Abstract: Red clay is one of the special soils which contains cementing material. Compared with common cohesive soils, the strength characteristics of red clay are more complicated. Some direct shear tests are performed on the unsaturated red clay samples with water and propanetriol solution; and the experimental results show that the strength and cohesion of samples exhibit double peaks of strength increasing with the increase of solution content; while the internal friction angle does not change. Besides, the strength of samples diminishes obviously with the propanetriol solution instead of water. Based on Bishop's effective stress for unsaturated soil and the characteristic of pore distribution acquired by NMR tests, the impact mechanism of pore distribution characteristic on strength is discucsed, and a conclusion is drawn that pore distribution characteristic has much effect on strength of red clay samples.

Key words: red clay , shear strength, unsaturation