Creep characteristics and creep model of deep buried marble at Jinping underground laboratory

In order to ensure the long-term stability of Jinping Underground Laboratory ( CJPL) caverns,the creep characteristics of 2 400 m deep-buried marble were studied. The multi-stage loading creep tests were conducted on the basis of convention triaxial compression tests to analyze the law of axial and circumferential deformation of marble dur- ing the creep process and the difference of creep characteristics of marble under different confining pressures (5 and 64 MPa). Then the long-term strength of marble was discussed by using the isochronous stress-strain curve method and a creep model based on fractional derivative was proposed to describe the process of creep deformation. The results show that under the confining pressures of 13 MPa and 27 MPa,the axial stress-strain curves of marble drop rapidly af- ter reaching the peak stress and under confining pressures of 40,53 and 64 MPa,the strain curves near the peak stress show obvious plateau section,which indicates that the deformation behavior of deep marble in CJPL tends to change from brittleness to ductility with the increase of confining pressures. Compared with low stress level,the marble under high stress level is more prone to creep,the circumferential creep phenomenon is more significant and the dilatancy phenomenon during creep process is more obvious. The volume creep deformation of specimens under 64 MPa confi- ning pressure is 16. 3 times of that under 5 MPa confining pressure. During the creep loading process,the deformation modulus of marble increases first and then decreases. In the stage of increasing modulus of deformation,the increasing range under high confining pressure is smaller than that under low confining pressure. The increase of deformation modulus under 64 MPa confining pressure is 1. 8 GPa,less than 3. 6 GPa under 5 MPa confining pressure,which indi- cates that the specimen has been partially compacted under high confining pressure. With the increase of stress level, the modulus of deformation decreases and the decrease range under high confining pressure is greater than that under low confining pressure. The decrease of deformation modulus of specimens under confining pressure of 64 MPa is 9． 4 GPa,which is about 22% of the peak deformation modulus. The decrease of specimen under confining pressure of 5 MPa is only 1． 8 GPa,which is about 4% of the peak deformation modulus. These indicates that the crack initiation and propagation of specimens under high confining pressure are more intense and the rock deterioration is more serious before failure. Under the same deviating stress condition,the creep rate of specimens with larger confining pressure is smaller,but the deformation is larger and the dilatancy phenomenon is remarkable when the failure occurs. It shows that under the same external load condition,when the environmental stress level of deep surrounding rock is higher,the deformation of surrounding rock is difficult to converge,and it is prone to aging large deformation failure. When the confining pressure is 5 and 64 MPa,the long-term strength of marble is 170 and 290 MPa,respectively,which are 82% and 73% of the corresponding triaxial compressive strength. Based on the fractional derivative,a new viscous-e- lastic-plastic damage creep model of marble was established and it was proved that the model is simple in form and can well describe the characteristics of non-linear acceleration in marble creep process.