| The advancement of urbanization accelerates the construction of subway,it makes the city vigorously develop underground space to meet the normal operation of urban rail transit,and thus increase the probability of crossing construction with existing subway lines and underground pipelines.However,due to the limitation of construction conditions and geological conditions,short-distance crossing construction has gradually become the choice of subway line construction.So far,there are more and more cases of short-distance crossing construction,but there are few engineering examples of shield tunneling through existing tunnels in soft-flow plastic strata,so it is necessary to study the influence of tunnel underpass construction on existing tunnels in this stratum.Due to the poor self-stabilization ability of the surrounding rock and the low strength of the medium cementation,the soft-flow plastic formation has great disturbance after excavation and unloading,which will threaten the safety and service performance of the existing structures in the upper part of the soft-flow plastic formation.Based on the model test and numerical analysis,the deformation characteristics and control techniques of existing tunnels and strata during the construction of the new tunnel are studied based on the tunnel project of No.5 Shield under Line 1 of Nanjing Metro.The method of model test and numerical analysis is used to study the deformation characteristics and control techniques of the existing tunnels and strata during the construction of the new tunnel.The main results are as follows:(1)The model test of shield tunnel under the existing tunnel is carried out by controlling the net distance of tunnel and the rate of formation loss,and the experimental data are analyzed,and the influence degree of two kinds of factors on the ground settlement、deformation of arch roof of existing tunnel structure and surrounding rock pressure around existing tunnel are obtained.and then the law of variation with excavation process is summarized as follows: the deformation curve of surface and existing tunnel arch roof is "V" type.The formation at the midline position is greatly disturbed by the secondary disturbance.The pressure deformation curve of the surrounding rock is "W" type,and the soil mass in the middle of the new tunnel plays the role of "pillar".(2)In order to further study the influence of shield construction on stratum and existing tunnel deformation,finite element software is used to carry out numerical calculation of shield construction process.The lateral strata deformation law of no existing tunnel and existing tunnel,and the vertical and horizontal deformation law of arch roof,arch bottom and arch waist of existing tunnel are analyzed: the existence of existing tunnel has a certain reinforcement effect on the stratum;When the tunnel is close to the new tunnel,the vertical displacement curve is "W" type,and the vertical displacement curve is "V" type when the tunnel is far away,and the horizontal displacement of the tunnel is smaller,and its law changes with the movement of the excavation surface.(3)The numerical results are compared with the model test results in order to obtain more reliable results.Based on this,the influence of new tunnel construction on the existing tunnel under different tunnel net distance and different surrounding rock conditions is analyzed: the deformation of existing tunnel decreases gradually with the increase of tunnel net distance,and the larger the surrounding rock strength,the smaller the deformation value of existing tunnel.(4)Through comparing and analyzing the strengthening effect of three kinds of prestrengthening measures,the optimum scheme of strengthening effect is obtained: the pipe shed cement-soil mixing pile and sleeve valve grouting combined reinforcement.On this basis,the influence of construction parameters,soil chamber pressure and grouting pressure,on the deformation of existing tunnels is studied,and a reasonable parameter range is given: the soil cabin pressure is controlled in 0.19 MPa or a little more than 0.19 MPa.The grouting pressure is controlled at about 1.0 times vertical earth pressure. |
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