The influence of infilling material on rock samples often becomes a problem in determining the shear strength. In fact, discontinuity such as joint is often encountered in rock mass. The presence of discontinuity will change the behavior of rock mass. This phenomena become worst when the joint filled by infilling material. There many problems in the design and construction due to the presence of filled joint rock Concerning with the signification the effect of filled joint to its shear strength it is necessary to carry out tests on rock samples. A series of laboratory experimentation is carried out to model the filled joint by using Tuff as a joint block and clay infilling material. In these tests is varying parameters of normal stress, infilling thickness and roughness of joint surface. From the test it is showed that the shear strength of a joint decreased significantly with the presence of infilling material in the aperture of a joint. The effect of asperities has influence 25–50% in shear strength of the thickness of the asperities. In tropical country as Indonesia, it is evident that asperities influence of Tuff on the shear strength in tropical country higher than non tropical country, the reduction of shear strength depends on the thickness of infilling material are cohesion of sample filled 0.25 cm of thickness and sample filled 0.5 cm of thickness decrease 73.63% and 86.91% of natural sample condition.

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