Study On Slurry Fracturing And Stratum Stability Of Slurry Shield Tunnels

With the construction of large-scale slurry shield projects in our country,such as river-crossing or sea crossing tunnels and urban underground tunnels,the complexity of geological conditions is gradually increasing,therefore it is more difficult to ensure the stability of strata during construction.In this paper,the slurry fracturing in excavation surface and the stability of overlying strata of shield tail when segment comes out during the construction of shield tunnels are systematically studied by means of numerical simulation,model tests and theoretical analysis.The main research contents and results are as follows:(1)Model tests on slurry fracturing of shield tunnels in sandy strata were conducted to verify the self-developed numerical simulation program of slurry fracturing.Based on the finite element method(FEM)and the volume of fluid technique(VOF),the program considers the couplings of stress distribution,seepage field,and fracturing to simulate the dynamic processes of fracture initiation and propagation,and provides the results of fracture pressure,fracture morphology and the displacements in strata.The model tests on slurry fracturing of shield tunnels in sandy strata were conducted,and the developed program was used to simulate the model tests.The numerical results obtained are consistent with the model test results,which verify the numerical simulation program of slurry fracturing.(2)By 2D numerical simulation of slurry fracturing,taking the originating section of Nanjing Dinghuaimen Yangtze River tunnel project as an example,the law of slurry fracturing of shield tunnels in saturated soft clay strata was investigated.The fracturing process in soft clay strata is that the fracture initiates on the top of the excavation surface and propagate upward,and the surface tensile failure develops downward,then they approach and form a connected fracture surface in a gradual manner.For large diameter tunnels,with the increase of the cover depth,the fracture initiates on the top of the excavation surface,and then propagate upward in acclivitous direction directly or form spherical failure zone and then propagate upward in acclivitous direction.The displacement of strata induced by slurry fracturing changes continuously.The vertical displacements in strata for shallow or deep cover depth tunnels mainly distributes in a triangle region or ellipse region from the top of the excavation surface to the surface,and position of the maximum vertical displacement is on the surface or near the excavation surface,respectively.The fracture pressure increases with the increase of the cover depth.Under the condition of the same cover depth,compared with the large diameter tunnel,the small diameter tunnel has higher fracture pressure,faster fracture propagation speed,and the morphology of the fracture propagation shows the characteristics of vertical and multi-branch.Based on the law of the variation of the displacement of strata above the excavated face,the early warning mechanism of the initiation and propagation of slurry fracturing for shallow cover depth shield tunnels in saturated soft clay strata was proposed.(3)Through 3D numerical simulation of slurry fracturing,the law of slurry fracturing of shield tunnels in different geological strata was investigated.In the 3D fracturing process,firstly the fracture surface of the excavation surface propagates laterally around the excavation surface,and then the shear slip surface forms longitudinally in the tunnel roof strata.The fracture surface of the excavation surface and the shear slip surface in the tunnel roof strata successively propagate to the surface and form the blowout block in the tunnel roof strata.The vertical displacement induced by slurry fracturing mainly distributes in the blowout block in the tunnel roof strata.Furthermore,the laws of fracturing morphology and fracturing pressure of small diameter and large diameter tunnels with the thickness-span ratio of 1 under different cohesion intercept,angle of internal friction and initial modulus conditions were analyzed.Compared with the small diameter tunnel,the fracture surface of the excavation surface of the large diameter tunnel is more upright in most cases.The fracture initiation and extending pressure of large-diameter tunnel are greatly influenced by the angle of internal friction.When the angle of internal friction is small,the fracture initiation and extending pressure of large-diameter tunnel is small.In general,the fracture expanding pressure is higher than the fracture initiation pressure,and the pressure difference between the fracture expanding and the fracture initiation pressure of the large diameter tunnel is small.Therefore,the engineering suggestion was put forward that the prevention of fracturing initiation is the key point for large diameter shield tunnels.(4)The limit analysis method and numerical approach for investigating the stability of overlying strata of shield tail when segment comes out under the condition of steady seepage flow were presented.The functional equation and the variational equation about the collapsing block were derived from the upper-bound theorem and the extremum principle,and the analytical solution of the pore water pressure distribution around the tunnel under the condition of steady seepage was employed.The Runge-Katte algorithm and the traversal search method were used to solve the equations,and the stability of the overlying strata was investigated and the expression of the detaching surface of the collapsing block was obtained.According to the different stability of the overlying strata,the method for calculating the grouting pressure of shield tail was proposed.Furthermore,the influence of soil parameter on the shape of the collapsing block was analyzed by the limit analysis method,and the influence on the grouting pressure of the shield tail was obtained.The results show that when the compressive and tensile strength of soil mass,tunnel diameter,hydraulic head difference between the ground surface and the tunnel circumference increase or when the depth of tunnel axis decreases,the grouting pressure of shield tail should increase correspondingly.When parameters A and B of Hoek-Brown criterion and saturated unit weight of soil masses change,it has little influence on the grouting pressure of shield tail.