Study On Vertical Deformation Characteristics Of Underground Structure Sharing Common Walls With Deep Excavation

The development of underground space in high-density underground structure area will often encounter the situation that the excavation of foundation pit and underground structure share common walls.Subsequently,excavation of foundation pits will lead to excessive uneven uplift and subsidence of underground structure.The existing research on deep excavations mainly focuses on the influence on the adjacent underground structure.In constrast,the deformation characteristics of the underground structure sharing common walls with deep excavation is less studied.The vertical deformation characteristics of underground structure caused by deep foundation pit excavation are studied.The vertical deformation mechanism of common wall caused by excavation is revealed,and prediction method and control measures for vertical deformation are proposed.Results provide theoretical basis and technical guidance for engineering practice.In this paper,numerical analysis,theoretical derivation and field testing are used to study the vertical deformation characteristics of common wall underground structure caused by excavation of deep foundation pit.The mechanism and influence characteristic of vertical deformation of underground structure are discussed.A theoretical prediction method for the vertical displacement of wall caused by excavation of foundation pit is presented.Aiming at the problem of underground structure passing through foundation pit,this paper proposes protection measures,such as setting horizontal partition wall,tied piles and soil improvement,and these measures are evaluated by numerical method.The vertical deformation characteristics of underground structure caused by excavation of foundation pit are elucidated on the basis of engineering cases and measured data.The main contents and conclusions are as follows:(1)The mechanism of vertical deformation of underground structure caused by excavation is revealed.By analyzing the soil displacement under wall,wall-soil relative displacement and vertical displacement of the wall,the mechanism of vertical deformation for underground structure is resolved.The vertical displacement of diaphragh wall could be divided into two parts: the bottom soil rebound and the wall-soil relative slip displacment.The bottom soil rebound depends on soil parameters and the wall-soil relative slip displacment depends on contact face property.Analysis show that in the caly model,the vertical displacement of bottom soil is relatively small and the wallsoil relative displacement is large,which lead to the diaphragm wall subsided.In the sand model,the rebound of bottom soil is relatively large and the wallsoil relative displacement is small,which would cause the diaphragm wall uplifted.When the wall is coupled with other underground structures,its own deformation is affected by the adjacent structures.When the surrounding structure is subsiding,the uplift of the wall will decrease.(2)A practical theoretical calculation method for vertical displacement of underground structure caused by excavation of foundation pit is proposed.Using the Mindlin solution to solve the soil stress field and displacement field caused by excavation unloading,combined with the load transfer model of wall soil,a practical calculation method for the vertical displacement of underground structure caused by excavation excavation is proposed.The analysis shows that,with the increase of soil modulus,the vertical displacement of the soil mass in the pit decreases,which leads to the decrease of the frictional resistance of the wall upward,thus reducing the uplift of the wall.The greater the stiffness of the contact surface,the smaller the frictional resistance on both sides of the wall,the smaller the vertical uplift of the wall.With the increase of the stiffness ratio of the contact surface inside and outside the pit,the frictional resistance of the wall is increased and the vertical displacement of the wall increases.(3)In this paper,the displacement control and protection measures of underground structure under the influence of excavation are proposed and the comprehensive evaluation is carried out.In order to protect the underground structure during the deep excavation on its both side,protection measures such as setting transverse partition wall,bulid tied pile and foundation reinforcement are adopted.A quasi three-dimensional numerical analysis is used in the comprehensive assessment for those measures.The results show that setting partition wall could be increase the underground structure stiffness,but the existence of short partition wall will increase the displacement of underground structure which is connect to the short partition walls during the excavation.It is recommended that the short partiton walls can be build at the end of the large excavation.By setting the tie piles,it can effectively control the uplift of the underground structure.The effect of tie piles for controling the underground structure deformation can be increase with the the depth.Soil reinforcement can effectively reduce the vertical displacement of the soil in the pit,and lead to reduceing the uplift of the underground structure.(4)Based on the measured analysis,the characteristics and development mechansim of the underground structure sharing common wall are explained.Based on the actual engineering cases,the field test is carried out,and results show that the vertical displacement curve of the cut-and-cover underground structure is bell shape during the deep excavation.When the large foundation pit is excavated,the wall is raised by the rebound of the soil in the pit,which leads to the uplift of the underground structure.During the excavation of the small foundation pit,the vertical displacement of the underground structure is raised due to the rebound of soil in the small foundation pit.In addition,the symmetrical unloading scheme can effectively limit the lateral deformation and inclination degree of cut-and cover underground structure.