The problems existing in the present sampling method for weak layers in exploratory adits are discussed. At present,the confining pressure effect on engineering properties of weak layers is often neglected,but the argillizational effect under ground water is emphasized. Ordinarily,samples of weak layers are obtained in exploratory adits. But the weak layers are easily affected by the stress relaxation after the excavation of exploratory adits. So the engineering properties of weak layers,based on laboratory or in-situ tests,are very poor. Obviously,the test results of samples from exploratory adits cannot reflect their engineering properties under natural ground stresses. To avoid the effect of stress relaxation on weak layers,a special sampling method in exploratory adits is adopted,which reveals that weak layers are of good engineering properties under confining pressure. Taking the weak intercalations at a giant hydroelectric station on Jinshajiang River as an example,the correlations between physical and mechanical indices and confining pressure indicate that confining pressure is the most important environmental factor to control the engineering properties of weak layers.