Study on Failure Mechanism of Roadway Sidewall Based on Rock Crack Damage Effect
Author:Zhang Yu Bao
Supervisor:tan yun liang zhao tong bin
In recent years,coal mining in China has entered deep strata.In the deep underground coal mine under high in situ stress,the damages of rock masses are typical brittle failure processes,therefore,understanding the properties of brittlness and strength for rock is of great importance to revel the damage mechanism of rock in coal sidewall.By applying filed survey,theoretical analysis,laboratory test and numerical simulation,this study investigates the mechanical characteristics of rock including the threshold stress,brittle evaluation,crack damage under cyclic loading/unloading and constitutive model,and revels the progressive failure mechanism of roadway sidewall.The obtained main conclusions can be summarized as follows:(1)Based on the full stress-strain curve,a brittleness index BIN is proposed by considering the factors including energy dissipation pre-peak,decrease of stress post-peak,residual strength and residual strain.Its validity is verified by comparing with the results obtained by uniaxial and triaxial tests on rock specimens.The brittleness index can be applied to the post-peak curves of Class Ⅱ curve,strain softening,ideal plastic and strain hardening.The proposed brittleness index can be divided into 5 levels:when BIN≤0,rock material is of extra-high brittleness;when 0<BIn≤0.20,rock material is of high brittleness;when 0.2<BIN≤0.75,rock material is of brittleness;when 0.75<BIN<≤1.50,rock material is of low brittleness;when BIN>1.50,rock material is of extra-low brittleness.(2)Based on the energy dissipation theory,a new method to measure the threshold stress is proposed by using the dissipated energy and energy dissipation index.Results show that both the threshold stresses for crack closure(qcc/qf)and threshold stresses for crack initiation(qcl/qf)slightly decrease with increasing the strength of rock,and the decrease tendency is not obvious.The threshold stress for crack damage(cd/qf)increases with increasing UCS.As the confining pressure increases.qcc/qf decreases and qcd/qr increases,while no unified variation tendency is observed for the change of with respective to different rock materials.The increment of condining pressure enhances the closure of interior micro-crack,resulting in the decrement of qcc/qf.On the contrary,the transverse propagation of crack is inhibited by the increasing confining pressure,thus a large shear stress is needed to unsteadily extend the crack.thereby increasing qcd/qr.(3)To analyze the effect of mining-induced stress on the damage mechanics of rock,a series of cyclic loading and unloading triaxial compression tests were conducted.Results show that the post-peak behaviour of stress-strain curve is the composite curve of Class I and Class Ⅱ.The slope of post-peak curve can be positive and/or negative values.The maximum dissipated energy occurs after reaching the peak stress.In the pre-peak stage,the confining pressure has relatively small influence on the magnitude of energy dissipation index K,while in the post-peak stage,K increases with increasing confining pressure.The crack damage variable D is defined as the ratio of accumulated dissipated energy to total dissipated energy.In the pre-peak stage,the effect of confining pressure on D can be negligible,but in the post-peak stage,the increase rate of D decreases with increasing axial strain under a larger confining pressure.The crack damage variable in the pre-peak stage can be estimated.(4)The constitutive model D-CWFS considering the crack damage effect is developed by introducing Dpre into the CWFS model.This model considers the non-simultaneously approach to the critical value for cohesion and internal friction angle,which reflects the natural damage characteristics of brittle rock.The D-CWFS model is compiled based on the secondary development of FLAC3D,the validity of which is verified by comparing the numerical results with the experimental results.(5)Based on the field survey of damage characteristics of coal and rock roadway sidewalls,the progressive failure mode is obtained from the engineering point of view,in which the spalling rib is the main damage response.The developed D-CWFS constitutive model is used to simulate the failure of roadway sidewall with different influencing factors.The simulation results show that the height-width ratio,lateral pressure coefficient,the internal friction angle and cohesion of surrounding rock play an important role in the deformation and failure of roadway.At the same time,the support design of rock roadway in Datai Coal Mine is optimized.The integrity of surrounding rock is good and there is no obvious fracture after excavation and support.The control effect of support for roadway sidewalls is good.