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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.

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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