The Research On The Calculation Method For Geosynthetic-encased Gravel Pile Composite Foundation

Gravel pile can raise the foundation bearing capacity, accelerating the soft foundation consolidation settlement for the convenient materials, simple construction and low cost, so gravel pile is used widely in soft foundation reinforcement project. Gravel pile body’s formation and the bearing capacity of pile play are subject to the lateral binding limits of soils around the pile. Under vertical load, when the strength of soil around pile is lower and can not provide adequate lateral binding, gravel pile easily happen lateral deformation and caused the whole composite foundation failure. Studies have shown that:gravel pile deformation is easily occurred in lateral deformation from pile top1-3times the depth of pile diameter scope. In order to fully exert gravel pile bearing potential and advantages, can consider parcelling geogrid sleeve around the gravel pile top near certain depth range to form stiffened gravel pile, effective control of pile lateral deformation, so as to improve the single pile bearing capacity and gravel pile composite foundation bearing capacity. This paper is based on the concept of that, the concept of geosynthetic-encased gravel piles is proposed,and researched the calculation method of geosynthetic-encased gravel piles composite foundationThis paper firstly analyzed reinforcement mechanism of gravel pile composite foundation. According to the gravel pile’s vertical and radial deformation characteristics, concluded geosynthetic-encased gravel pile’s load-bearing characteristics and deformation characteristics, and discussed geosynthetic-encased gravel piles composite foundation’s load transfer mechanism. Based on this, it further analyzed the action mechanism of cushion of gravel pile composite foundation, and discussed work principle between pile-soil-cushion under the load of embankment etc.Secondly, in view of deformation characteristics of geosynthetic-encased gravel piles composite foundation, researched the effection of foundation stiffness to the upward puncture of piles. On this basis to explore friction of pile side and pile-soil deformation, then according to the relationship of deformation compatibility condition between pile-soil-cushion, introduced the concept of "neutral point". On the basis of load transfer method, the piles and the surrounding soil were assumed to be linear elastic masses, and the distributions of pile-lateral frictions above and under the equal settlement section were assumed to be linear along the pile, respectively. On the basis of the deformation coordination of pile-soil-cushion and the load transfer method, a calculation formula of the pile-soil stress ratio was proposed taking into consideration of positive and negative skin friction resistance on piles, the upward puncture and the bottom puncture deformations of piles, diameter and length of piles and soil layer property, etc.Again, through comparing deformation characteristics between common gravel pile composite foundation and geosynthetic-encased gravel piles composite foundation, several kinds of failure modes was investigated. Failure mode1is mainly controlled by the strength of geogrid sleeve, and failure mode2is controlled by the pile-lateral friction of geosynthetic-encased stone column and the end bearing capacity of common stone column before the damage of drum deformation. Based on brauns’calculation theory of the ultimate bearing capacity of single pile, and according to limit equilibrium theory in different failure modes a calculation formula of the ultimate bearing capacity of geosynthetic-encased stone columns was proposed. And the least calculation result in several different failure modes is as the result of eosynthetic-encased gravel piles’bearing capacity.Finally, through an actual project of eosynthetic-encased gravel piles composite foundation, calculation formula of bearing capacity have be verified of reasonable. The influencing factor to bearing capacity of eosynthetic-encased gravel piles composite foundation was analyzed, and load-bearing characteristics in different failure modes of eosynthetic-encased gravel piles composite foundation was studied, to provide reliable basis and experience for the relevant design and construction.