A Simplified Method for the Stress Analysis of Underground Transfer Structures Crossing Multiple Subway Tunnels

According to the design specifications,the construction of extended piles involves traversing the tunnel’s upper region and extending to the underlying rock layer.To address this challenge,a subterranean transfer structure spanning multiple subway tunnels was proposed.Deliberating on the function of piles in the transfer structure as springs with axial and bending stiffness,and taking into account the force balance and deformation coordination conditions of beams and plates within the transfer structure,we established a simplified mechanical model that incorporates soil stratification by combining it with the Winkler elastic foundation beam model.The resolved established simplifiedmechanicalmodel employed finite difference technology and the Newton-Simpsonmethod,elucidating the mechanical mechanism of the transfer structure.The research findings suggest that the load carried by the upper structural columns can be transferred to the pile foundation beneath the beams through the transfer structure,subsequently reaching the deep soil layer and ensuring minimal impact on adjacent tunnels.The established simplified analysis method can be used for stress analysis of the transfer structure,concurrently considering soil stratification,pile foundation behavior,and plate action.The pile length,pile section size,and beam section size within the transfer structure should account for the characteristics of the upper load,ensuring an even distribution of the beam bending moment.

Failure precursor of surrounding rock mass around cross tunnel in high-steep rock slope

The stability of the surrounding rock mass around cross tunnel in the right bank slope of Dagangshan hydropower station, in the southwestern China, was analyzed by microseismic monitoring as well as numerical simulations. The realistic failure process analysis code （abbreviated as RFPA3D） was employed to reproduce the initiation, propagation, coalescence and interactions of micro-fractures, the evolution of associated stress fields and acoustic emission （AE） activities during the whole failure processes of the surrounding rock mass around cross tunnel. Combined with microseismic activities by microseismic monitoring on the fight bank slope, the spatial-temporal evolution and the micro-fracture precursor characteristics during the complete process of progressive failure of the surrounding rock mass around cross tunnel were discussed and the energy release law of the surrounding rock mass around the cross tunnel was obtained. The result shows that the precursor characteristic of microfractures occurring in rock mass is an effective approach to early warn catastrophic damage of rock mass around cross tunnel. Moreover, the heterogeneity of rock mass is the source and internal cause of the failure precursor of rock mass.

Effect of insufficient tunnel crown thickness on the post-tensioned concrete lining of the Yellow River Crossing Tunnel and its strengthening schemes

The effect of deficiency in tunnel crown thickness on the Yellow River Crossing Tunnel with post-tensioned concrete inner lining was investigated by the elasto-plastic finite element method. Changes in the deformations and circumferential stresses of the post-tensioned concrete inner lining with the gradual decrease of the tunnel crown thickness were compared, and the potential bearing risk of insufficient tunnel crown thickness for the Yellow River Crossing Tunnel was revealed. Based on the finite element calculation results of circumferential stresses under different defective cases, the corresponding reinforcement schemes were proposed. The calculation results show that the inner lining can still maintain a satisfactory stress state when the tunnel crown thickness is equal to or greater than 0. 28 m. When the tunnel crown thickness decreases below 0.28 m, the external surface of the crown and internal surface of the crown＇s adjacent areas may be under tension. The tension stresses will incrementally increase and ultimately exceed the tensile strength of the inner lining concrete as the tunnel crown thickness further decreases gradually. Then, the Yellow River Crossing Tunnel cannot operate normally, and severe cracking, leaking or even failure may occur. When the tunnel crown thickness is equal to or greater than 0.28 m, the reinforcement suggestions are that the void spaces between the inner lining and the outer lining should be back-filled with concrete. When the tunnel crown thickness is less than 0. 28 m, the inner lining should be reinforced by steel plates after concrete back-filling.

Interaction analyses between tunnel and landslide in mountain area

This paper focuses on the analytical derivation and the numerical simulation analyses to predict the interaction influences between a landslide and a new tunnel in mountain areas. Based on the slip-line theory, the disturbance range induced by tunneling and the minimum safe distance between the tunnel vault and the sliding belt are obtained in consideration of the mechanical analyses of relaxed rocks over the tunnel opening. The influence factors for the minimum safe crossing distance are conducted,including the tunnel radius, the friction angle of surrounding rocks, the inclination angle of sliding belt,and the friction coefficient of surrounding rocks. Secondly, taking account of the compressive zone and relaxed rocks caused by tunneling, the Sarma method is employed to calculate the safety factor of landslide. Finally, the analytical solutions for interaction between the tunnel and the landslide are compared with a series of numerical simulations, considering the cases for different perpendicular distances between the tunnel vault and the sliding belt. Resultsshow that the distance between the tunnel vault and the slip zone has significant influence on the rock stress and strain. For the case of the minimum crossing distance, a plastic zone in the landslide traversed by tunneling would be formed with rather large range, which seriously threatens the stability of landslide. This work demonstrates that the minimum safe crossing distance obtained from numerical simulation is in a good agreement with that calculated by the proposed analytical solutions.