Deformation Behavior Of Deep Excavation In Soil-rock Composite Stratum Under Complicated Conditions

In areas with thin coastal cover,such as Xiamen,Guangzhou,Qingdao,etc.,the excavation of the foundation pit often encounters a thin upper layer,and the alluvial(mainly alluvial)and residual soil layers under the fill layer,reaching the undulating granite down.This typical geological conditions of the layer,is so-called soil-rock composite formation.Under such geological conditions,the middle and lower rock layers are undulating,and different soil-rock interface depths result in different foundation pit deformation results,and the design and construction are difficult.Compared with the soft soil area,domestic scholars have less research on the foundation pit of the soil and rock binary stratum structure.There are only some empirical analysis,and a small amount of research results are far from the actual needs of the engineering construction.This paper relies on a deep foundation pit of a soil-rock composite stratum in Xiamen area.The maximum height difference of the base rock surface of the foundation pit is more than 6.5m.The bottom of the high bedrock surface is 7m above the bottom line of the pit.The bottom of the low bedrock surface is below the bottom line of the pit.2m.Based on field measurement and numerical simulation analysis,the deformation behavior of foundation pit was studied.The main research contents and conclusions are as follows:(1)Using the field measurement method to obtain various deformation data of the foundation pit construction process.Through analysis,the variation law of deformation characteristics of foundation pit under unfavorable geological conditions and construction conditions is obtained.The maximum horizontal displacement of the measured high rock face pile is 70.39 mm,about 0.40%H,and the maximum surface settlement is-26.81 mm.The maximum horizontal displacement of the low rock face pile is 105.42 mm,about 0.58%H,and the maximum surface settlement is-45.95 mm.The lower the bedrock surface is,the more the deformation is affected by the excavation of the foundation pit and the disturbance of the surrounding soil.Underthe unfavorable condition of the anchorage of the first road,the deformation of the retaining pile with insufficient depth of consolidation is cantilevered inwardly inclined.The deformation distribution combined with the translation type,the retaining pile with sufficient embedded depth is inwardly deformed;under complicated working conditions,if the support is unfavorable,the excavation surface is exposed,and the soil reinforcement outside the pit increases the deformation amount.(2)According to the actual working conditions and geological conditions of the foundation pit,FLAC3 D is used to establish a refined three-dimensional numerical model considering the undulation of the bedrock surface,and simulate the deformation of the retaining structure and the surrounding soil during the foundation pit excavation process.The numerical simulation results show that the horizontal displacement of the largest pile on the low rock face is 77.01 mm,and the high rock face area is 112.16 mm,which is in good agreement with the measured results.The rationality of the numerical model is verified.The displacement of the soil after the pile shows the deformation consistent with the retaining pile.Regularity,the deformation of the foundation pit shows obvious spatial characteristics.On this basis,the deep foundation pit and soil deformation modes of the composite rock stratum with large undulations on the bedrock surface are obtained.(3)Studying the influence of various parameters of pile-anchor combined retaining structure on the deformation of foundation pit under the condition of soil-rock composite stratum,the results show that the pre-stress of anchor cable is an important factor affecting the horizontal displacement of the retaining structure.The locking foot force of the lock foot anchor has obvious effect on reducing the horizontal displacement of the pile bottom;the first anchor cable has the largest deformation effect on the restraining structure;the horizontal displacement of the pile top is linear with the increase of the rock-socket depth and the reserved rock shoulder width.The relationship of diminishing;finally,on this basis,the relevant suggestions for the control of foundation pit deformation during construction are proposed.