Evolution Of Loosened Zone And Stability Analysis For Large-span Small-distance Tunnel

The small spacing,large span and bad geology condition always appear in two-hole eight-lane tunnel portal.Under shallow buried and unsymmetrical pressure,the surrounding rock is broken,tunnel excavation causes loosened zone and induced slip surface with obvious displacement in upper rock mass,range of loosened zone will impact stability of surrounding rock.Based on a large-section and double-hole tunnel project in Guangzhou,excavate section of tunnel portal with both side drift method in accordance of actual spacing by using finite difference numerical analysis software FLAC 3D,draw the range of loosened zone during tunnel excavation with improved tangential stress disturbance factor and analyze distortion and displacement of surrounding rock,plastic area,then excavate tunnel with 0.75 times and 0.5 times span and contrast with actual monitoring data.Then excavate tunnel under hydrous condition by using fluid-solid coupling function in FLAC 3D,draw the area of loosened zone and compare the displacement,stress,plastic zone with that under anhydrous condition.The analysis shows that,development of loosened zone shows a regular variation with both side drift excavation method,loosened zone caused by left and right side heading excavation are lager and converge above tunnel vault,influence of core soil excavation is less.The loosened zone is asymmetric between left and right side,range of loosened zone is larger in deep side.Range of loosened zone above two tunnels reach surface,pressure arch do not exist in surrounding rock.By practical project,the loosened zone of two holes is not confluent,whole distortion present flattened trend.After reducing spacing to 0.75 times and 0.5 times span,two part of loosened zone become close and confluent,compressive stress decline and tension stress increase in surround rock,key points displacement in right side of left tunnel and left side of right tunnel increase 20%~63%.Surface subsidence above two tunnels increase 12%~60%.Plastic range become close and confluent,too.Through comprehensive consideration to stress and displacement of two tunnels,space of tunnels should not be less than 0.75 times span.Considering the precipitation and seepage condition,tunnel excavation causes double funnel in seepage,loosened zone area of two tunnels under fluid-solid coupling condition increase about 30% and 42%.Comparing with result of anhydrous condition,tension stress concentration increase in arch foots of tunnels,the increase of loosened zone in deeper buried side causes larger unsymmetrical pressure and deformation around tunnels.Lateral displacement of tunnels increase six times,surface subsidence increase up to 33%.The plastic zone appear a large increase and confluent between two tunnels.Tunnel stability is badly affected by precipitation and seepage,must take waterproof measure during the project.