| Due to the special structure and water sensitivity of sandy loess.Those make the shallow-buried unsymmetrical pressure sandy loess tunnel have the characteristics of many weak structural surfaces,not easy to arch and uneven force.resulting in poor self-stability of the tunnel surrounding rock.As a result,the tunnel surrounding rock has a poor self-stabilization ability.The initial support and secondary lining of the tunnel are not uniformly stressed.During the construction process,it is easy to cause large-scale settlement and cracking on the surface of the tunnel.These factors have an important impact on the safety and stability of the tunnel.We analyze the stability of large-section,shallow-buried,unsymmetrical loess tunnels.At the same time,research on optimization of excavation method has theoretical value and practical engineering significance.This paper takes the Yinxi high-speed railway Huianbao shallow buried unsymmetrical pressure sandy loess tunnel as the research background.Laboratory tests on the physical and mechanical properties of sandy loess were carried out.Using a combination of model testing and numerical simulation.The mechanical properties and deformation laws of surrounding rock during the excavation of shallow-buried unbiased sandy loess tunnels were studied.And the optimization plan of the arch ring three-step method is proposed.The result shows:(1)The basic physical and mechanical characteristic parameters of sandy loess in the Huianbao tunnel are obtained through laboratory tests.To provide data support for the establishment of tunnel numerical calculations.(2)Using FLAC3D numerical simulation software,the changes of stress,strain and ground settlement of the tunnel under different construction methods are calculated.(3)According to the actual situation of the Huianbao tunnel and using similarity theory,a tunnel similarity model was established.Completed the three-step reserved core soil method and three-step temporary inverted arch method of the tunnel construction process model experiments,and compared with the numerical simulation results.By comparing the results,the optimal excavation method of the Hui’anbao Tunnel was selected.Considering the difficulty of construction and construction efficiency during the actual construction of the tunnel,it is recommended that the Hui’anbao tunnel adopts the three-step reserved core soil method for construction.(4)Both experimental results show that the location of the highest tunnel stress is on the vault,followed by the vault.The vault is under tension in both cases,while the arch is under compression.The displacement of each key point of the three-step reserved core soil method is greater than that of the three-step temporary invert method.(5)An excavation method suitable for shallow-buried and biased tunnels is proposed.The three-step reserved core soil method ensures the safety of construction although the core soil is reserved during excavation.However,during construction,all steps are excavated at the same time on the left and right sides,which has a greater impact on the soil around the tunnel and the arching effect is worse than that of the three-step temporary invert method.The arch ring three-step method can control the deformation of the tunnel while reducing the tensile stress that is not conducive to the stability of the vault.(6)Unsymmetrical pressure of the overlying soil of the tunnel has a greater impact on the deformation of the surrounding rock.Stress concentration occurs on the larger side of the buried depth of the tunnel vault,which makes the surface settlement and deformation asymmetrical.Therefore,the control measures of the bias should be strengthened during the construction. |
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