Abstract: The problems existing in the present sampling method for weak layers in exploratory adits, are discussed in this paper. Because the weak layers are easily affected by the relaxation after the excavation of exploratory adits, the test results of samples taken from exploratory adits cannot reflect the engineering properties of weak layers under natural ground stress. But the stress relaxation effect on weak layers can be avoided effectively if a special sampling method in exploratory adits is adopted.Taking the intercalations of a certain dam site in Jinshajiang River as an example, the correlations between physical and mechanical indices and confining pressure indicate that ground stress is the most important environmental factor, which controls the engineering properties of weak layers. The confining pressure effect on weak layers has a realistic significance for the construction and stability assessment of rock mass projects too.