Abstract: Understanding the mechanical behavior and constitutive characterization of cemented fault gouge is critical for ensuring the safety of rock tunnel construction in China.This study investigates the microstructural features and mineral composition of fault gouge in the Daheishan Tunnel,Yunnan Province,using scanning electron microscopy (SEM) and X-ray diffraction (XRD).Stress-strain behaviors under varied conditions (water content,clay content,and confining pressure) are determined through consolidated drained triaxial shear tests.The disintegration mechanism of cementation in fault gouge during underground water softening and shearing deformation is thoroughly examined,revealing significant influences on the deformation and mechanical behavior of fractured rock masses.Employing a critical state constitutive modeling approach for structured soils,a novel parameter,the “cementation degree” is introduced to represent cementitious effects in fault gouge.This leads to a constitutive model integrating cementation effects.Integrated into ABAQUS using the UMAT subroutine,this model effectively simulates the mechanical softening and volume yield behavior observed in laboratory tests.The model provides a theoretical framework for accurately characterizing the mechanical behavior of cemented fault gouge under engineering disturbances,offering crucial insights into the safety of rock tunnel constructions.