Previous studies of shear behaviour at different suctions and temperatures focused mainly on recompacted soils. Limited attention has been paid to intact soil. In this paper, the shear behaviour of intact and recompacted loess was studied using a suction- and temperature-controlled direct shear apparatus. A scanning electron microscope and mercury intrusion porosimeter were used to evaluate the microstructure of both intact and recompacted loess. For a given suction and temperature, intact specimens exhibit a higher shear stiffness and larger dilatancy than recompacted specimens. This is mainly because clay particles in intact specimens accumulate near grain contacts and stick to the grain surfaces, as revealed in scanning electron microscopy and mercury intrusion porosimetry tests. This type of clay particle distribution in intact specimens results in a more resistant structure and higher yield stress. At suctions of 0 and 200 kPa, the shear stiffness and dilatancy of recompacted specimens increase with temperature. The shear stiffness and maximum dilatancy increase by as much as 47% and 63% at suctions of 0 and 200 kPa, respectively. These increases in shear stiffness and dilatancy are attributed mainly to the continuous plastic volumetric contraction during heating, which induced strain-hardening effects. On the contrary, thermal effects on the stress–strain behaviour of intact specimens are negligible at zero suction. At a suction of 200 kPa, as soil temperature rises from 20 to 60 °C, the shear stiffness and maximum dilatancy of intact specimens decrease by 35% and 68%, respectively. It is evident that intact specimens and recompacted specimens experience opposite thermal effects.
Previous investigations of the volume change of soil with different fabric patterns have been mostly carried out at a constant temperature. To investigate the influence of the specimen preparation method on the volume change of saturated clay under cyclic thermal loads, reconstituted, intact and recompacted specimens were tested. Thermal axial strains of these specimens in a normally consolidated state were measured using a temperature-controlled invar oedometer apparatus. The soil fabric of each specimen was evaluated using a scanning electron microscope (SEM) and a mercury intrusion porosimeter (MIP). All specimens showed continuous contraction as the number of thermal cycles increased, albeit at a decreasing rate. After five heating and cooling cycles with temperatures ranging from 15 to 70°C, the accumulated plastic axial strain of the reconstituted specimen was 38% and 68% larger than those of the intact and recompacted specimens, respectively. The SEM visualisations and MIP measurements demonstrate that these observed differences can likely be attributed to different distributions of clay particles in the soil specimens (with a 28% clay content). In the intact and recompacted specimens, most of the clay particles formed silt-size aggregates. In the reconstituted specimen, the clay particles filled the spaces between silt particles and the soil fabric was homogeneous overall.
The time domain reflectometry (TDR) penetrometer, which can measure both the apparent dielectric permittivity and the bulk electrical conductivity of soils, is an important tool for the site investigation of contaminated land. This paper presents a theoretical method for evaluating the measurement sensitivity and an improved design of the TDR penetrometer. The sensitivity evaluation method is based on a spatial weighting analysis of the electromagnetic field using a seepage analysis software. This method is used to quantify the measurement sensitivity for the three types of TDR penetrometers reported in literature as well as guide the design improvement of the TDR penetrometer. The improved design includes the use of semicircle-shaped conductors and the optimization of the conductor diameter. The measurement sensitivity to the targeted medium for the improved TDR penetrometer is evaluated to be greater than those of the three types of TDR penetrometers reported in literature. The performance of the improved TDR penetrometer was demonstrated by conducting an experimental calibration of the probe and penetration tests in a chamber containing a silty soil column. The experimental results demonstrate that the measurements from the improved TDR penetrometer are able to capture the variation in the water content profiles as well as the leachate contaminated soil.
In situ wetting–drying cycles significantly and repeatedly influence the soil water content in the active zone but not in the steady zone. To investigate effects of in situ wetting–drying cycles on the mechanical behaviour of intact loess, a series of pressure plate tests was carried out with an intact loess sampled from the active and steady zones. Results show that the specimens from the steady zone have higher yield stresses at given suctions and a larger inclination of normalized loading collapse (LC) curve than the specimens from the active zone. As evidenced by the scanning electron microscopy results, the clay particles accumulating around the contacts between silt particles are widely observed in the former specimens but not in the latter specimens. The distribution of clay particles in the former specimens results in a stabilized skeleton with mechanical properties sensitive to suction variations. On the other hand, the maximum collapse potential of the specimen from the steady zone is 37% larger than that of the specimen from the active zone, mainly because of the larger inclination of normalized LC curve owned by the former specimen.
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