Study On Mechanics Properties Of Shallow Buried Tunnels With Small Clear Spacing At Bottom Of River When Double Holes Are Coexisting

In recent years,with the rapid development of underwater traffic tunnels,the small clear spacing traffic tunnels has also expanded from land to the bottom of river,and there has been a small clear spacing traffic tunnels at the bottom of river.Compared with the small clear spacing traffic tunnels on land,the small clear spacing traffic tunnels at the bottom of river should consider not only the interaction of the double hole's stress field,the interaction of the double hole's seepage field and the influence of the seepage-stress coupling,but also consider the impact of changes in river water levels.Mechanics properties of the small clear spacing traffic tunnels at the bottom of river were extremely complicated because of the combination of many factors,and related research needs to be deepened and improved.Mechanics properties of the small clear spacing traffic tunnels built by different methods such as mining method,shield method and immersed tube method were very different.Taking Liu Yang river highway tunnels as the engineering background,this paper researched the mechanics properties of the shallow buried highway tunnels at the bottom of river with small clear spacing in mining method while double holes coexisted.Considering the interaction of the double holes in the small clear spacing tunnels,mechanics properties of the double holes under different clear spacing were analyzed.Analytic calculation and numerical simulation(both verified by field test)were used to study the mechanics properties of surrounding rock and initial lining during construction period,the model test was used to study the mechanics properties of the second lining in service period,so that engineering suggestions for the whole life of the tunnel are proposed.The main research contents and research results of this paper are as follows:(1)Analytical calculation formulas for the surrounding rock pressure of shallow buried tunnels with small clear spacing at bottom of river influenced by river water pressure and double hole's excavation sequence were studied.And factors such as tunnel clear spacing,tunnel depth and river water level were calculated and analyzed.The analytical results showed that the smaller the tunnel clear spacing or the greater the buried depth of the tunnel,the more obvious the local bias phenomenon of the tunnel.With the increase the river water level,the lateral pressure and vertical pressure of each part of the tunnel increase almost linearly.(2)A relatively complex three-dimensional numerical model of seepage stress coupling was established to analyze the mechanics properties of initial lining and surrounding rock during the construction period: measured water level curve of the river was embedded in the numerical model for calculation.The numerical model was divided into 92 mainly large construction steps.The large construction steps and 312 small construction steps had carried out a refined simulation of the construction period.The numerical model considers the simultaneous occurrence of two factors that river water level change and change of the excavation surface position during the construction period.The numerical simulation results showed that under the influence of the adjacent hole construction,when the upper step of a hole is applied but the lower step of the hole is not applied,maximum compressive stress of initial lining of the upper step appears in the arch.The maximum effective plastic strain,the initial settlement of the vault,and the maximum compressive stress of initial lining increased with the decrease of clear spacing.It was recommended that when designing a similar tunnel section,the clear spacing between double hole's can be reduced from 1.5B to 1.0B(B is the width of the excavation section)while keeping the supporting parameters constant.(3)The model test considered the influence of river water level's cyclic change,which was divided into two categories: impervious second lining and pervious second lining.Each category was subdivided into 8 conditions according to the clear spacing of 2.0B,1.5B,1.0B and 0.5B.The water and soil pressures of the second lining and the axial force and bending moment of the second lining were measured.The model test results showed that in the four clear spacing of permeable conditions,the water pressure behind the second lining fluctuated with the fluctuation of the river level above the tunnel.The water pressure behind the second lining was less than the corresponding hydrostatic pressure,which means,the water pressure was reduced.The fluctuation of the soil pressure behind the second lining was not decreased with the fluctuation of the river water level.The bending moment at the bottom of second lining arch has a large fluctuation range in different clear spacing,and values of the bending moment were in a large numerical value,this position may become the first part of damage during the service period of the tunnel.When second lining was permeable,the soil pressure,the axial force of second lining,and the bending moment of second lining had different degrees of increment compared with the impermeable condition of second lining.It was recommended to reserve grouting pipe behind the second lining when designing a similar tunnel.