The stress states of suspended cut-off wall and strong permeable foundation plays an important role in the evolution of piping. A series of seepage-stress coupling experiments on the suspended cut-off wall and piping-typed sand gravel foundation under different stress states are carried out using a newly designed soil-structure contact shear permeameter. The results show that the stress state has a great influence on the critical piping hydraulic gradient of the suspended cut-off wall and sand gravel foundation; the larger the confining pressure is

the larger the critical piping hydraulic gradient is. A linear empirical formula on the critical piping hydraulic gradient denoted by the confining pressure is established based on the experiment results. The results show the intensive influence of the seepage-stress coupling effect on the seepage stability of the cut-off wall and sand gravel foundation

so as to provide an important theoretical basis for determining the reasonable penetration depth of the suspended cut-off wall in deep overburden foundation.