Undrained Shear-Strength Variation of Salt-Rich Clay: Implications for Fast-Moving Clay Landslides

It is well recognized that the shear strength of salt-rich clay is reduced after leaching and under drained conditions, as commonly occurs in reactivated landslides or in slow-moving landslides of such clay. This is due to loss of the physical and/or chemical bonds provided by the salt, demonstrating the physic-chemical effect of pore water on the shear strength of the clay. However, is there a similar effect when such clay is subjected to undrained conditions after leaching, as in fast-moving landslides in the clay following rainstorms or irrigation? To gain better understanding of the mechanisms of fast-moving landslides of salt-rich clay, the shear strength of a salt-rich clay under undrained conditions was investigated after being leached repeatedly. The clay was weathered mudstone from Heifangtai, Gansu, China. Test results showed that the clay’s undrained shear strength was remarkably reduced after leaching, mainly through water-soil physical-chemical interactions. The effective friction angle and cohesion of the clay may be reduced due to loss of the bonding salts, similar to the variation of its strength parameter under drained conditions. In addition, higher pore water pressure could build up during shearing due to an increase in the clay fraction caused by disaggregation of silt-size particles, leading to a decrease in effective normal stress and then shear strength.