microbes for soil improvement has recently gained attention due to their ubiquitous and versatile characteristics in subsurface soil. Microbes produce soft and sticky extracellular polymeric substances (or biopolymers) that are known to alter the hydrological characteristics of soils; however, the mechanisms and extent of such soft biopolymers in altering soil erosion resistance remain scarcely explored. This study explored the role of microbial biopolymers in soil erosion resistance. The surface erosion resistance of sandy soils was evaluated by using a hybrid erosion function apparatus, in which the model bacteria Leuconostoc mesenteroides were stimulated to produce an insoluble biopolymer. The results revealed that the microbial biopolymer formation increased the critical shear stress and surface erosion resistance, which the researchers attributed to the increased cohesion by grain-coating biopolymer slimes and the reduced seepage flows due to pore clogging. This study provides baseline but promising results on how microbially grown biopolymers can be used to improve soil erosion resistance.