Stability Analysis Of The Utility Tunnel Foundation Over Crossing The Existing Cancel In Beijing 5th Ring Road

This paper is based on the actual project which the utility tunnel over crossing an existing underground canal,Meanwhile,the software called MIDAS GTS NX is used to calculate whether the slope of foundation pit structure meets the stability requirements and to reduce the potential risk of accidents.In addition,the paper provides an estimate for the following construction.This paper also researches the influence degree between the excavation of foundation pit and the deformation of the canal and evaluate the safety of the structure of the canal used to transfer water from south to north.Finally,this paper provides the experience for similar projects in the future.This paper draws the conclusion by using the software to calculate the foundation pit bottom bounce with the different length-width ratio of soil blocks,that the smaller the length-width ratio,the steadier the foundation pit is.When the length-width ratio reaches a certain ratio,the foundation pit bottom bounce to be stable.When the length-width ratio achieves 3:1,it can keep the balances between the stress,the strain and the displacement.Meanwhile introduce the protective layer of the tunnel which will affect the tunnel seriously.The range of protective layer is 1.2 times of the canal diameter in Beijing area.If tunnel protective layers and foundation pit protective layers have overlap,the tunnel may have the larger sudden deformation.In addition,this paper also discusses how to calculate the additional stress of foundation pit excavation on the existing canal by Mindlin theory,and how to solve the vertical deformation of the existing canal by beam theory on elastic foundation pit.At last,several simple calculation formulas are briefly introduced.When the foundation pit is not supported,the stress concentration phenomenon and the through plastic deformation zone will appear in the soil on both sides.It will result in the instability of foundation pit.However,using bolt support will improve the stability of foundation pit.After foundation pit excavation,The stress at the bottom of the canal is the greatest,and the pressure at the top is the least,so the displacement and deformation are the most serious.There been haven residual deformation in the canal and the upward trend has not changed when the construction completed.In addition,different excavation techniques may lead to "tension" at the top of the canal.However,as the soil body is a loose particle,it cannot provide tension.Therefore,when the phenomenon of "tension" appears in the canal,it indicates that the overburden soil and the canal have substantially separated,and the upper overburden soil can no longer play roles in inhibiting the upward deformation of the canal.Under this condition,the existing canal is prone to sudden deformation.