Propagation Law And Evolution Mechanism Of Shield Tunnel Face Instability In Soft Clay Stratum

Soil sliding failure occurs if the excavation face loses stability during shield tunnelling in soft clays.It is of great significance to gain a deep understanding of the change rules of soil movement and earth pressure during face instability for safe construction of shield tunnels.The current researches are mainly focused on the final face instability shapes,modes,corresponded theorical models and the determinations of limit face supporting pressure.The researches on face instability processes are not detailed enough,which include the space-time evolution laws of both soil movement and earth pressure.This dissertation has carried out researches on the propagation laws of soil movement,the space-time change laws of earth pressure and the evolution mechanisms of soil arching during face instability of shield tunnel in soft clays by using indoor model tests,centrifuge model tests,numerical simulations and theorical analysis.The research contents are as follows:1.A set of model equipment for face instability of shield tunnel in transparent clay was designed.The designed equipment was composed of transparent clay materials,model box system,data measurement system and data calculation system.The traditional data measurement method was improved.A scaled horizontal guide rail and platform board were designed and installed on a self-leveling optical platform.The model box which contained transparent clay on the platform board could be accurately translated with the guide rail.The digital-photographically measurement of multiple cross-sections and longitudinal-sections could be performed simultaneously with undisturbed and stable accuracy.The preparation method of transparent clay with optimal visibility was proposed by controlling the volume ratio of 15#white oil and n-dodecane and the solid-liquid mass ratio of amorphous silicon powder and mixed liquid being 10:0.8 and 1:0.6,respectively.The obtained optimal visible thickness of transparent clay was greater than 15 cm.The large batch preparation technology of transparent clay was proposed ensuring successful progress of the model tests.The technology included precise volume and mass ratios,complete dissolution,high and stable vacuum degree,preloading and adding tracer.2.A series of model tests on face instability of shield tunnel by using transparent clay were carried out.The propagation laws of soil movement during face collapse process were analyzed and the characteristics of staged face collapse and instable influencing zones were investigated.The nature of face instability was a gradual process of soil movement which can be described as three stages.The clays in front of tunnel face slipped into the tunnel vertically downward at an angle together with those in the upper portion in the initial deformational stage.The surrounding clays were disturbed and loosed continuously and the extent of soil movement increased in the deformational-extended stage.The clays slipped rapidly and the overall face instability occurred in the collapse stage.The three-staged gradual face collapse processes including soil loosening,disturbance expansion and slipping failure were revealed based on the characteristics of soil movement.The three stages were classified by soil loosening in the upper portion of excavation face,soil movement expansion in space and doubled increasing of soil movement in the front of excavation face,respectively.The contours of influencing zones of face instability including both the loosen zone and disturbance zone were determined based on indexes of the maximum displacement gradient line and the vertical displacement vector boundary line.The scopes of the loosen zone and disturbance zone were obtained quantitively.The shape changes of transverse and longitudinal surface settlement troughs were analyzed.The range of the transversal settlement trough width parameter were calculated.3.The centrifuge model tests and numerical simulations of face instability of shield tunnel in soft clays were carried out to clarify the change laws of face supporting pressure during the face instability of excavation face.The change laws of vertical,longitudinal and transversal earth pressures during face instability process were investigated based on the centrifuge model tests and numerical simulations.The face supporting pressures during face instability process followed three stages being rapidly-decreasing,slow-decreasing and stabilization.No rebound occurred which existed in the sandy soil and the required retreat displacement of the face excavation panel to attain a limit value was 25%of tunnel diameter,which was longer than that in the sandy soil.The limit supporting pressure was 5～7%percentage of the initial overburden pressure.In the soil slipping failure zone,the vertical earth pressures decreased at different rates along the depth direction,the shield tunnel driving direction and the cross-sectional direction as the excavation face gradually collapsed.The vertical earth pressure gradually decreased with the excavation panel receding and the vertical earth pressure concentration ratio was less than 1.0.The vertical earth pressure concentration ratio increased with the distance from the tunnel section increasing.The longitudinal earth pressure concentration ratio was less than the corresponded the vertical earth pressure concentration ratio at the height of the central axis of shield tunnel.The longitudinal and transverse earth pressures decreased near the excavation face but increased far away from the excavation face along the depth direction.The longitudinal earth pressure and transversal earth pressure showed two different trends of decreasing and increasing with the retreat of the excavation panel along the depth direction.The longitudinal earth pressure and transverse earth pressure changed more obviously than the vertical earth pressure in the scope out of range of 0.5D from the central axis of the tunnel.The study on the face instability of shield tunnel based on the change laws of earth pressure should consider the three-dimensional earth pressure.The research results showed that in addition to the vertical earth pressure targeted in the traditional method,the influence of the time-spatial changes of longitudinal and transversal earth pressure should be comprehensively considered in researches about face stability shield tunnel.4.The generation condition and evolution mechanism of spatial soil arching effect were analyzed based on the coefficient of lateral longitudinal and transverse pressures during face instability process.The spatial locations of arch foots and axes of arch structure,loose zone and stability zone were determined qualitatively and the spatial arch boundary was obtained quantitatively.The changes of coefficients of the longitudinal and transversal lateral earth pressure resulted in the continuous evolution of spatial soil arching effect,by which the spatial locations of the loose zone,stability zone and the arch foots and axes of arch structure were determined.The coefficients of longitudinal and transverse earth pressure in the loose zone were both less than the coefficient of lateral earth pressure at rest,K₀.The coefficients of longitudinal and transverse earth pressure in the arching zone were greater than the coefficient of lateral earth pressure at rest.The coefficients of longitudinal and transverse earth pressure in the disturbed zone outside the arching zone changed in the range of K₀-0.1and K₀+0.1.The coefficients of longitudinal and transverse earth pressure along the buried depth direction,the coefficient of longitudinal earth pressure along the shield driving direction and the coefficient of transverse earth pressure along the transverse direction all show S-shaped curves.The outer boundary of the spatial soil arch was obtained according to the inflection point of the vertical earth pressure along the depth direction and the longitudinal earth pressure along the tunnel driving direction.The inner boundary of the spatial soil arch was determined by parameters of percentage changes of vertical earth pressures along the vertical direction being 1～2%and longitudinal earth pressures along the longitudinal direction being 4～5%.5.The face instability models considering the gradual evolution of arching effect and characteristics of soil movement and earth pressure during face instability process under different cover depth to diameter ratios were established.The face instability combined-model included a cylinder body in the upper portion and a logarithmic spiral body in the lower portion considering the overall soil slipping failure was obtained when C/D=1.0.The two-stage face instability models were proposed based on the evolution of upper soil arching shapes and local soil slipping failure when C/D=2.0.The first-stage combined-model was composed of a catenary rotating body in the upper portion and a logarithmic spiral body in the lower portion.The second-stage combined-model consisted of a cone body in the upper portion and a logarithmic spiral body in the lower portion.The earth pressure at bottom of the upper cylinder body was calculated based on the Terzaghi loose earth pressure calculation method.The earth pressures at bottom of the upper catenary rotating body and cone body according to the arch theory.The carrying capacity at top of the logarithmic spiral body was calculated according to the slicing method considering the intersection between the logarithmic spiral body and tunnel section was the entire cross-section.The limit equilibrium equation and the calculation model by using Matlab were established and solved to obtain the numerical solution of the limit supporting pressure of shield tunnel excavation face.The reliability of the theoretical calculation method based on arch theory and the validity of face instability combined-model were verified by comparing the calculated values with the measured values of the centrifuge model tests,numerical simulation results and existed research results.The influence of parameters of cohesion,frictional angle and cover depth to diameter ratio on limit face supporting pressure was analyzed.The results provided a basis for determination of the limit face supporting pressure and controlling measurement during shield tunnelling at different stages of face instability.