This paper studied the seismic stability of the 1# surge chamber in the Baihetan hydropower plant under the influence of interlayer shear weakness zone (ISWZ) C2 regarded as the large dominating geological discontinuity. Based on the dependent behaviour of normal stress

a nonlinear continuous yielding (CY) model was adopted to describe the complex mechanical properties of the ISWZ C2 under static and seismic dynamics. In this model

the deformation characteristics of the discontinuity surface were expressed in terms of a power function

and the progressive destruction of the strength during shear failure was also considered. Besides

3DEC software was applied to verify the CY model. Then the applicability of the CY model was proved by comparing experimental results with theoretical solutions. Three ground motion waveforms were utilised to conduct the seismic analysis of the #1 surge chamber after the special response spectrum matching process. The seismic analysis confirmed the control effect of ISWZ C2 on the seismic stability of the cavern. The seismic displacement of the cavern was mainly the elastic body movement and was supplemented by the plastic deformation. Furthermore

most of the deformations were caused by the contact deformation of C2. For the contact deformation of C2

the magnitude of permanent shear deformation was larger than that of the normal deformation. The magnitude of permanent shear deformation was more obvious along the strike direction of C2

and the permanent normal displacement of C2 mainly occurred along the dip direction of C2. Finally

the seismic stability of the cavern was determined by the overload method. The measured seismic safety factor of the cavern was approximately 2~3. The findings in this study may provide helpful references for the seismic design of the underground caverns.