Interlayer protection significantly influences the anti-seepage strength of deep overburden soil,and it should be considered in the evaluation of seepage stability,however this influence can't be quantified now.Aiming at a typical strata structure of"②-2,③-1,④"in Luding hydropower station,a list of single-layered and three-layered seepage tests were performed to investigate the influence of interlayer protection.The results indicate that there are two different interlayer protection mechanisms between base soil and overlying strata.When the retention ratio meets the needs of filter and drainage,the overlying strata is the same as the artificially designed filter,and a continuous hydraulic transition is formed to give full play to the anti-seepage ability of the base soil.When the retention ratio only meets the need of filter,the overlying strata mainly undertake the pressure to protect the base soil.Based on the experimental results,the influence of interlayer protection on the allowable gradient was quantified,and then a method determining the allowable gradient was established.Under the protection of strata of ③-1 and④,the allowable gradient of stratum ②-2 can increase to two times as the initial allowable gradient;Under the protection of stratum of ④,the allowable gradient of stratum ③-1 can increase to 2.3 times as the initial value.The results provide important supports for the solution of abnormal leakage and safety evaluation of Luding hydropower station,and they will provide reference for other similar projects.
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