In this paper, the interactions between the weathered red mudstone and pore water in oft-com:office:smarttags" />Lanzhou area, China were explored by means of direct shear testing. It was found that under the fast shearing condition, mechanical (pore water pressure), physical and chemical effects function together making shear strength variation of the weathered red mudstone, and that mechanical effect is the strongest. The physical and chemical effects are closely associated with abundance of clay minerals and soluble salt within the weathered red mudstone.oft-com:office:office" />