Abstract：Possible effects of changes in soil shear strength induced by acid rain on landslide formation are investigated. Soil samples from freshly exposed slip surface and adjacent surface have been studied at two landslides in the Three Gorges Reservoir area，China. A visibly higher smectite content at the slip surface and a dominant content of illite in the adjacent surface are observed. In laboratory，remolded soils are subjected to acid water immersion and tested for changes in their mineral compositions and shear strength after immersion with different durations. The largest change occurs in the internal friction angle，which drops over one-third，going with the change of the original smectitic soil with a substantial illite component into completely smectite dominated soil. Subsequently the internal friction angle rises back，simultaneously with neoformation of kaolinite at the expense of the smectite component. Changes in cohesion are much less significant. The original koalinite-rich and illite-rich samples are found to be much more stable. A chemo-mechanical model of clay is discussed at last.
