Small strain shear moduli of unsaturated natural and compacted loess

Abstract

Suction and microstructure play critical roles in the mechanical behaviour of loess, yet their effects on small strain (from 0·001 to 1%) shear moduli are not fully understood. In this study, shear moduli of loess with different microstructures and at different suctions are investigated and reported. Suction-controlled triaxial tests were carried out on two compacted and two intact loess specimens. Testing procedures include isotropic compression, wetting and shearing at constant mean net stress and constant suction. In addition, the soil microstructure was studied through scanning electron microscope images. At a given suction, the intact specimens have larger shear moduli than the compacted specimens at deviatoric strain below 0·025%, mainly because more clay aggregates accumulate at grain contacts in the intact loess. As suction decreases (i.e. wetting), contrary to the typical behaviour of compacted soils, the small strain shear moduli increase in the compacted loess. The increase is most probably because of significant wetting-induced contraction during the wetting process. In contrast, the shear moduli of the intact specimens remain unchanged, owing to slight swelling observed during wetting. The behaviour of the intact loess is likely to be dominated by the soil microstructure, and suction effects are relatively minor. With continued shearing, the differences resulting from the specific microstructures and wetting-induced volume changes (i.e. suction) diminished rapidly.