Ⅵ级围岩浅埋隧道竖向旋喷桩-双层管棚-CRD进洞施工力学特性研究OA

To address the high susceptibility of tunnel portal construction in Grade VI surrounding rock to water and sand inrush,support structure deformation,and even portal collapse,this study proposes a tunnel entrance construction scheme combining vertical jet grouting piles,double-layer pipe roofing,and the Cross Diaphragm(CRD)method,based on a single-bore bidirectional tunnel project.A nu-merical comparison model using the single-layer pipe roofing-Center Diaphragm(CD)method is also established.The mechanical parameters of sandy loess are determined through laboratory tests,and construction mechanical simulations of the full tunnel excavation process are conducted together with field monitoring.The results show that reinforcing the portal slope with vertical jet grouting piles ar-ranged in a quincunx pattern,together with double-layer advance pipe roofing to form a composite canopy support system,can substantially improve the overall stability of the portal slope and enhance the pre-support capacity of the surrounding rock ahead of the tunnel face.Compared with the original single-layer pipe roofing-CD method,the load carried by the pipe roofing decreases from 58.54 kN to 27.29 kN,representing a reduction of 53.38%;the temporary support stress decreases to 2.95 MPa;and the extrusion deformation of the tunnel face decreases from 10.47 cm to 3.17 cm,a reduction of 69.72%.These improvements significantly mitigate the risks of surrounding rock instability,water in-rush,and collapse.Tunnel excavation induces stress redistribution in the surrounding rock,which can readily lead to a sharp increase in support stress.Following the construction principle of"rapid excava-tion and rapid support,sectional excavation and ring closure,"the CRD method enables rapid closure of the primary support structure and accelerates the convergence of stress and deformation in the sur-rounding rock–support system,demonstrating a pronounced effect in controlling deformation in ex-tremely weak rock tunnels.Compared with the original CD method,the surrounding rock stress de-creases from 6.95 MPa to 1.55 MPa,the internal force of the steel arch decreases from 93.94 kN to 63.74 kN,and the primary support stress decreases from 5.59 MPa to 1.25 MPa,corresponding to a reduction of 77.64%.The combined support system achieves three-dimensional pre-reinforcement of the surrounding rock through vertical jet grouting piles and double-layer pipe roofing,while the CRD excavation method effectively reduces the exposed sliding range of the tunnel face.Under these com-bined effects,extrusion deformation of the tunnel face is effectively constrained,thereby reducing the risk of tunnel collapse at the source.Field monitoring data indicate that,under the combined support measures,the peak values of crown settlement,surrounding rock stress,and steel arch internal force are 15.76 mm,1.58 MPa,and 61.16 kN,respectively,and the overall support system remains safe and stable.The numerical simulation results agree well with the field monitoring results,verifying the reliability of the vertical jet grouting pile-double-layer pipe roofing-CRD tunnel entrance construction technique for shallow-buried tunnels in Grade VI surrounding rock.