Laboratory Tests On Encapsulated Stone Columns Composite Foundation

Typical stone column is a common approach for foundation treatment. However, the column consists of stones without cohesive force among them. Typical stone column gains vertical bearing capacity from cylindrical constraint imposed by the surrounding soil. The engineering practice proved that the stone column is limited for the soft soil whose undrained shear strength is less than 20kPa. Therefore, encapsulated column is proposed which is formed by encasing single stone column with geotextile. The encapsulated column behaves better in stiffness and bearing capacity than the typical stone column, for it could gain cylindrical constraint from geotextile with high tensile strength.In order to study the influence of the tensile strength and the length of the geotextile encasement to the performances of the composite foundation with encapsulated stone column, the bearing capacity of the composite foundation with encapsulated stone column, deformation characteristic, failure mode and so on were studied. A model experiment which consisted of a series of composite foundation tests with single encapsulated stone column in a soft soil bed were carried out. The main conclusions as following:(1)The limited bearing capacity of composite foundation of encapsulated stone columns is about 2.5-4 times that of typical stone column and about6.5-9.5times of soft soil, and the pile-soil stress ratio of the former is about 5-6.5 times that of the latter.(2)The pile-soil stress ratio and bearing capacity increase with increase in the tensile strength of the geotextile encasement. The encapsulated stone column behaves bad for partially encased case with bearing stratum while the column length is less than critical length.(3) It exists negative skin friction within the depth of twice the diameter of the column. The maximum vertical pressure of the surrounding soil is just under the bearing plate, and the maximum horizontal pressure exists at the depth of twice the diameter of the column.(4)The maximum bulging at a depth of 2 to 2.5 times the diameter for typical stone column, while at a depth of 2.5 to 3.5 times the column diameter for encapsulated stone columns. The influence depth of deformation for typical stone column is only 4.5 times diameter of the column. While the deformation exists along the whole column (the length of the column is equal to 6.5 times the column diameter) for encapsulated stone column, and the deformation is relatively well-distributed along the column. The encasement gains about 1%～2% tensile strains in the forming process. So the encapsulated stone column bulges at a larger vertical load with the pre-constraining in radial gained from piling.(5) Encapsulated stone column and typical stone column can effectively drain away the water in the soft soil and dissipate the excess pore water pressure in the soil under the upper loads. So it can speed up the consolidation of foundation.(6) The failure mode of encapsulated stone columns composite foundation is local shear failure. The bulge and penetration failure below the encasement for partially encased stone columns; The penetration into the cushion for fully encased columns.What’s more, base on the results of experiment and the correlated research, reinforcement effect of the encapsulated stone column on the soft soil was discussed in the aspects of bearing mechanism, deformation mechanism of composite foundation, influence depth of column, failure mode and so on. The calculation method of limited bearing capacity for the composite foundation of encapsulated stone column was also studied, and a comparison between the results of calculation and experiment were conducted. The calculation results of the ultimate bearing capacity are influenced greatly by the tensile strength of the packing materials which should be reduced according to the particular case.