Dynamic responses and failure mechanisms of the existing tunnel under transient excavation unloading of an adjacent tunnel

Engineering disasters(e.g.rock slabbing and rockburst)of the tunnel groups induced by the transient excavation of an adjacent tunnel threaten the stability of the existing tunnel,especially for those excavated by using the drill and blast tunneling(D&B).However,the dynamic response and failure mechanism of surrounding rocks of the existing tunnel caused by adjacent transient excavation are not clear due to the difficulty in conducting field tests and laboratory experiments.Therefore,a novel transient unloading experimental system for deep tunnel excavation was proposed in this study.The real stress path and the unloading rate can be reproduced by using this proposed system.The experiments were conducted for observing the dynamic response of the existing tunnel induced by adjacent transient excavation under different lateral pressure coefficients l(?0.4,0.6,0.8,1,1.2,1.4,1.6,1.8)with a polymethyl methacrylate(PMMA)specimen.The propagation of the impact wave and unloading surface wave was detected through the digital image correlation(DIC)analysis.The reflection of the unloading surface wave on the incident side of the existing tunnel(tunnel-E)was observed and analyzed.Moreover,the dynamic characteristics of the stress redistribution,the particle displacement and vibration velocity of surrounding rocks of tunnel-E were analyzed and summarized.In addition,the Mohr-Coulomb(MeC)failure criterion with tension cut-off was adopted to evaluate the stability of the existing tunnel under adjacent transient excavation.The results indicate that the incident side of the existing tunnel under the dynamic disturbance of transient excavation of an adjacent tunnel was more prone to fail,followed by the shadow side and the top/bottom side.

Analytical solution for a circular roadway considering the transient effect of excavation unloading

The rocks surrounding a roadway exhibit some special and complex phenomena with increasing depth of excavation in underground engineering.Quasi-static analysis cannot adequately explain these engineering problems.The computational model of a circular roadway considering the transient effect of excavation unloading is established for these problems.The time factor makes the solution of the problem difficult.Thus,the computational model is divided into a dynamic model and a static model.The Laplace integral transform and inverse transform are performed to solve the dynamic model and elasticity theory is used to analyze the static model.The results from an example show that circumferential stress increases and radial stress decreases with time.The stress difference becomes large gradually in this progress.The displacement increases with unloading time and decreases with the radial depth of surrounding rocks.It can be seen that the development trend of unloading and displacement is similar by comparing their rates.Finally,the results of ANSYS are used to verify the analytical solution.The contrast indicates that the laws of the two methods are basically in agreement.Thus,the analysis can provide a reference for further study.

3D DEM simulation of hard rock fracture in deep tunnel excavation induced by changes in principal stress magnitude and orientation

To achieve the loading of the stress path of hard rock,the spherical discrete element model(DEM)and the new flexible membrane technology were utilized to realize the transient loading of three principal stresses with arbitrary magnitudes and orientations.Furthermore,based on the deep tunnel of China Jinping Underground Laboratory II(CJPL-II),the deformation and fracture evolution characteristics of deep hard rock induced by excavation stress path were analyzed,and the mechanisms of transient loading-unloading and stress rotation-induced fractures were revealed from a mesoscopic perspective.The results indicated that the stressestrain curve exhibits different trends and degrees of sudden changes when subjected to transient changes in principal stress,accompanied by sudden changes in strain rate.Stress rotation induces spatially directional deformation,resulting in fractures of different degrees and orientations,and increasing the degree of deformation anisotropy.The correlation between the degree of induced fracture and the unloading magnitude of minimum principal stress,as well as its initial level is significant and positive.The process of mechanical response during transient unloading exhibits clear nonlinearity and directivity.After transient unloading,both the minimum principal stress and minimum principal strain rate decrease sharply and then tend to stabilize.This occurs from the edge to the interior and from the direction of the minimum principal stress to the direction of the maximum principal stress on theε1-ε3 plane.Transient unloading will induce a tensile stress wave.The ability to induce fractures due to changes in principal stress magnitude,orientation and rotation paths gradually increases.The analysis indicates a positive correlation between the abrupt change amplitude of strain rate and the maximum unloading magnitude,which is determined by the magnitude and rotation of principal stress.A high tensile strain rate is more likely to induce fractures under low minimum principal stress.