Application And Research Of Thin Sand Layer As Bearing Layer Of PHC Pipe Pile Composite Foundation

Rigid pile composite foundation is a kind of vertical reinforcement composite foundation,which usually adopts cement base cement material,sometimes composed of concrete or concrete and other admixtures,and the strength of pile body is high.In order to ensure pile-soil interaction,a certain thickness cushion is usually set at the top of the pile.Rigid pile composite foundation has higher bearing capacity and compression modulus than flexible pile composite foundation,and the bearing capacity of composite foundation also has a larger adjustment range.A construction project is located in Liaoning Province,including five high-rise buildings with 24-31 floors and small high-rise buildings in three buildings.According to the geological survey,most of the site soil of the project is the cohesive soil,and only the thin layer of the coarse sand and the close to 50 meters of the coarse sand layer are seen in 20 meters,but the thick sand layer with a depth of 20 meters is buried,the thickness is relatively thin,and the thickness variation is large,so that the foundation type selection of the high-rise building is greatly disturbed.Through the calculation and comparison of the project cost,the project selects the phc pipe pile composite foundation scheme with the thin layer of coarse sand as the pile end bearing layer.As to whether the thin sand layer can be used as the bearing force layer of the pile end,the method of combining the specification method and the on-site load test is adopted to verify that the thickness of the sand layer at the selected site area of the test pile and the thickness of the lower bearing layer at the end of the pile are smaller,The test results show that the bearing capacity can meet the design requirement under the condition that the thickness of the bearing layer at the end of the pile is 1.0m.After the bearing capacity characteristic value of the single pile is determined,the composite foundation bearing capacity and the settlement deformation are calculated,and the results meet the design requirements of the high-rise building.After the treatment of the composite foundation,the static load test of the pipe pile of the composite foundation is carried out.It is verified by the static load test that the characteristic value of the vertical compressive bearing capacity of the single pile and the corresponding settlement deformation meet the requirements.The results of low strain dynamic test are qualified.According to the settlement observation results of five buildings,it can be seen that one year after the completion of the project,the average settlement of 24-story tall buildings is 27.2mm,and the average settlement of 31-story tall buildings is 32.3 mm,which meets the requirements of the national code.It shows that the phc pile composite foundation with coarse sand 5 thin layer as pile end bearing layer can meet the foundation bearing capacity and deformation requirements of high-rise buildings.According to the relevant data of the engineering example,this paper analyzes andstudies the finite element analysis of the thin sand layer as the bearing layer at the end of phc pile composite foundation with abaqus software,and the research results are in good agreement with the engineering examples.When the bearing layer(sand layer)is different,the corresponding bearing capacity meets the design requirements,and the influence of changing the sand layer thickness on deformation is obvious.With the increase of sand layer thickness,the final settlement of foundation decreases.Based on the actual engineering case,through the whole process of engineering geological investigation,scheme design,pile foundation detection,settlement monitoring and numerical simulation,this paper studies the application of thin sand layer(1.2 ? 3.3 m)in thick clay soil as the bearing layer at the end of phc pile composite foundation of high-rise building.The research results show that the scheme is advanced in technology,remarkable in economic benefit,short in construction period and reliable in foundation reinforcement method.It provides valuable experience for similar engineering design in the future.