Deformation Mechanism Of Subway Tunnel Induced By Groundwater Level Variation And Its Interaction With Tunnel

Groundwater extraction-induced depression and settlement cones not only threaten the healthy development of the economy and society,but also make a major impact on underground construction,subway tunnel for example.This dissertation takes the typical southern part of the Yangtze River Delta as a research area for the construction of a metro project.The deformation mechanism of the subway tunnel induced by the groundwater level variations and its interaction with the subway tunnel are researched,taking comprehensive analysis method including theoretical analysis,numerical simulation,model tests,field tests and other means.The main conclusions can be drawn as follows:(1)Based on a thorough investigation of occurrence law of groundwater extraction-land subsidence in the southern Yangtze River Delta region,sine function,geometric progression function and logarithmic function are recommended in the article to formulate five typical generalized water level fluctuation modes from the southern Yangtze River Delta region deposits.Meanwhile some key fitting parameters are given,including the period ? and amplitude ?h of the sine function,the common ratio q of geometric function.(2)Based on the condition of radial flow along the cylindrical surface around the steadystate penetrating probe and the negative exponential distribution of the initial pore pressure,a determination method of the hydraulic conductivity is put forward.Using the conventional qualitative,quantitative analysis method and relative error index,and the cumulative frequency curve methods,the measured data of the seven sites for the Yangtze Delta deposits indicate that the methods of Elsworth and Chai greatly underestimated the hydraulic conductivity of the soil and this method is more reliable for the Yangtze Delta deposits.Taking into account the classical theoretical method curve shapes and a certain degree of subjectivity to distinguish of undrained and partial drainage boundary lines,three experience prediction curves,including arc,parabolic or elliptic curve are proposed.The number of curves and the statistical indicators reveal the best prediction curve is elliptical curve.(3)The vertical and horizontal deformation analysis of the tunnel are perfomed under the condition of continuous rising or falling of water level through a large-scale model test method.The results reveal that under the same conditions,the value of maximum settlement(Smax)/lift amount(Lmax)at the tunnel longitudinal profile center is proved to be between 2 and 5,which is close to the rebound modulus(Er)/elastic modulus(E).Then the stress and deformation of soil and subway tunnel under the above-mentioned five typical groundwater level fluctuation conditions(Condition 1-approximately constant-amplitude fluctuation,Condition 2-continuous wavelike decline,Condition 3-small wavelike rise,Condition 4-small wavelike decline,and Condition 5-continuous wavelike rise)are carried out using numerical simulation means.The results indicate that:With the increase of the soil depth,the deformation at the center of the cone climbs up first and then declines,in which the maximum value occurs between 15 m and 20 m of the soil depth.The deformation at the depth of 25 m is about 9.4% more than the ground deformation.The vertical deformation,axial force and the bending moment of the tunnel along longitudinal direction basically satisfy the modified Gaussian curve.While the horizontal deformation of tunnels in each condition shows a cubic curve that increases first and then decreases to both ends.Moreover,the maximum value exists near the inflection point.In the five conditions,the continuous wavelike decline of condition 2 is to exert influence on the tunnel.(4)The additional stress distribution caused by engineering dewatering can be simplified to be trapezoidal.Combined with the two-stage analytical method,the analytic solution of the soil and the tunnel deformations is derived based on the Mindlin solution.The results show that:The results of the theoretical analysis show to be in good agreement with the numerical simulation,with a difference of about 10%.The longitudinal bending moment of the tunnel is affected lager by the contact coefficient and the width of the force,but less by the elastic modulus of soils,the buried depth of the tunnel and the buried depth of the force.Yet,the transverse bending moment of the tunnel is influenced greatly by the depth of force,contact coefficient and eccentricity ration,but less by elasticity modulus and the buried depth of the tunnel.(5)Assuming that the head variation pattern in the significant influence zone indued by the underground barrier is a straight line or a parabola,five theoretical analytical formulae for the barrier effect of underground structures on the seepage field are deduced.In addition,the proposed formulae are validated and compared by using model tests and numerical simulation methods.Moreover,the influence on the barrier effect are analyzed including aquifer types,aquifer parameters,and spatial position relationships.The calculation results show that: Formula 4 and 5 give better accuracy and simplicity in five equations.The type of aquifer has limited influence on the barrier effect.The barrier effect within phreatic water is smaller than that within aquifer,and the difference gradually increases as the insert depth of the barrier.The hydraulic conductivity of the barrier kb and that of the soil below it kbs have a great influence on the barrier effect,while the soil permeability coefficient does a little influence.The spatial position gives little effect on it.