Experimental Study On The Damage Of Brittle Rockmass Anchored By The Bent Rockbolt Under Tension-Shear Stress

In the deep underground engineering,the rockbolt has been widely used as a very effective support method.However the complex geostress stress environment often leads different damage forms of the bolt,such as rupture,cut,collapse,and cap,etc.The development of anchoring theory laid the foundation for the understanding of anchoring mechanism.The introduction of the theory of maximum horizontal stress has led researchers to gradually deepen their understanding of the axial and radial effects of rockbolts.Correspondingly,the failure of the rockbolts under the combined stress of tensile and shear is also a common form in the deep hard and brittle rock mass.Under the support of the National Natural Science Foundation project(51674116),In this paper,through the indoor similar simulation test and numerical simulation method,the anchor specimens under different anchoring conditions(such as straight rockbolts and bent rockbolt)in the shearing state are studied,including bearing capacity,crack propagation evolution law,stress distribution and acoustic emission characteristics of anchor specimens.The purpose is to provide a basis for the application of the bent rockbolt in deep hard and brittle rock mass support.The main contents and conclusions of this paper are as follows:(1)Considering the deformation effect of the bending section of the bent rockbolt and the ultimate deformation rate of the rebar material.In this paper,the bending element of 18 mm bolt diameter is designed on the premise that the bolt body does not suffer permanent damage.The tensile tests were carried out on the bent rockbolt and the straight rockbolt respectively,and the advantages of the bent rockbolt were analyzed from the angle of the axial forcedeformation of the bolt.(2)In terms of load-bearing characteristics and deformation of specimens: The bearing capacity of anchored specimens is generally higher than that of non-anchored specimens,which the straight rockbolt with the full length anchorage is first,the bent rockbolt with the reservation anchorage is second,and the straight rockbolt with the reservation anchorage is third.The loadcarrying capacity of the anchorless specimens decreases rapidly after reaching the peak strength,which exhibits properties similar to brittle materials.While the load-carrying capacity of the anchored specimens decreases slowly after the peak,and the strength and deformation performance of the bent rockbolt after the peak are the best.Comparing the time of two main cracks in each anchorage specimen,the straight rockbolt with the full length anchorage is 18.265 s,the straight rockbolt with the reservation anchorage is2.033 s,and the bent rockbolt with the reservation anchorage is 35.103 s.So the bent rockbolt can delay the crack generation compared with straight rockbolt.(3)In terms of numerical simulation analysis,The shear state analysis shows that the bent rockbolt has less stress under the same shear displacement than the straight rockbolt,and the difference becomes more and more obvious with the increase of shear displacement.Therefore,in terms of coordinating the shear displacement of the surrounding rock,the bent rockbolt can allow the displacement of the surrounding rock to be larger than that of the straight rockbolt,which effectively avoids the failure of straight bolt when resisting shear displacement of surrounding rock.Analysis of tension-shear state shows that the anchoring method of the bent rockbolt can change the stress distribution of the surrounding rock and effectively reduce the stress value of the sheared area.Therefore,the stress in tension-shear region with the bent rockbolt is weakened,which avoid failure of rockbolt due to excessive load(4)In terms of acoustic emission,The load-bearing characteristics-ring counting,crack propagation and ring counting are analyzed respectively.The bent rockbolt has greater loadbearing capacity during the period when the bolt plays its role after the peak,and the duration of bell counting is longer than that of the straight rockbolt.The bent rockbolt has more advantages in the post-peak stage which is reflected in the load-bearing performance and the action time of the bolt.The crack penetration will lead to the increase of acoustic emission activity,at this time the rockbolt begins to play a role.At this stage,the amount of reduction in the bearing capacity of specimen with the bent rockbolt is minimized.