By using the FEM software ANSYS, in which the modulus of cushion, the modulus ration between soil on bottom of pile and soil surrounded pile are simulated both in single-pile model and multi-pile model,the regulation of bearing performance of the composite foundation improved by flexible piles are founded:1.for the composite foundation improved by flexible piles, there is the critical modulus for cushion. When modulus of cushion is at critical state, the stress ratio is maximum.2 The more modulus ratio, the bigger stress ratio, in which the function of substitution is made full of use for the composite foundation improved by flexible piles and the bearing performance of the initial foundation is improved.
In the numerical simulation of the composite foundation improved by flexible pile, the bearing performance of it, influenced by factors, such as (1)the cushion, (2)the soil on bottom of pile and the soil surrounded pile, can be reflected by the changes of their modulus. By using the FEM software ANSYS to simulate the modulus' changes of factors, the regulation of bearing performance of the composite foundation improved by single flexible pile and by multi flexible piles is got according to the changes of factors.
For the composite foundation improved by single flexible pile, the one dimensional semi-model is set up to simply calculation, in which the unit is solid 42. Following measures are accepted to make sure the calculation accuracy:
mapped mesh while refining mesh in pile and soil surrounded pile,
calculation limits:1.5 width of cushion cap from the axis to horizontal direction and 2.0 length of pile from the axis to vertical direction[1~2],
there is no horizontal displacement of lateral wall of model and no displacement of bottom of model. The single pile model and meshing is shown in fig.1.Initial value of model's parameters:
deformation modulus: cushion cap 100Mpa, cushion 50Mpa, the soil surrounded pile 6.4Mpa, the soil on bottom of pile 11.8Mpa, pile 330Mpa,
Poisson's ratio: cushion cap 0.15, cushion 0.25, the soil surrounded pile 0.38, the soil on bottom of pile 0.33, pile 0.30,
Cohesive strength: the soil surrounded pile 18.0kPa, the soil on bottom of pile 26.0kPa,
Internal friction angle: the soil surrounded pile 28°, the soil on bottom of pile 28°. For the composite foundation improved by multi piles, the three dimensional one-fourth-model is set up to simply calculation, in which the unit is solid 82, and other initial conditions same as the single pile model. To simply calculation, it is necessary to give some assumptions for the single and multi pile model:
for the same material, it is homogeneous and isotropic;
there are linear elastic for the cushion cap, cushion and pile;
it is the Druker-Prager model for soil;
there is no relative displacement between pile and soil, cushion cap and cushion.