Study On The Mechanism About Vertical Shaft Resistance Of Root-reinforced Pile

With the rapid development of urbanization in our country,the modern urban scale continues to expand,in particular the scale of the high-rise building,which requires to control foundation bearing capacity and settlement deformation in complex geological conditions,our country has many kinds of complicated geological conditions,which means the reinforcement treatments of foundation design are putting forward to new and higher requirements.The method of using pile foundation to treat the complex soft soil foundation can shorten the construction period,reduce the settlement of the building and improve the bearing capacity of the foundation,while the root-reinforced pile combines the advantages of the high strength of the concrete core pile and the high lateral friction resistance of the wrapping material,giving full play to the advantages of the two,which has attracted extensive attention in recent years.In this paper,the model tests are carried out by designing root-reinforced piles with different sizes of expansion,and using the finite element numerical simulation method to study the effects of expansion size and material on the lateral friction mechanism of each pile under vertical load.According to the static load test of three piles in a certain engineering project in Zhengzhou,the corresponding pile-soil model was established by combining with the field conditions and the numerical simulation,the vertical bearing characteristics of the pile type were studied,and the method of determining the lateral friction coefficient of the expanded root piles was proposed.The following research results were obtained:?1?According to the length,diameter and material of the extension,the vertical ultimate bearing capacity of single pile can be significantly increased.The increase of the extension length has limited effect on improving the ultimate bearing capacity of a single pile,and it can reduces the load proportion of the tip resistance to a certain extent,but it has little effect on the side friction resistance.The increase of the diameter of the extension can significantly improve the ultimate bearing capacity of a single pile,and the load ratio of the tip resistance increases slightly.When the expansion material is concrete or cement mortar,the bearing capacity of the root pile is basically the same,while the cement soil is used as the expansion material,the settlement of the pile increases more than others,the development of the lateral friction resistance in the upper part of the expansion section is limited,and the proportion of the tip resistance also decreases.?2?The corresponding pile-soil model was established by using the finite element software combined with the field static load test results,and the validity of the model was verified by the comparison of the load-settlement curves,and the development process and variation law of the pile axial force,pile lateral friction resistance and pile tip resistance in the loading process were analyzed.The results show that the root-reinforced pile in this project is friction pile.In the early stage of loading,the lateral friction resistance at the lower part of the unexpanded body section plays a low role,while it tends to increase at the later stage of loading.The lateral friction resistance of the expanded part and non-expanded part are almost synchronous in the loading process.The pile lateral friction resistance in the expanded section is significantly increased,and the expanded section has a significant effect on improving the ultimate bearing capacity of single pile.?3?According to the test and numerical simulation results,the bearing capacity formula is used to check the performance coefficient of?_sj for the expanded section.The result shows that the variation of the length of the expanded section has little impact on the performance coefficient of?_sj.There is an optimal expansion length that can make?_sj reach the maximum value,and the increase of expansion diameter can significantly improve?_sj.According to the results of field static load test and finite element calculation,the compaction and diameter expanding effects of the piles in the construction process are significant in improving the vertical compressive bearing capacity.