Stress-path-dependent behaviour of compacted loess

This paper describes an experimental study aimed at evaluating the effects of mechanical and hydraulic paths on the compressibility and water retention behaviour of compacted loess from Xi'an, China. Suction-controlled oedometer tests, performed on statically compacted samples, are combined with mercury intrusion porosimetry (MIP) tests to evaluate the volumetric behaviour as well as the evolution of soil fabric for the compacted material. Oedometer test results are analyzed in the degree of saturation versus suction plane, using the water retention curve (WRC) for as-compacted material as reference. Negligible volume change, and hence no movement of the as-compacted WRC, is observed upon drying paths under low stresses. Wetting paths cause plastic deformations and hence the movement of the water retention curve. Soil densification shifts the WRC rightwards (in the degree of saturation versus suction plane), enlarging the air entry/air occlusion values and promoting steeper wetting and drying branches. MIP tests show that specimens subjected to drying and loading paths retain their as-compacted bi-modal pore size distribution (PSD). However, re-arrangement in soil fabric towards predominant mono-modal PSDs is observed in specimens wetted and loaded. Soil suction increases the compressibility and the yield stress of compacted loess. These experimental features are properly captured using an elasto-plastic constitutive model for nonactive soils that accounts for the evolution of the loading-collapse (LC) yield locus with plastic deformation.