Most of debris-flows are triggered by landslide in Jiangjia Ravine

widespread shear failure of slope is the first step in the three processes of debris-flows triggered by landslide. There is widely distributed gravelly soil in the triggering and sedimentary area. The strength behaviour of the gravelly soil is important for mechanism of debris-flows triggering in Jiangjia Ravine. Because the large grain size

coarse-grained content and specimen size have much effect on the gravelly soil strength behaviour

the large scale direct shear apparatus is used to test direct shear strength behaviour of the gravelly soil with different coarse-grained contents. As a result

it is found that there is no peak in the curve of relationship between shear stress and horizontal displacement at dried density equal 2.0 g/cm3 and water content equal 7 %

except for the specimen of coarse-grained contents equal 74.31 % under lower upright stress(50 kPa

100 kPa). The shear modulus when horizontal shear strain is 0.02 increase with increment of upright stress at the same coarse-grained contents. And increase with increment of coarse-grained contents at the same upright stress

which is larger at lower upright stress than higher stress. Friction angle increase with increment of coarse-grained contents. Cohesion drop to the minimum at coarse-grained contents equal 48.7 %

and decreases with increment of coarse-grained contents as coarse-grained contents litter than 48.7 %

however cohesion increases obviously with increment of coarse-grained contents when coarse-grained contents larger than 48.7 %.