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Abstract
Mechanical properties of rock joints govern the strength and deformational behaviour of a rock mass. The responses of a rock joint to shear and normal loadings highly depend on its surface properties, block size and matching state. Rock joints can become mismatched or opened due to underground excavation, earthquake vibration and nearby blasting. Previous studies indicate that the opening along joint walls significantly decreases the normal and shear stiffness of a joint, which undermines the stability of rock masses. Nevertheless, the opening effect has not been well considered before in developing constitutive models for the rock joint behaviour.
The primary objective of this dissertation is to provide a comprehensive understanding of the mechanical behaviour of field-scale opened joints. A new constitutive model for rock joints with various degrees of opening under both normal and shear loadings is presented. The normal deformability is represented by a semi-empirical non-linear relationship that accounts for the stiffness reduction due to joint opening. A formulation of a joint shear model is developed based on the wear fundamentals. Roughness degradation depends on the relative normal stress and the geometry of the asperity involved in shear. Numerical investigation using UDEC with Voronoi tessellation quantifies how much these parameters affect the dilation behaviour during shear. Experimental curves from both triangular and JRC-shaped joint replicas agree well with the analytical predictions. Scale dependence of the joint roughness is investigated by postulating that a natural joint profile is a self-affine fractal. The proposed scaling laws show that both slopes of waviness and unevenness become gentler with increased sampling length, which has been validated by morphological examinations of several 400 mm long rock joints.
The proposed constitutive law has been incorporated into UDEC via the built-in language, FISH. The deformation of three rock structures is assessed by numerical simulations in which the mechanical behaviour of rock joints is represented by the model developed in this study. Good agreement between predictions and field measurements demonstrates the capability of the new joint model for stability analysis of rock masses with opened joints at the field scale.