Research On Key Technology Of Pile-Anchor Retaining And Protection Structure System For Soil Rock Combination Deep Excavations

In the mountainous,hilly and coastal areas of China,the bedrock depth is generally shallow,and the excavations often involves the soil-rock composite strata with soil on the top and rock on the bottom,which are called soil rock combination excavations.Although such excavations are widely distributed,but the calculation methods and theories are not mature,and a few research results are difficult to meet the actual demand of engineering construction.This paper comprehensively applies the methods of field measurement,numerical simulation,model test and theoretical derivation to study the pile-anchor retaining and protection structure system for soil rock combination deep excavations,including the law of deformation,key influence factors,wedge bearing characteristics and the calculation method of the retaining and protection structure,etc.The main conclusions and innovative results are as follows.(1)Based on a soil rock combination deep excavations project with a depth of38.5m in Qingdao,the on-site test data such as prestressed anchor axial force,pile top displacement,ground settlement and excavation conditions in 4 typical sections show that: with the excavation,the axial force of rock body anchor varies very little,within5% of the locking value;Without considering the structure of the rock body,simplifying the design of rock body into soil according to the equivalent internal friction angle is defective;The horizontal displacement of pile top increases linearly with the excavation depth and the pile top tends to settle with the excavation;The ground settlement increases with the excavation,and the settlement curve is groove-shaped.(2)Based on the measured data,an accurate numerical simulation analysis method is found.Through the finite element numerical simulation of 4 typical sections,the deformation law of pile-anchor retaining and protection structure in soil rock combination deep excavations is deeply studied.The results show that: when the excavations is dug to the base,the curve of the horizontal displacement of pile body is roughly arched toward the pit.The influence of three key factors on the excavations deformation is analyzed.The results show that the horizontal displacement of pile body and ground settlement decrease with the increase of rock-socketed depth of pile end,and the suitable rock-socketed depth of pile end is 1.2d～～1.5d(pile diameter);The same is true of the influence of the width of rock shoulder,and there are suitable rock shoulder widths ranging from 1.0m to 1.5m.The horizontal displacement of pile body increases with the decrease of prestress of anchor,and the magnitude and range of displacement change are related to the rock-socketed depth and the width of rock shoulder.(3)The sliding damage mode of the wedge in the lower part of soil rock combination deep excavations was investigated by using 1:20 model test.By varying five factors such as soil thickness,wedge size,soil type,ridge inclination and structural surface angle,12 groups of tests were carried out.The results show that:when the cohesion is zero,the failure surface has nothing to do with the thickness of the filling soil,and all of them are part of a certain definite surface in space,and an analytical expression for the surface is proposed;On this basis,the soil sliding failure surface above the top surface of the wedge is modeled.(4)Based on the calculation theory of elastic foundation beam,it is proposed that the calculation of internal force and displacement of soil rock combination deep excavation retaining and protection structure should be calculated according to the different depth ranges of the excavation surface in the pit,and should be calculated in4 stages by considering the constraints under the pile and rock shoulder resistance under different working conditions;The ground settlement is related to the horizontal displacement of pile body,which can be calculated by skew distribution curve model,and its parameters w are suggested to be 0.5～0.6;It is proposed that the calculation method of the rock shoulder resistance considering the single structural surface of rock body,the analytical formula of the wedge stability calculation,and the solution method of soil pressure on the wedge under the limit equilibrium state.