Indoor and outdoor combined method for determining in-situ critical hydraulic gradient

The invention discloses a method for determining an in-situ critical hydraulic gradient by combining indoor and outdoor conditions. The method comprises the following steps: testing a soil body to obtain various parameters of a soil body sample; carrying out a seepage failure test under a stress condition indoors to obtain a critical hydraulic gradient under a certain soil body sample length, consolidation pressure and seepage direction; determining a hydraulic slope reduction coefficient and a standardized stress according to the parameters, and performing numerical fitting on the hydraulic slope reduction coefficient and the standardized stress to obtain a fitting relationship; according to the in-situ stress of the to-be-determined sample, obtaining in-situ standardized stress, and obtaining an in-situ hydraulic gradient reduction coefficient through the fitting relationship; and obtaining the in-situ critical hydraulic gradient according to the obtained in-situ hydraulic gradient reduction coefficient and