Effect Of Layered Joints On Rockburst In Deep Tunnels

Engineering rock mass is a discontinuous medium body composed of joints,fissures and other structures.The rockburst tendency of layered jointed rock mass is an important subject of deep engineering safety and stability research.Based on the PFC numerical simulation test method,this paper studies the strain-type rockburst induced by stress concentration in the hard and brittle surrounding rock with layered joints in deep underground tunnels after excavation,and the occurrence of excavated caverns induced by blasting of adjacent caverns.From the perspective of energy theory and fracture failure theory,the influence of different initial in-situ stresses,different joint dip angles and RQD on the rockburst tendency of the surrounding rock of the cavern was studied,and the layered joints were revealed.Mechanism of rockburst formation in rock mass tunnels.Research indicates:(1)After the excavation of the underground chamber,the rockburst process of the surrounding rock with layered joints can be divided into four stages:layered fracture of the surface rock mass,splitting of the internal rock mass into slabs,fracture of the internal rock layer and fragmentation and ejection into blocks,and showed the form of layer-by-layer damage from the cave wall to the interior,and the final damage area was V-shaped.At the same time,according to the hoop stress changes in the surrounding rock,three deformation modes of the surrounding rock can be judged,and the quantitative indicators that the initial in-situ stress,RQD and joint dip angle affect the rockburst tendency under the preset test conditions are proposed.(2)The magnitude of RQD and rock strength determines the rock mass’s compressive strength and the ability to store elastic strain energy.When the RQD and rock strength are small,the compressive strength of the rock mass is small and the energy storage capacity is poor,and the rock mass will be destroyed when the pressure and the accumulated strain energy of the rock mass are not enough for rock burst;When the rock mass is large,the strength of the rock mass is high and the upper limit of energy storage is high,If the pressure und accumuated energy are not enough to destroy the rock mass,the rock burst will not occur,but when the pressure and accumulated energy are large enough,Rockbursts will also be more violent.(3)Under the condition of static excavation,in-situ stress is the source of elastic strain energy of surrounding rock,and rockburst will occur only when enough energy is accumulated in the surrounding rock.When σx=σy,the increase of initial in-situ stress will make the kinetic energy of rockburst increase nonlinearly,and the intensity of rockburst and the damage degree of surrounding rock will increase greatly.When σx≠σy,whether the lateral pressure σx increases(50MPa～75MPa)or decreases(50MPa～25MPa),the strength of the rock burst will increase;when subjected to stress waves,the greater the initial in-situ stress,the more serious the surrounding rock damage.After being disturbed by stress waves,the larger the initial in-situ stress is,the larger the hoop stress increment is,and the more strain energy is accumulated,which greatly increases the risk of rockburst.(4)Under the condition of static excavation,the inclination angle of the j oint is not the main controlling factor for the occurrence of rockburst,but the main controlling factor for the location of the rockburst.The rockburst always occurs perpendicular to the direction of the joint.It not only affects the location of rockburst,but also is one of the main controlling factors affecting whether rockburst occurs in the tunnel.The closer the inclination angle of the joint is to 90°,the more likely the rockburst occurs in the surrounding rock.