ObjectivesExisting studies on zonal fracture of surrounding rock mainly focus on single tunnels， whereas the zonal fracture mechanism of surrounding rock in deep-buried multiple tunnels remains unclear in practical engineering. Therefore， the zonal fracture mechanism of surrounding rock during excavation of deep-buried multiple tunnels was investigated.MethodsTaking the diversion tunnels of the Jinping II Hydropower Station as the engineering background， a numerical model based on the strain-softening constitutive model was established. The variation laws of zonal fracture morphology， maximum radial distance of fracture zones， and number of fracture zones with strength parameters， deformation parameters， and dilation angle after excavation unloading were analyzed. In addition， the influences of these parameters on the zonal fracture characteristics of surrounding rock were compared.ResultsThe results show that the fracture patterns around the four tunnels exhibit a certain symmetrical distribution after excavation unloading. More severe zonal fracture phenomena occur in the surrounding rock of Tunnel No. 2， and the zonal fracture on the right side of Tunnel No. 1 is more significant than that on the left side. With decreasing internal friction angle， cohesion， and elastic modulus， or increasing dilation angle， the zonal fracture becomes more pronounced. Poisson's ratio mainly affects the locations of fractured zones. As Poisson's ratio decreases， the fractured zones on both sides of the tunnel gradually migrate deeper into the surrounding rock， whereas the fractured zones near the crown and invert move closer to the tunnel wall. Among all parameters， the strength parameters have the greatest influence on the zonal fracture morphology， followed by the deformation parameters， while the effect of dilation angle is relatively small.ConclusionsThe study reveals the zonal fracture mechanism of surrounding rock after excavation unloading of deep-buried multiple tunnels and provides theoretical support for surrounding rock support design in deep underground multi-cavern excavation engineering.
Objectives To analyze the influence of karst caves on the construction of large-section tunnels,and to investigate the deformation,failure,and water inrush characteristics of tunnel surrounding rock under various occurrence states of karst caves.Methods Based on the Shuangbei Tunnel project,considering the stress-damage-seepage coupling effect of the surrounding rock,numerical simulations were conducted using FLAC3D software.The simulations analyzed the displacement,failure zone,water-conducting fracture channels,and water inflow characteristics of the large-section tunnel surrounding rock under five factors and four levels:the angle between the karst cave and the tunnel center,the clear distance between the karst cave and the tunnel,the diameter of the karst cave,the water pressure in the karst cave,and the initial permeability coefficient of the rock mass surrounding the karst cave.The range analysis method was system-atically used to evaluate the impact of karst caves on the deformation,failure,and water inrush degree of the tunnel surrounding rock.Results The results show that:under conditions without a karst cave,the maximum displacement of the tunnel surrounding rock is 104.1 mm,the failure zone exhibits a"butterfly-shaped"distribution,with failure width and height of 7.0 m and 25.9 m,respectively.When a dry(water-free)karst cavity exists near the tunnel,the plastic zone of the surrounding rock expands and extends toward the karst cave,and the maximum displacement increases by approximately 30%～42%.When a water-rich,high-pressure karst cave exists near the tunnel,one or several water-conducting fracture channels form in the rock mass between the tunnel and the karst cave,and the maximum displacement of the surrounding rock exceeds 1,000 mm.The range values for the influence of the five karst cave occurrence parameters on the maximum displacement of the tunnel surrounding rock are 215.8,443.4,453.1,869.3,and 227.5 mm,respectively.The range values for the influence on the failure area of the tunnel surrounding rock are 351.8,102.1,372.5,243.3,and 166.4 m²,respectively.The range values for the influence on the total water inflow of the tunnel surrounding rock are 1 797.3,2 829.4,1 747.6,2 336.5,and 2 816.0 m³/h,re-spectively.Conclusions When karst caves near the tunnel contain no groundwater,their impact on the sta-bility of the large-section tunnel surrounding rock is relatively small.When groundwater is present,there exists a critical water pressure value that triggers the formation of water-conducting fracture channels between the tunnel and the karst cave,leading to overall sliding instability and water inrush.Among the five karst cave occurrence parameters,the water pressure in the karst cave determines the final displacement magnitude of the tunnel surrounding rock.The diameter of the karst cave and the angle between the karst cave and the tunnel center are key parameters affecting the extent and degree of failure of the tunnel surrounding rock.The clear distance between the karst cave and the tunnel,the initial permeability coefficient of the rock mass surrounding the karst cave,and the water pressure in the karst cave are the most important parameters controlling local and overall water inflow into the tunnel.
ObjectivesTo clarify the mechanical response of fractured sandstone under coupled freeze-thaw cycling and confining pressure conditions， the effects of freeze-thaw cycles and confining pressure on its mechanical properties and failure mechanisms were investigated.MethodsGray sandstone was selected as the research material. Intact， single-fractured， and cross-fractured specimens were prepared and subjected to 0， 30， 60， and 90 freeze-thaw cycles. Uniaxial compression tests and triaxial compression tests under confining pressures of 5 MPa and 10 MPa were conducted.. Stress-strain curves were obtained， and characteristic stresses including crack closure stress， crack initiation stress， and peak stress were extracted. Combined with analyses fracture distribution and failure patterns， the mechanical evolution of different specimens under freeze-thaw and confining pressure coupling was systematically investigated.ResultsWith increasing freeze-thaw cycles， the characteristic stresses of all specimen types decreased， and the overall stiffness was reduced. Intact specimens exhibited significant early-stage damage followed by a gradual stabilization. Single-fractured specimens showed rapid crack propagation in the initial stage and slower development thereafter. Cross-fractured specimens exhibited the most complex damage evolution due to interactions among fractures. Under low confining pressure， multiple crack types and complex failure patterns were observed. Under high confining pressure， the failure mode gradually transitioned from shear-dominated failure to shear-tensile composite failure. Freeze-thaw cycling intensified stress concentration at fracture intersections. Under uniaxial loading， failure tended to be tensile-dominated or disintegrative， while under triaxial conditions with low confining pressure， shear-tensile composite failure was dominant.ConclusionsFreeze-thaw cycling and confining pressure jointly govern the mechanical evolution of fractured sandstone. Freeze-thaw cycling significantly weakens rock strength and promotes crack coalescence， while confining pressure partially suppresses deterioration and inhibits tensile cracking. Compared with intact specimens， cross-fractured specimens exhibit greater reductions in peak strength and characteristic stresses. Their fractures tend to propagate in multiple directions and form interconnected fracture networks， thereby enhancing the sensitivity of fractured sandstone to freez-thaw damage and mechanical loading. These results provide theoretical support for tunnel construction， slope stability evaluation， and disaster prevention in cold-regions engineering.
ObjectivesTo investigate the effects of altitude， gas emission rate， distance between the air supply outlet and the tunnel face， and ventilation velocity on the diffusion and migration of gas in high-altitude gas tunnels during construction， a multi-factor coupled prediction model for gas concentration at the tunnel face was established.MethodsA three-dimensional numerical model was established using computational fluid dynamics （CFD） software. Nineteen working conditions were designed using a single-factor control variable method to analyze the influence of each key factor on gas migration in the tunnel. Based on a four-factor， four-level orthogonal experimental design and multivariate nonlinear regression analysis， a multi-factor coupled prediction model for gas concentration at the tunnel face was constructed. Field measurements and numerical simulation results were used to validate the model accuracy.ResultsThe results show that the gas concentration at the tunnel face increases exponentially with altitude. When altitude increases from 0 km to 5 km， the average gas concentration in the stable flow region rises from 0.072% to 0.128%. A strong positive linear correlation exists between gas concentration and gas emission rate. When the gas emission rate increases from 1 m³/min to 7 m³/min， the average gas concentration at different sections in the stable flow region stabilized within the range of 0.03%-0.23%. The minimum gas concentration at the tunnel face occurs when the distance between the air duct outlet and the face is 10 m. Gas concentration shows a clear inverse power-law relationship with ventilation velocity， and a reduction of up to 44.4% is observed when velocity exceeds 16 m/s. The proposed model achieves a coefficient of determination of 0.903， and the relative prediction error is within 2.79%-4.54%.ConclusionsThe proposed model can effectively predict the gas concentration at the tunnel face under varying altitudes， gas emission rates， ventilation velocities， and air duct positions， providing a scientific basis for ventilation design and gas hazard prevention in high-altitude gas tunnels.
ObjectivesTo safely dispose of the large amount of radioactive waste generated by nuclear energy， the permeability and mechanical properties of barrier materials under optimal proportions were investigated.MethodsThe main properties of bentonite-gravel-sand buffer materials under the optimal proportion were studied， including the relationship between the material proportion and the maximum bulk density. Simulation tests were carried out to explore the bearing capacity and permeability of an artificial simulated foundation.ResultsThe results show that the maximum bulk density of the material is positively correlated with the fine particle content within a certain range. When the fine particle content increases from 5% to 20%， the maximum bulk density increases. The maximum dry density of the material is related to the bentonite content and water content. The lower the bentonite content，the lower the maximum dry density； the higher the water content， the lower the maximum dry density. Under high compaction， the buffer material exhibits high foundation bearing capacity， with no proportional limit in the bearing capacity curve， and a low permeability coefficient. The average permeability coefficient is 1.577 8×10-5 cm/s，indicating a weak permeable layer with good impermeability.ConclusionsThe optimal proportion of the buffer material is related to the content of each component， mainly controlling the maximum dry density by affecting the porosity of the material. The optimal proportion is 10% bentonite + 24.4% manufactured sand + 65.6% gravel. The bearing capacity of the buffer material is related to its compactness. Under the optimal proportion， the buffer material exhibits high bearing capacity and better impermeability， and is classified as a weak permeable layer.
