| With the further improvement of the national transportation network,there are more and more urban tunnel projects.As a result,more and more tunnels pass under ancient buildings.Due to the special physical and mechanical properties of loess and its strong regionality,the construction technology requirements for large-section shallow-buried tunnels passing through ancient buildings are high and the risk factor is high.Large-section shallow-buried loess tunnels passing through ancient buildings can easily induce surface subsidence and affect the normal use of buildings.The existing studies on large-section shallow buried tunnels have not considered the effects of stratum properties,tunnel section shape and buried depth on surface settlement.This article takes the Baitashan Tunnel,a key project in Lanzhou City,as the engineering background.Revealing the settlement of tunnel vault,tunnel convergence,surface settlement and deformation law of ancient buildings after excavation by double-side pilot pit method through on-site monitoring.Numerical simulation is used to study the dynamic excavation of shallow tunnels passing through ancient buildings,so as to grasp the dynamic change law of surface subsidence and the deformation mechanism of building foundations.According to the monitoring data of surface subsidence on site,the theory of surface subsidence during tunnel excavation was explored.A theoretical prediction model of surface subsidence suitable for large-section loess tunnels was constructed and verified.Finally,the final surface subsidence of the tunnel passing through the ancient buildings is predicted.The main research contents and results are as follows:（1）Analysis of Surface Subsidence Variation Law of Section NK+602 and Section NK+580 Through Field Monitoring.The shape of the lateral surface settlement curve caused by the excavation of the large-section shallow-buried loess tunnel by the double-side-wall pilot pit method is V-shaped or U-shaped.The maximum surface settlement is 35.72mm and 31.55mm respectively at the central axis position.The closer to the central axis,the greater the cumulative value of the final surface settlement.Comparing the monitoring results of the two monitoring sections,it is found that with the increase of the buried depth of the tunnel,the cumulative value of the maximum surface settlement decreases.Surface subsidence curve transitions from narrow and deep to wide and shallow.The initial support vault settlement of the large-section shallow-buried loess tunnel is smaller than the corresponding surface settlement.The surface settlement caused by excavating the pilot pit first is less than that of excavating the pilot pit later.Excavating the upper pilot pit has a greater impact on the vault settlement and surface settlement than the lower pilot pit.（2）This paper studies the theory of surface settlement in tunnel excavation.If the random medium theory is used to predict the surface settlement of large-section loess tunnel,the uneven convergence should be 2ΔR=1.2u0,and the tangent value of the main influence angle of the stratum is 0.7-0.91,but the error in predicting the surface settlement is large when it is far from the central axis.In view of the large error of the classical Peck formula in predicting the surface settlement of large-section shallow-buried loess tunnel,the average relative error of the modified Peck formula considering the influence of tunnel section shape and buried depth is 8.3%,and the maximum relative error is 18.4%,which shows that the modified Peck formula has good application effect in large-section shallow-buried loess tunnel.（3）Numerical simulation research was carried out on the excavation of a large-section shallow loess tunnel passing through an ancient building under the dynamic double-side-wall pilot pit method.The calculation results show that:the stress distribution law of the surrounding rock excavated by the double-side-wall pilot pit method is vault>arch waist>two side walls.The dangerous construction section of the large-section loess tunnel passing under the building is to excavate the upper right pilot pit and the upper middle pilot pit.Ancient buildings will produce large surface settlement,which is easy to cause uneven settlement and cause ancient buildings to tilt.（4）The analysis of predicting the final surface settlement of ancient buildings shows that when using the stochastic medium theory to predict the settlement of buildings,the main stratum influence angle should be selected as 0.9,and the uneven convergence value should be selected as 2ΔR=2u0.The predicted maximum surface settlement value is 77.4mm,and the maximum settlement difference in the core area is34.2mm.When the modified Peck formula is used to predict the surface subsidence,the stratum loss rate should be 1%,the predicted maximum surface subsidence value is 78.51 mm,and the maximum subsidence difference in the core area is 59.86 mm. |
