Abstract：Through experiments on 16 sets of samples of red beds collected from 16 reservoirs，the law that the softening coefficients of long-term water-soaked redbeds decrease is obtained. Rock weakening is the result of long-term softening and weathering co-action in lower water level of the rock in the region between the highest and lower water levels. Analytical results show that the softening coefficient of mudstone can be reduced 10%–20% after 30–50 years soaking compared with that obtained in laboratory，while the softening coefficient of silty mudstone can be reduced about 15% after 30–50 years soaking compared with that obtained in laboratory.
