Study On Action Mechanism And Bearing Characteristics Of Composite Ground With The Load Of Embankment In Soft Loess Area

Composite ground technology has been widely used in expressway construction. But the theoretical study on composite ground is lagged behind engineering practice, especially the study on working mechanism of composite ground under embankment or flexible foundation. Mechanical and deformation behaviors of composite ground are studied with the means of geotechnical centrifugal model test, static loading test, prototype test and numerical analysis. At last the methods of calculating bearing capacity of composite ground are proposed.Load transfer mechanism and relative displacement mode of different kinds of composite ground and the influence of foundation stiffness and substratum thickness are studied on the base of geotechnical centrifugal model test. The distribution of upward frictional resistance of suspended pile under rigid foundation shows inverted triangle shape when the load-level is lower and S shape when the load-level is higher. Pile and hemispherical soil mass at pile tip piece into the substratum together. The failure of composite ground under rigid foundation is caused by piles piecing into substratum. There is downward frictional resistance over the depth of 1/6~1/3 times of pile length and upward frictional resistance below that depth under flexible foundation. The distribution of upward frictional resistance shows inverted triangle shape when the load-level is lower and shows saddle shape when the load-level is higher. The neutral point at which the settlement of pile and that of soil among piles is equal appears at the depth of 1/6~1/3 times of pile length. The distribution of downward frictional resistance of bearing pile under flexible foundation shows inverted triangle shape when the load-level is lower and shows inverted trapezoid shape when the load-level is higher. There is obvious upward penetration at the top of pile.Deformation and bearing characteristics of composite ground under different size of loading plate is revealed through a series of in situ tests on original soil, soil among piles, single pile, from one pile compound to six piles compound. Original saturated loess and soil among piles show obvious second yield phenomenon, which can be used in the design of composite ground. The ultimate bearing capacity of soil among piles is 2 times of that of original soil. The characteristic bearing capacity of composite decreases with the growth of size of loading plate and the properties of site have obvious effect on it. Because of the cushion under loading plate, the pile-soil stress ratio of static loading test presents the properties of pile-stress ratio under flexible foundation. The stress-bearing capacity ratio of soil among piles is about 1.0~1.5, and that of piles is 0.3~0.8. Relative displacement mode between soil and pile dynamically changes with the load-level improving. The bearing capacity of soil among piles at road shoulder and toe of slope is underutilization. Working behaviors of composite ground under different size of loading plate are investigated using numerical analysis. The relation between settlement of composite ground and width of loading plate can be described by hyperbolic curve. The neutral point at which the settlement of pile and that of soil among piles is equal moves up and the growth point of settlement downward shifts with the increasing of width of loading plate. With the growth of loading plate width, stress under rigid foundation decreases first then increases and that under flexible foundation increases first then decreases. Pile-soil stress ratio at the top of pile under rigid foundation increases first then decreases and that under flexible foundation decreases first then increases. The pile-soil stress ratio at the bottom of pile under both kinds of foundation decreases with the growth of loading plate width.The limitations of the methods, static loading test and calculation formula, to determine the bearing capacity of composite ground is pointed out. And two kinds of modified methods which are suitable to determining the bearing capacity of composite ground under embankment or flexible foundation are proposed.