Study On Deformation Mechanism Of Underlying Tunnel Caused By Excavation And Unloading Of Long Distance Foundation Pit In Residual Soil Layer

Based on the rapid reconstruction project of Guimiao Road in Shenzhen,by field observation and numerical simulation,this thesis performs comprehensive investigation of the evolution and generation mechanism of the underlying tunnel deformation,and corresponding influential factors due to the long-collinear excavation.In addition,measures for mitigating the tunnel deformations are also proposed.The findings of this thesis can provide reference for controlling the tunnel deformation in similar scenarios.The main contributions are as follows:(1)Based on interpreting the field observations of the case history,the mechanism behind the influence of the long-collinear excavation on the underlying tunnel is revealed.It shows that normalized tunnel heave(i.e.,tunnel heaveδ_t/initial cover depth C)and unloading ratio(i.e.,excavation depth H_e/C)exhibit an exponential relationship,which is further adopted as an empirical formulation for estimating tunnel heave.The excavation-induced tunnel heave exhibits appreciable time effects.The tunnel heave continuous to develop after the excavation is completed,and the average post-excavation tunnel heave accounts for about 48%of the final value.Furthermore,the severity of water leakages of the tunnel lining shows positive correlation with the magnitudes of tunnel convergence.(2)Based on explicit numerical simulation,evolution mechanism behind the ground stress,excess pore-water pressure and tunnel convergence due to the step-by-step overlying excavation are revealed.It indicates that negative excess pore-water pressure of soils below the excavation base increases.Effective vertical stress of soils above and below the tunnel gradually decreases,while the differences between soils above and below the tunnel increases.Earth pressures acting on the tunnel decreases,and the reduction amplitudes nearby crown and invert are much greater than these neighboring springlines.Tunnel heave and convergence increases,and the transverse shape of the tunnel transfers from“horizontal oval”to“vertical oval”.After excavation,negative excess pore-water pressure gradually dissipates and the effective/total vertical stress on the tunnel decreases,associated with growing tunnel convergence.As the superstructure begins to be constructed,effective vertical stress of soils below and above tunnel exhibit appreciable increases,notably that above tunnel crown.Besides,negative excess pore-water pressure of soils below excavation bases becomes to be positive.Earth pressures on the tunnel show significant growth,especially these nearby tunnel crown and invert,which mitigating the tunnel vertical convergence.(3)By numerical analyses,arch zone and loosened zone below the excavation base are depicted based on existing methods,and the transfer mechanism of the ground stress within arch zone is revealed.It is found that excavation can cause evident soil arching effects,notably for narrow excavation(width of 2.5-5.5D,D is tunnel diameter).Based on the generation mechanism,two types of arch zone(arch A and arch B)exists below the excavation base.Arch A and arch B can transfer the ground stress at arch foot to arch crown.Curve of ground heave below the base versus burial depth shows inflection point,beyond which is referred to as loosened zone.The disturbed zone evaluated by shear strain level is close to the loosened zone.Curves of tunnel deformation versus relative position with respect to loosened zone have three pieces.As the tunnel is entirely located within loosened zone,the tunnel heave and vertical convergence experiences appreciable growth.(4)Influential factors,including excavation size,excavation-tunnel relative position,soil permeability,and construction time of superstructure after excavation,of the long-term tunnel deformation after the excavation is completed are analyzed.It indicates that the excavation width and depth exhibit positive correlation with the stable time for tunnel heave,while soil permeability and vertical distance of tunnel axis from excavation base show negative correlation.Besides,the growth amplitudes of tunnel heave after excavation is completed increases with increasing excavation depth and construction time of superstructure after excavation.In comparison,vertical distance of the tunnel axis from the excavation base shows negative correlation with the growth amplitudes,and the soil permeability has negligible impacts.Based upon the aforementioned results,this thesis proposes several measures for mitigating the tunnel deformation in terms of the pre-excavation,excavation,and post-excavation stages.