Abstract This study investigates linkages between volume change, pore fluid drainage, shear wave velocity, and temperature of soft clays using a thermal triaxial cell equipped with bender elements, a measurement approach that has not been explored widely in past thermo-mechanical studies. Two kaolinite specimens were consolidated mechanically to a normally consolidated state and then subjected to drained and undrained heating-cooling cycles, respectively. After cooling, the specimens were subjected to further mechanical consolidation to evaluate changes in apparent preconsolidation stress. Both specimens showed net contractive thermal strains after a heating-cooling cycle and overconsolidated behavior during mechanical compression immediately after cooling. The shear wave velocity increased during drained heating, but negligible changes were observed during drained cooling, indicating permanent hardening because of thermal consolidation during the heating-cooling cycle. The shear wave velocity decreased during undrained heating because of a reduction in effective stress associated with thermal pressurization of the pore fluid but subsequently increased when drainage was permitted at elevated temperature. The shear wave velocity increased slightly during undrained cooling but decreased when drainage was permitted at room temperature. Net increases in small-strain shear modulus of 17 and 11 % after heating-cooling cycles under drained and undrained (with drainage after reaching stable temperatures) conditions, respectively, provide further evidence to the potential of thermal soil improvement of normally consolidated clays. Transient changes in shear modulus also highlight the importance of considering drainage conditions and corresponding changes in effective stress state during heating-cooling cycles.
This study evaluated the strength properties of compacted lateritic soils reinforced with polypropylene (PP) waste strips cut from recycled plastic packing with the goal of promoting sustainability through using local materials for engineering work and reusing waste materials as low-cost reinforcements. Waste PP strips with widths of 15 mm and different lengths were uniformly mixed with clayey sand (SC) and clay (CL) soils with the goal of using these materials as low-cost fiber reinforcements. The impact of different PP strip contents (0.25% to 2.0%) and lengths (10, 15, 20, and 30 mm) on the unconfined compressive strength (UCS) of the soils revealed an optimum combination of PP strip content and length. Statistical analysis showed that PP strip content has a greater effect than the PP strip length on the UCS for both soils. Results led to the definition of an empirical equation to estimate the UCS of strip-reinforced soils. The results from direct shear tests indicate that the SC soil showed an increase in both apparent cohesion and friction angle after reinforcement, while the CL soil only showed an increase in friction angle after reinforcement. California bearing ratio (CBR) tests indicate that the SC soil experienced a 70% increase in CBR after reinforcement, while the CBR of the CL soil was not affected by strip inclusion.
ABSTRACT: This paper provides theoretical background, laboratory data and full-scale measurements useful in understanding the interaction between soils and geosynthetics under unsaturated conditions. It also includes an evaluation of the current state of knowledge regarding the hydraulic properties of porous geosynthetics under unsaturated conditions relevant for geosynthetic capillary barrier design. These properties include the water retention curve and the hydraulic conductivity function. In addition, the mechanisms involved in the development of capillary barriers are evaluated to explain the storage of water at the interface between materials with contrasting hydraulic conductivity (e.g. a fine-grained soil and a nonwoven geotextile). Finally, specific applications are presented to illustrate new opportunities and applications that may result from a better understanding of the unsaturated hydraulic properties of geosynthetics. Experimental data are provided illustrating that geosynthetic capillary barriers are superior to soil-only capillary barriers. Based on this observation, it is emphasized that no capillary barrier should be designed without consideration of the enhanced performance offered by the inclusion of nonwoven geotextiles under the fine-grained soil component of the cover.
Abstract This paper presents an experimental methodology for using multistage, drained triaxial tests on compacted soils under unsaturated conditions to estimate soil-specific relationships between mean effective stress and matric suction. Tests were performed by applying a matric suction to a soil specimen in a triaxial cell using the axis translation technique with back-pressure, then shearing the specimen under drained conditions until reaching stress-path tangency. The specimen was then unloaded, a new suction was applied, and the shearing process was repeated. The points of maximum principal stress difference for the unsaturated specimen were plotted versus mean effective stress, defined using the degree of saturation as the effective stress parameter, and they were found to correspond well with the critical state line defined from triaxial tests on saturated specimens. The suction stress for the compacted soil tested in this study was observed to increase nonlinearly with suction, tending toward a constant value with increasing suction. Although there are potential changes in soil structure in the specimen during loading, unloading, and reloading, the results indicate that the multistage testing method may be useful for estimating soil-specific effective stress parameters for compacted soils in unsaturated conditions. Furthermore, the fact that differences in the soil-water retention curve of soil specimens subjected to different net confining pressures were observed for the soil tested in this study emphasizes the importance of using soil-specific tests to validate predictive relationships between suction stress and matric suction.
Abstract This paper focuses on the impact of elevated temperatures on the adsorptive and capillarity water retention mechanisms of unsaturated soils under constrained (constant volume) conditions. This topic is critical for simulating the thermo-hydraulic behavior of soils in hydrogeological or geotechnical applications, including climate change effects on near surface soils, energy piles or soil borehole thermal energy storage systems in unsaturated soil layers, and buffers for geological nuclear waste repositories. A nonisothermal soil water retention curve (SWRC) that separately considers the temperature-dependency of the key parameters governing adsorptive and capillarity water retention mechanisms and soil physical parameters (e.g., surface tension, contact angle, adsorption capacity, cation exchange capacity, mean cavitation suction, air entry value and equilibrium film thickness) was developed to provide insights into the impact of temperature on water retention over the full suction range. The nonisothermal SWRC was validated using experimental data on high plasticity clays, with a good prediction of temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils.
ABSTRACT: The water retention curve (WRC) of unsaturated nonwoven geotextiles is necessary for the prediction of transient water flow in unsaturated soil–geotextile systems. A test method is introduced in this study to measure the WRC of deformable, unsaturated nonwoven geotextiles. This test method combines the axis-translation technique with water flow control using a flow pump. Specifically, a nonwoven geotextile specimen is placed within a flexible wall permeameter atop a cellulose membrane having high air entry and low impedance, and the flow pump is used to impose upward or downward water flow across the membrane. A control loop is used to reach stable suction values at the boundary of the suction in order to ensure that points on the WRC are defined for equilibrium flow conditions. A piston is used to apply a vertical load on the nonwoven geotextile specimen in order to replicate field loading conditions, and was also useful for measurement of changes in specimen volume. The combination of flow control, suction stabilization, and height measurement allow this new approach to effectively consider the impact of normal stress and matric suction on the porosity of the geotextile during measurement of the WRC. Experimental results for typical tests indicate that this approach yields drying and rewetting WRC curves for a geotextile specimen consistent with those in the technical literature.
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