Rasearch On The Mechanical Behavior Of The Long-short Pile Foundation Under Vertical Load

Long-short pile foundation is a new type of foundation, it is base on the length of pile foundation, taking into account that two or more than two courses of soil can be used as the the bearing strata we may meet in design, if the pile is located to the shallow bearing stratum, the bearing capacity can satisfy the requirement but the settlement deformation is too large, if the pile is located in the deep bearing courses, then the bearing capacity and settlement will meet the requirements, but too many long piles or super long piles are needed, not only will increase the cost of the project but also will increase the construction difficulty. Therefore, scholars put forward the concept of long-short pile foundation, set up some short piles to fully utilize the bearing capacity of the shallow bearing stratum, and set up some long piles to reduce the foundation settlement deformation. The long-short pile foundation has a good adaptability to bearing capacity, but also has good characteristics in the control settlement, so it is more and more widely used in practical engineering. However, due to the long-short pile foundation is a new type of foundation, so for its theoretical research is not particularly mature. Therefore, Based on the theory of pile-raft foundation, using the finite element software AB AQUS, the stress characters of the length of pile foundation are studied and analyzed, this paper mainly do the following work:(1)Introduced the bearing capacity mechanical of single pile under vertical load, and the interaction between pile and soil is analyzed by the load transfer method. Introduced thin-plate theory,the stiffness matrix of the pile soil raft system is derived.(2) Briefly introduced the ABAQUS in rock and soil engineering application, and using ABAQUS finite element software to establish pile-raft foundation of 3D model, research plastic zone development of pile-raft foundation, the results show that under the vertical load, soil under the long pile advanced into the plastic zone, soil under the short pile second,the soil under raft finally into the plastic zone; The variation of average settlement, differential settlement, load sharing ratio of pile and soil as well as the length of long and short pile axial force along with long and short pile bearing strata, pile spacing and thickness of raft are studied in the study. The results show that with short pile bearing strata compression modulus growth, the average settlement and differential settlement of foundation is decreased, but the decreasing rate is small, the axial force of short pile is increased, the axial force of long pile is decreased, the load sharing ratio of short pile is increased, the load sharing ratio of long pile is decreases; with the long pile bearing strata compression modulus growth, the average settlement and differential settlement are reduced greatly. The axial force of short pile decreases, long pile axial force increases, short pile load sharing ratio decreases, the load sharing ratio of long pile increases; with the increase of pile spacing, which increases the average settlement and differential settlement of foundation, the axial force of the long and short piles increases, the load sharing ratio of long pile is slightly reduced, short pile load sharing increased slightly, but the magnitude is very small, it can be neglected; with the increase of thickness of raft foundation, the average settlement and differential settlement are also decreases, especially differential settlement, axial force of the short pile is decreased, the length of pile axial force is increased, the long pile load sharing slightly decreased and short pile load sharing is slightly increased, but the magnitude is very small, so we can ignore the effect.(3)Analysing an actual project that use the long-short pile foundation, the measured settlement data show that the long-short pile foundation can be a good control of the settlement of the building.