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.

Influence of the spatial distribution of underground tunnel group on its blasting vibration response

The spatial distribution of underground tunnels is significant to the stress redistribution in the surrounding rock masses and blast wave propagation.The field blasting tests were first carried out to study the propagation of blast-induced seismic waves through under-ground tunnels of the Xiluodu Hydropower Station in China.The results show that the peak horizontal particle vibration velocity can be used as a safety criterion for underground tunnels.The effects of in situ stresses and spatial distributions of the tunnel group on the vibra-tion velocities distribution is afterward investigated by numerical simulation.The results show that there is a significant amplification of the blasting vibrations in the adjacent tunnels,which depends on their vertical positions during the excavation of a tunnel.The peak vibration velocity decreases as the lateral separation between tunnels increases.When the separation between the tunnels exceeds the width of three tunnels,the impact of the blast waves on each part of the adjacent tunnel tends to be stable on the whole.In terms of the size of the tunnel,the blasting vibration velocity in the upper part of the straight wall on the front-blast side increases as the width increases(and then levels off),while the blasting vibration velocity in the lower part decreases as the width increases(and then levels off).Finally,a generalized formula of blasting vibration velocity considering the spatial distribution is established,which can well predict the vibration velocity of particles in underground tunnels.

Pinning-down molecules in their self-assemblies with multiple weak hydrogen bonds of C-H···F and C-H···N

Two-dimensional self-assemblies of four partially fluorinated molecules, 1,4-bis（2,6-difluoropyridin-4-yl）benzene, 4,4＇-bis（2,6-difluoropyridin-4-yl）-1,1＇-biphenyl, 4,4＇-bis（2,6-difluoropyridin-4-yl）-1,1＇：4＇,1＇-terphenyl and 4,4＇-bis（2,6-difluoropyridin-3-yl）-1,1＇-biphenyl, involving weak intermolecular C-H···F and C-H···N hydrogen bonds were systematically investigated on Au（111） with low-temperature scanning tunneling microscopy. The inter-molecular connecting modes and binding sites were closely related to the backbones of the building blocks, i.e., the molecule length determines its binding sites with neighboring molecules in the assemblies while the attaching positions of the N and F atoms dictate its approaching and docking angles. The experimental results demonstrate that multiple weak hydrogen bonds such as C-H···F and C-H···N can be efficiently applied to tune the molecular orientations and the self-assembly structures accordingly.