Three-dimensional DEM Analysis And Model Tests On Geosynthetic Encased Stone Columns

Geosynthetic encased stone columns(GESC)is a new ground improvement technique for soft soils,which is formed by encasing stone columns with geosynthetic encasement.The geosynthetic encasement could provide additional confinement.As a result,the capacity and stiffness of the columns can be increased and the settlement of the soil strata can be further reduced.GESC has been widely used to support a wide variety of structures including embankments for highways and railways and low-rising buildings on soft soils.However,the analytical studies of GESC are not well built.The research on performance of GESC,such as bearing capacity and deformation,is still required to carry on.This research was funded by two National Natural Science Foundation of China(No.51078138,51208191)and partially supported by China Scholarship Council(No.201406130006).The stresses and deformations of GESC were studied by model tests and three dimensional Discrete Element Method(DEM).An unconfined compression test was performed in the laboratory at Hunan University in China to investigate the effect of geogrid encasement on the behavior of stone columns.In addition,Ordinary Stone Columns(OSC)and GESC were installed in clay beds subjected to vertical plate.Two types of tests were conducted,one on an individual column and another on a composite foundation consisting of a column and its surrounding soil.The lateral deformations of the stone columns were measured using the strain gauges fixed on the geogrid ribs and the hoop displacement gauges wrapped around the unreinforccd columns.These gauges monitored the deformations at every load increment.The effective encasement depth was also investigated by changing the length of geogrid encasement along the stone columns.Earth pressure cells were installed to investigate the load transfer mechanisms between columns and soil and with depth.Three-dimensional particle flow code,PFC3I)based on the DEM was used in this study to model the unconfined compression test of GESC Aggregate was simulated using graded particles of diameters ranging from 30 to 5()mm and a biaxial geogrid as an encasement material was simulated using parallel-bonded particles.Micro-mechanical properties of the aggregate were determined using numerical triaxial tests at various confining stresses.Tensile propertes of the geogrid were determined by a multi-rib tensile test and the flexural rigidity of the geogrid was calibrated by a flexural bending test.The load-displacement response of the DEM model of the geogrid-encased aggregate sample closely agrees with the experimental results.The coefficient of radial stress,defined as the ratio of vertical stress to radial stress within the aggregate,varied from 0.6 to 2.7 during loading where the tensile strength of the geogrid encasement was not fully mobilized.The aggregate showed volume contraction at small deformation and then dilated with an increase of deformation.The interlocking effects between the aggregate and the geogrid were observed at the initial state.The particles within the middle portion of the column more likely slipped than those at other locations.Based on the previous study,seven DEM models were developed to investigate the effect of four important factors(i.e.,geogrid stiffness,column length,column diameter,and aggregate size)on the behavior of single geogrid-encased stone columns under unconfined compression.Both the macroscopic responses(e.g.,vertical pressure-strain curves)and the microscopic interactions(e.g.,contact force,coordination number,and sliding fraction)of the columns were analyzed.The numerical results show that the geogrid encasement with high tensile stiffness could provide high confining stresses and then effectively increased the bearing capacity of the column.The short column yielded quickly even though its column modulus at a small deformation was relatively high.The modulus of the column slightly decreased with an increase of the column diameter due to high circumferential strains mobilized in the geogrid encasement.The column with large aggregate was stiffer and deformed less than the column with small aggregate.Selecting aggregate with a size larger than the geogrid aperture size was an effective way to achieve better interlocking between the aggregate and the geogrid and to minimize mass loss for the geogrid-encased stone column under loading.Due to limited deformation allowed by the geogrid encasement,a coefficient of radial stress equal to half of the coefficient of passive earth pressure,was suggested to estimate the ultimate bearing capacity of the geosynthetic-encased stone column.DEM models for OSC and GESC with surrounding soils were also developed and verified by measured results from model tests.The encasement length of GESC considered in DEM simulation was two times the dimeter of the column.The surrounding soil was simulated using spherical particles of d iameters ranging from 18 to 20 mm.Weak contact bonds was generated at particle contacts to simulate the cohesion of surrounding soils.The strength change of soil at different depths was considered in DEM models due to compactions in model tests.The 1000 mm deep clay bed was divided into five layers and each layer was 200 mm thick.The load-settlement behaviors for OSC and GESC obtained from DEM models were compared with experimental results.The vertical and radial stresses of columns and soils were analyzed.The microscopic interactions(e.g.,contact force,porosity change,coordination number,and sliding fraction)of the columns were evaluated.The coefficient of radial stress for OSC exceeded the value of coefficient of passive earth pressure of columns at small settlement.The coefficient of radial stress for GESC was equal to half of the coefficient of passive earth pressure for encased portion and was higher than the coefficient of passive earth pressure for portion below the encasement.The main contact forces concentrated in the column for OSC and in the encased portion for GESC.The stress concentration ratio for GESC was much higher than that for OSC,especially in the encased portion.Based on the practical stresses and deformations conditions of GESC and the previous DEM results,a calculation method aimed at improving the determination of the bearing capacity of GESC was proposed.Due to limited deformation allowed by the geogrid encasement,the passive earth pressure state of the column was not reached.The coefficient of radial stress was much smaller than the passive earth pressure coefficient.Therefore,the calculation method suggested using the coefficient of radial stress equal to half of the passive earth pressure coefficient to estimate the ultimate bearing capacity of CESC.In addition,a field test was analyzed and the measured bearing capacity of GESC was compared with the calculation results based on this new calculation method.Good agreement between the measured and calculated results was achieved.which demonstrated the reasonability and effectiveness of this proposed calculation method.