Analysis And Application Of Composite Structure With Scattered Piles And Soil Nailing

The combined support structure of sparse row piles and soil nailing wall is a kind of foundation pit support structure which increases the distance between traditional row piles and uses appropriate soil nails to reinforce the soil between piles.This new type of support structure breaks through the limit condition that the distance between piles is no more than 2 times the diameter of piles.By reducing the number of piles and increasing the diameter of piles,it can improve the deflection of piles and control the deformation of foundation pit.In recent years,the combined structure of sparse row piles and soil nailing walls has been applied in foundation pit support.However,there is no systematic study on the applicable conditions,internal force and deformation of pile body,interaction between piles and soil nailing walls,impact on surrounding environment and comprehensive cost of this new type of support structure.Based on a 8m deep foundation pit project in front of the alluvial fan in Taihang Mountain,this paper establishes(1)0.8m pile diameter 1.6m pile spacing row piles,(2)1.2m pile diameter 3.6m pile spacing row piles(without soil nails between piles),(3)1.2m pile diameter 4.0m pile spacing row piles(without soil nails between piles),(4)1.2m pile diameter 4.4m pile spacing row piles(without soil nails between piles)and(5)1.2m pile diameter 4.4m pile spacing row piles(with soil nails between piles)Five working condition models are used to calculate and compare the parameters of pile excavation and concrete pouring volume,pile head displacement,maximum tensile stress of pile body,shear force of pile body,earth pressure in front of pile and surface settlement outside the pit.Based on the simulation results of FLAC3 D software and the actual monitoring results,the applicability,safety and economy of the combined support of sparse pile and soil nail wall are explored.The main results are as follows:1)The soil pressure in front of the sparse row pile is higher than that of the ordinary row pile,which is suitable for the stratum with high strength of the soil layer in the embedded section,especially in the upper part of the embedded section,which can provide a large reaction force.Moreover,the higher the soil resistance in the embedded section,the more obvious the advantage of this kind of support structure.The soil resistance of the embedded section is not fully developed when the traditional pile arrangement is adopted in the supporting project;2)Model(2),model(3)and model(4): the excavation volume of pile body,the volume of concrete pouring and the maximum tensile stress of pile body are continuously reduced compared with model(1),the soil resistance in front of pile is well developed,and the surface settlement outside the pit is increased,but all of them are within 32 mm allowed by the code;3)The arching effect of the soil behind the pile is the most significant at-6m position.The soil behind the pile is obviously divided into three parts: tensile failure area,arch body area and arch back area.The farther away from the depth,the less significant the arching effect of the soil is.The soil nails set between the piles have a better sharing effect on the stress of the sparse piles.4)The monitoring data show that the sparse pile is safe,economic and applicable to the supporting project.