Study On Surface Settlement Control In Shield Construction Of Upper Soft And Lower Hard Composite Strata

For shield tunneling through upper soft and lower hard composite strata,the difficulty of controlling the shield's attitude,the cutting degree of the cutter on the front rock and soil is not consistent,which can easily cause great stratum disturbance and cause large-scale and large-area surface settlement.In addition,the combination of soft and hard layers in space is changeable,and the distribution of surface settlement is more complex.It is difficult to control the surface settlement,which makes the construction safety of shield tunneling and the safety of existing structures above shield tunneling difficult to be guaranteed.This paper takes the shield tunneling project of Yaozhuang Station-Wenboyuan Station of Xuzhou Metro Line 2 as the background,aiming at the problem that the surface settlement is difficult to be effectively controlled when the tunnel passes through the upper soft and lower hard strata for a long distance.By means of theoretical analysis,field measurement and numerical simulation,the surface settlement caused by shield tunneling through the upper soft and lower hard strata is deeply analyzed,the control measures of surface settlement are discussed,and the corresponding control measures for surface settlement of shield tunneling in upper soft and lower hard strata of Yaozhuang Station-Wenboyuan Station of Xuzhou Metro Line 2 are formulated.Based on the comprehensive analysis of the geological characteristics of the upper soft and lower hard strata,the technical characteristics of shield construction,and the mechanism of surface settlement caused by shield,the influencing factors of surface settlement caused by shield crossing through the upper soft and lower hard strata are found out.It includes two aspects:geological factors and construction factors.Combined with the geological environment analysis of Yao-Wen interval,the main influencing factors of the ground subsidence caused by the interval shield construction in the upper soft and lower hard composite strata can be summed up as follows:the space combination of soft and hard rocks on the excavation section,the mechanical properties of the soft and weak strata in the upper part,the construction parameters:Shield thrust,the size of the unfilled clearance and over cut clearance.Based on the analysis of the surface settlement monitoring data in the upper soft and lower hard strata of Yao-Wen interval,it is found that the cross-section settlement distribution curve is similar to the normal distribution curve,and the regression analysis can be carried out by using the Peck formula.The fitting curve of the monitoring data obtained by regression analysis is compared with the prediction result of the traditional Peck formula,and the shape correction factor?₁,?₂ and the center position correction factor?₁ of the cross section settlement trough are introduced to modify the traditional Peck formula.The correction coefficient?₁ ranges from 0.63to 1.3083 for Yao-Wen interval,from 1.31 to 2.32 for?₂ and from-2.78 to 4.06 for?₁.The numerical simulation software FLAC3D is used to analyze the factors affecting the surface settlement of shield construction in soft and lower hard strata.The four main factors in Yao-Wen interval are analyzed.The conclusions are as follows:the higher the proportion of soft rock space in the upper half of tunnel,the higher the surface settlement is;due to the uneven soft-hard ratio in the upper and lower hard strata,the surface settlement caused by shield construction is not uniform.The higher the mechanical parameters of the soft layer,the smaller the surface settlement.The maximum settlement and the width of the settlement trough are mainly affected by the elastic modulus E and cohesion force c of the soft layer.When the supporting force of excavation surface is set to 1.53 times of theoretical calculation,the surface settlement caused by the insufficient support capacity can be effectively controlled.The larger the over-digging clearance and the unfilled shield-tail clearance in the construction process,the greater the surface settlement.Combined with the regular pattern of surface settlement obtained from the measured research and numerical simulation analysis,the corresponding control measures for the surface settlement of the upper and lower hard segments of Yao-Wen interval are formulated:taking the interval mileage K758.05^K795.91 as the longitudinal range,taking the horizontal extension of the center of the two-line tunnel as the transverse range,the soft and weak layers in the enveloping range are strengthened by grouting.The numerical simulation method is used to compare the ground settlement before and after the reinforcement measures are carried out,and it is proved that the reinforcement measures are effective in controlling the surface settlement.