Shear behavior of unsaturated intact and compacted loess: a comparison study

Understanding the shear behavior of unsaturated intact and compacted loess is essential for analyzing rainfall induced failure of man-made and natural loess slopes. So far, comparison studies on the shear behavior of unsaturated intact and compacted loess are still incomplete. This study aims to investigate the shear behavior of intact and compacted loess under unsaturated conditions. To achieve this objective, consolidated-drained triaxial tests were carried out with 36 intact and compacted loess. Results show that the failure envelope of intact and compacted loess at different dry densities and suctions could be unified through the effective stress concept of unsaturated soil together with the Mohr–Coulomb failure criteria. Comparing with intact loess, compacted loess shows a unified failure envelope with a 15% larger slope (corresponding to a 14% larger friction angle). As evidenced from Scanning Electron Microscope (SEM) observations, significant amount of aggregates was developed during the preparation of compacted loess. Those aggregates may enhance the interlocking in the compacted loess and hence result in a relatively large friction angle. On the other hand, suction contribution to the effective stress of compacted loess is more significant than that of intact loess. This can be explained by the fact that the compacted loess contains larger amount of air–water interface and contact area between water and soil particle because of higher water retention capacity. Therefore, the effective stress contributed by the surface tension (i.e., air–water interface) and the negative pore-water pressure acting on the contact area is higher for compacted loess.