Effect of rock bolt pretension on anchoring and bearing characteristics of fractured rock mass

Gravel bolting test is the basic test method to study the support mechanism of rock bolts for fractured rock mass. The compression zone formed by rock bolt pretension is usually used to explain the gravel bolting mechanism, the test results show that the gravel body can be successfully bolted under low or even no pretension, which shows that it is not appropriate, at least not sufficient, to explain the gravel bolting phenomenon using the compression zone. In order to obtain the influence mechanism of rock bolt pretension on the anchoring and bearing characteristics of fractured rock mass, the numerical simulation method of gravel bolting and the pretension imposing method were developed. Gravel bolting laboratory and numerical tests under low and high pretensions were carried out, respectively. Based on the above tests, the formation and instability process of bearing structure and the force evolution law of rock bolts were analyzed, and the gravel bolting mechanism and the bearing-instability characteristics of anchoring structure were fully revealed. The results show that: ① The proposed numerical simulation method of gravel bolting and the pretension imposing method are feasible, which can simulate the gravel bolting tests under different pretensions, and overcome the malpractice that the contact forces between gravel particles can not be fully obtained in laboratory tests. ② Under the conditions of low pretension and high pretension, the gravel can be successfully anchored by rock bolts, but the anchoring mechanisms under different pretensions are different. Under low pretension, the pressure arch bearing structure is formed in the bolted gravel, and the pressure arch carries all the weight of the gravel. The rock bolts mainly play the role of suspension and only bears the weight of the gravel below the pressure arch, with low forces in rock bolts. Under high pretension, the bearing structure of compression zone is formed in the bolted gravel. The rock bolts mainly play the role of pre-tightening and has a large force. ③ When the pressure arch structure is loaded, the height of the pressure arch decreases continuously, and the internal contact force in the pressure arch first increases and then decreases, until the pressure arch structure is completely destroyed, and the corresponding rock bolt force increases at first and then decreases. When the compression zone structure is loaded, the compression zone is destroyed at first, and the corresponding rock bolt force decreases, and then the bearing structure changes from the compression zone to the pressure arch structure, and the rock bolt force increases and then decreases. ④ There is a significant positive correlation between the bearing capacity of bolted gravel and the inner stress in the bolted gravel. The internal stress in the compression zone structure is much larger than that in the pressure arch structure, so that the peak value of the inner stress in the bolted gravel under high pretension is much higher than that under low pretension, which is the reason why the bearing capacity of compression zone is much larger than that of pressure arch.