Analysis Of The Influence Of The Undercut Tunnel Under The Underground Structure In The Soil-rock Combination Strata

In recent years,the underground space has been vigorously developed,resulting in frequent occurrence of new tunnels underneath existing structures.The mechanical response of a newly built tunnel through an existing structure to the existing structure is very complex.Especially in the soil-rock composite stratum,the tunnel is excavated in a narrow space enclosed on three sides composed of the existing structure and the retaining piles on both sides.The stability of the underground structure and the stress and deformation of the underground structure are more complicated.Based on the supporting municipal rail transit construction project in Jinan’s CBD,this paper uses field tests,numerical simulations and stratum deformation theory to analyze the ground deformation and existing structure deformation and stress caused by the construction of a tunnel excavated on three sides in a soil-rock combination stratum.Research on deformation and force.The main research contents include:First of all,by means of on-site monitoring methods to monitor the changes in the undercutting of urban roads,the settlement of underground passages,and the force and lateral convergence of the newly-built tunnels.The results show that the axial force of the steel bar gauge is under compression,the transverse convergence of the tunnel shows inward contraction,and the reading of the steel bar gauge and the convergent deformation of the upper step barely fluctuate when the lower step passes by,and the impact of the construction of the lower bench on the deformation and force of the upper bench is very limited;the superimposition effect of the two-line single-hole tunnel is extremely small.Obviously,the successive excavation of the two tunnels basically does not cause the relative tunnel stratum disturbance.Secondly,the use of finite element software to analyze the impact of the new single-hole tunnel and the horizontal net distance between the retaining piles,the vertical net distance between the vault and the bottom plate,and the tunnel excavation surface area on the ground displacement deformation and settlement trough in the limited space under the three-sided enclosure condition.The influence of the width;and discuss the changes of soil parameters in the“soil-rock”combination strata and the proportion of soft soil in the excavation surface.Studies have shown that,compared with semi-infinite space,the existing structure has weaker restraint influence on the internal strata,and its stratum deformation curve has a smaller difference,only in the value of settlement;compared with semi-infinite space,enclosed The inner stratum and the width of the settlement trough have similar changes,and both show the characteristics of increasing the width of the settlement trough as the relative buried depth z/z₀（z is the stratum depth;z₀ is the buried depth of the tunnel）decreases,and the width of the settlement trough in the enclosure is larger than that in the semi-infinite space.When the vertical clearance between the top and bottom plate is greater than 1 times the tunnel span,it will not be affected by the superstructure.After the excavation of the upper step is completed,the lower the soil strength of the stratum where the anchor bolts are arranged,the greater the settlement of the tunnel vault.For this reason,it is recommended that when using the upper and lower steps to excavate the tunnel,the anchor bolts should be laid in the stratum with higher soil strength after the upper step excavationFinally,the equivalent radius of the non-circular cavern is introduced into the Verruijt formula,and the Park formula is used to modify the values of the ellipticization and radial shrinkage parameters.The actual measurement verifies the feasibility of the modified formula;then the modified formula is introduced into the Pasternak foundation beam in the model,the force and deformation of the underground structure are estimated.In addition,a variety of equivalent methods are used to discuss the equivalent stiffness of existing structures of different materials and different cross-sections,which are verified by numerical simulations,and the results show that the existing structure deformation and additional bending moment calculated by the equivalent foundation beam of tubular structure wall thickness or box structure bottom plate are highly consistent with the numerical results.