Abstract: The completely weathered tuff is encountered in the dam site area of Beikeng Reservoir, and its feasibility as a dam foundation is an important factor that affecting the project investment and the engineering safety. In-situ large-scale direct shear tests, laboratory direct shear tests and triaxial compression test are performed on the completely weathered tuff to study the shear strength characteristics and provide the basis of shear strength parameters for the dam foundation stability analysis. By comparing and analyzing the test results of different test methods and different water contents, the following shear deformation and strength characteristics of the completely weathered tuff are observed: 1) With the increase of the shear stress, the shear stiffness decreases and the shear strain increases rapidly. The normal deformation increases with the increase of shear stress, indicating the shear shrinkage characteristics. 2) The internal friction angle decreases with the increase of the water content. 3) The in-situ direct shear is essentially unconsolidated undrained shear, and the test results of the internal friction angle is lower than that of the laboratory consolidated quick shear. 4) Affected by the original rock structure, the cohesion of the in-situ direct shear is significantly greater than that of the laboratory direct shear and the triaxial shear tests.

Key words: Situ Test, Tuff, Completely Weathered, Water Content, Shear Strength Parameters