The conventional triaxial compression test was carried out on the weathered rock in immersed tunnel section of the Shenzhen-Zhongshan Bridge in natural and water-soaked states to reveal the strength change law of the weathered rock after encountering water under different conditions. The strength of weathered rock will decrease significantly when it encounters water

showing obvious softening behavior. When other conditions are certain

the shear strength of the specimen with deviator stress consolidation is significantly lower than that with isotropic consolidation. With the increases of confining pressure and deviator stress

the strain softening phenomenon of weathered rock gradually weakens

and the peak strength and residual strength gradually converge. The elastic modulus of the rock mass before the peak strength is regarded as a constant

and the plastic internal variable when the internal friction angle and cohesive force reach the residual values is used as the variable. The fitting of the cohesive force and internal friction angle to the plastic internal variable is given. The functional relationship expression improves the constitutive model considering the strain softening of weathered rock after encountering water; through regression verification

the theoretical calculation and test results of the model are consistent

indicating that the model can describe the deformation characteristics of weathered rock softened by water. Based on this model

the softening modulus of the weathered rock is obtained

and the softening coefficient is proposed as a reference for the strength and stiffness reduction of the weathered rock when it encounters water

which can provide a theoretical basis for the design and construction of similar projects such as Shenzhen-Zhongshan bridge.