| With the mass operation of the metro tunnel,the cut-off pile has a broad prospect for application in the current large construction environment.At present,there are few experimental studies and practical applications on the influence of foundation excavation controlled by cut-off piles on adjacent tunnels.Therefore,it is necessary to do exploratory study on the influence of foundation excavation controlled by cut-off piles on adjacent tunnels,so as to provide related basis for its application in foundation pit engineering of adjacent tunnel.Based on the governing equation,this paper deduces the similarity of tunnel and enclosure structure,this paper conducts model tests on the excavation of the inner convex shape of foundation pit of soft soil layer and cantilever arm type of foundation pit of sandy soil respectively.Under the two conditions of foundation and according to the layout characteristics of cut-off piles,the conditions of non-isolated pile,cut-off pile without deeply buried and cut-off pile with deeply buried are respectively set for grouping model tests.The main findings are as follows:(1)Based on thin plate bending differential equation and Winkler elastic foundation beam deflection differential equation,this paper deduces and analyzes the similarity of shield tunnel segment.The effective ratio of circumferential and vertical stiffness of shield tunnel are summarized.Based on the deflection differential equation,this paper analyzes and derives the similarity of retaining structure.(2)Through the foundation excavation tests under various working conditions,the horizontal displacement of the convex foundation pit is parabola.The horizontal displacement of sand cantilever foundation retaining structure is cantilever.The measured data show that the addition of the cut-off pile makes the bending moment and horizontal displacement of the retaining structure decrease,and the control efficiency of the cut-off pile without deeply buried is obviously superior to the cut-off pile with deeply buried.The analysis shows that the effect of cut-off pile reduces the bending moment and horizontal displacement of the retaining structure.Compared to the cut-off pile without deeply buried,the lateral barrier effect of the cut-off pile with deeply buried have a greater role.(3)Through the foundation excavation tests under various working conditions,it is found that the horizontal displacement and vertical displacement of the tunnel increase with the increase of the tunnel depth ratio(foundation pit depth/tunnel depth).After the excavation of the foundation is completed,the horizontal displacement and vertical displacement of the tunnel increase by 23.1 % and 38.6 % respectively with the addition of the cut-off pile without deeply buried in the parabolic foundation of soft soil,and the horizontal displacement and vertical displacement of the tunnel decrease by 71.8 % and 63.0 % respectively with the addition of cut-off pile with deeply buried.The analysis shows that the cut-off pile without deeply buried are located in the arch horizontal deformation area,the length of the pile is larger,the tunnel is mainly located in the active earth pressure development area,and the horizontal displacement and vertical displacement of the tunnel increase.The pile length of the cut-off pile with deeply buried is reduced in the horizontal deformation area of the arch.At this time,the tunnel is located in the passive earth pressure development area,and the horizontal displacement and vertical displacement of the tunnel are both small.The horizontal displacement and vertical displacement of the tunnel were reduced by 29.5 % and 18.1 % respectively with the addition of the cut-off pile without buried depth in the cantilever foundation of sandy soil,and the horizontal displacement and vertical displacement of the tunnel were reduced by 51.2 % and 60.2 % respectively with the addition of the cut-off pile with buried depth.The analysis shows that the cut-off pile without deeply buried is located in the cantilever horizontal deformation zone,the length of the pile is large,the embedded section of the pile body reduces the buried depth of the pile tip,and the horizontal displacement and vertical displacement of the tunnel are reduced.The length of the cut-off pile with deeply buried in the cantilever horizontal deformation zone decreases,and the horizontal displacement and vertical displacement of the tunnel further decrease.(4)Through foundation excavation tests under various working conditions,the change rule of the tunnel circumferential bending moment with the tunnel depth ratio is obtained.The measured data show that after the excavation of the foundation pit,the maximum value of the circumferential bending moment in the tunnel increases by 3.8 % and the minimum value of the bending moment decreases by 132.6 %,and the maximum value of the circumferential bending moment increases by 13.2 % and the minimum value of the bending moment decreases by 48.8 % with the addition of the cut-off pile with deeply buried.After the foundation excavation is completed,the maximum circumferential circumferential bending moment of the tunnel decreases by 45.8% and the minimum bending moment increases by 25.2 % with the addition of the cantilever foundation of cut-off pile without deeply buried in sandy soil.The maximum circumferential circumferential bending moment of the tunnel decreases by 53.0 % and the minimum bending moment increases by 48.3 % with the addition of cut-off pile with deeply buried.The analysis shows that the additional circumferential bending moment of the tunnel can be reduced to some extent when the tunnel is located in the passive earth pressure development zone,and the circumferential bending moment of the tunnel is increased when the tunnel is located in multiple active and passive development zones.(5)Based on the measured results,and the Rankine earth pressure theory,the action mechanism of the cut-off pile is analyzed.When the tunnel is located in the active earth pressure development zone,the horizontal displacement and vertical displacement of the tunnel increase.When the tunnel is located in the passive earth pressure development zone,the horizontal displacement and vertical displacement of the tunnel decrease.It is proposed that the tunnel should be placed in the passive earth pressure development zone when the pile top is set deeply,and the tunnel is located in the zone as a whole,and the balance between the tunnel displacement and the annular bending moment control rate should be considered.(6)In combination with the measured data and the domestic 6 foundation engineerings which set the cut-off pile,this paper adopts the cut-off pile statistics to analyze the retaining structure horizontal displacement factors,obtains the deep pit pile length ratio(foundation pit depth/ cut-off pile length),the ratio of the hole depth and pit length(the foundation pit depth/ cut-off pile and hole length),and cut-off pile bending stiffness equivalent thickness increase and the horizontal displacement of retaining structures decrease;The horizontal displacement of the retaining structure increases with the depth of the pile. |
