The Analysis Of Working Mechanisms Of The Bi-directional Reinforced Composite Foundation

Problematic soft grounds were highly concerned with the prevailing construction of highways and high-speed railways.Bi-directional reinforced composite foundation(BDRCF)was extensively adopted to improve the performance of soft grounds by combining the horizontal reinforcement and vertical columns,the design theory and experimental investigation for BDRCF thrived as well.Because of the diversity of types and complicated mechanism,exclusive investigation on this ground improvement technique is very necessary and crucial.With the support of National High Technology Research and Development Program(863 Program)titled as “Bi-directional reinforcement treatment technique for massive uneven roadway soft ground controlled by settlement(2006AA11Z104)”,researches involving the bearing capacity,load and deformation,consolidation and laboratory model tests for BDRCF under embankment were carried out respectively.The working behaviors of components comprising of the BDRCF under embankment were analyzed;the calculation model was established by considering the embankment-reinforced cushion-columns-soils as an entirety.The column-soil stress ratio above and under the reinforced cushion was derived through the static equilibrium of column and soil elements and boundary conditions,and by accounting for the net effect of reinforced cushion,the equal settlement level inside the stabilized zone,the skin friction characterized by Berrum formula.On the basis of theoretical computation results,the bearing characteristic of BDRCF as a whole was furtherly evaluated.The existing embankment soil arching theories were essentially defected by th e deviated calculation results from each other induced by the selection of soil arching models and the position of plastic hinge and the plastic state.The Hewlett soil arching theory cannot take the cohesion of overlying fillings into account,and was imp roved by introducing the dual shear strength theory.This improved soil arching theory was used to determine the load bearing ratio when the plastic hinge located at the arch top and the column top respectively by considering two different boundary conditions in analyzing plastic hinge at the column top.Calculation results were found to agree well with the measurements for a practical case.The horizontal reinforced cushion was idealized and modeled by a thin plate overlying the elastic ground.When the ground settlement is moderately small subjected to the embankment loads,the small deflection thin plate theory was used to solve the deformation of reinforced cushion based on the reciprocal theorem of work and the finite difference principle.The modelling effectiveness was verified in a comparison with measured results of a practical case.When the ground settlement is excessively large due to the high embankment or deficient ground properties,the reinforced cushion could deform with a significant deflect ion.The large deflection thin plate theory was used to solve the deformation of the reinforced cushion by using varying parameter iteration.Good convergence efficiency can be achieved by normalization on the equation and boundary conditions and transform ing the solutions back to dimensional expression.The calculated results were compared with the measured data in a practical case and good agreement in between was found.The energy method was used to analyze the column supported composite foundation by accounting for the impact of negative skin friction on its working behavior.The upper part and lower part column divided by the neutral surface were analyzed respectively.The tri-linear model was found to be more appropriate to model the column-soil interaction induced by the decreasing potential accompanying the settlement of column and soil.The equations for nodal forces and nodal displacements of column were established by using energy method on column element.Iteration technique was adopted to solve t he equations and drive the column-soil load bearing ratio,the position of neutral points,the distribution of axial forces and the distribution of skin friction for column.The above theoretical findings were incorporated into the load transfer mechanism and deformation compatibility from the base of embankment to the stabilized zone by considering the embankment-reinforced cushion-column-soil as a integral system.The essential solutions for embankment settlement,the settlements of column and soil,the column-soil load bearing ratio,axial force distribution and skin friction distribution as well the neutral point position of column were derived using the interation.The subgrade of roadway was discretized into meshes in horizontal and vertical directions based on the finite difference principle.Combining with the initial conditions,the horizontal and vertical pore pressures of each single grid point at arbitrary time were determined.The consolidation degree of subgrade was calculated by using Carrillo theory,and time-dependent behaviors of load transfer and settlement for BDRCF subjected to embankment loads were evaluated furtherly.Laboratory model tests for nine groups of testing conditions were carried out based on the similarity theory.The work performances of geocell reinforced cushion,the sand drain,the stone column and flexible column supported composite foundation were evaluated respectively from three aspects as bearing capacity,settlement and consolidation.The above evaluations were incor porated to the comparative analyses on the working behaviors of various geosynthetic reinforced column supported composite foundations characterized by “geogrid + stone column”,“geocell + stone column”,“geogrid + flexible column”,“geocell + flexible column”.The results can provide some insights into the application of BDRCF in engineering practice.