Currently

most definitions of the damage variable for jointed rock mass only use geometrical factors of joints such as the length and dip angle

but cannot consider joint shear strength. Due to this limitation

a new damage variable formulae is firstly deduced to calculate rock mass with a non-persistent closed joint

based on the connection between the increment of additional strain energy caused by the existence of one joint in fracture mechanics and the emission of damaged strain energy in damage mechanics. Secondly

the calculation method of the stress intensity factor (SIF) of a single joint tip under biaxial compression is studied according to fracture mechanics theory

and then the calculation SIF formulae of KⅠ and KⅡ are obtained. Thirdly

the calculation formulae of the tip SIF of a set of one or more-rowed joints are given by considering the interaction among the joints. Finally

a biaxial compression damage constitutive model for the jointed rock mass is developed

which is further employed to analyze an example. It is found that for rock mass with a single non-persistent closed joint

the strength of rock mass is the same as that of the intact rock

and the damage is 0 when the joint dip angle is less than its internal friction angle. Furthermore

with the increase in joint dip angle

the change laws of rock mass strength and the damage with the joint dip angle are parabola with the hatch up and down

respectively. The strength of rock mass is the lowest and its damage is the highest when the joint dip angle is about 60°. With the increase of joint length

the damage of rock mass increases; while with the increase of the internal friction angle of joint

the damage of rock mass decreases. For the rock mass with a set of one-rowed non-persistent closed joint

when the total length of the joint is the same

the damage of rock mass gradually decreases with the decrease of a single joint length and the increase of the joint number

however the relationship between the decrease amplitude and the joint number exhibits nonlinear response.