FEATURE ARTICLE Overview of the Geotextile-Encased Column Foundation System There are a few alternatives for the construction of foundations in soft ground. Among others, soil improvement techniques include granular columns, and more recently, geotextile-encased columns (GEC). The latter has similar features to its stone or sand counterparts but with the advantage of yielding smaller settlements, furthermore it can be used in very weak than 300 lb/ft (15 kN/m ). The concept was first developed and implemented in Europe in 1994 and has since been successfully used in many projects in Germany, Sweden, Holland, Poland, Turkey, Brazil and California. The system, comprised of a high- 2 soils with undrained shear strengths of less 2 encasement diameter ranging from 1.3 to 3.3 ft (0.4 m to 1.0 m). The performance of the system, in terms of controlling settlement and general stability, can be adjusted by varying the ratio of column area to its influence area (i.e., column diameter and spacing), the axial stiffness of the geotextile encasement and shear strength of the column fill. The analysis model used for designing strength horizontal geotextile, high- strength geotextile encasements, aggregate columns and surrounding soft ground, transmits the loads through the weak stratum to a bearing layer. As the loads increase, the bulging of the aggregate columns is limited by the encasement, and to a certain degree by the soft ground, thus reducing absolute and differential settlement when c ompa r ed t o uncased applications. The encasement also prevents contamination of the granular fill by migration of sur rounding soi l particles into the column, thus ensuring that the friction angle will not decline with time. The GEC foundation is built in a triangular grid pattern with constant center-to-center spacing, usually between 5 and 8 ft (1.5 and 2.4 m) and AUTHOR Lilma Schimmel, P.E., Huesker Inc. 82 • DEEP FOUNDATIONS • JULY/AUG 2015 the foundation is shown in Figure 1. It illustrates the vertical stress distribution over a column and surrounding soft stratum and respective horizontal stresses in the aggregate column, in the geotextile encasement and soft ground. Due to its higher stiffness, the columns bear higher stresses than the soil. The interaction between the column and the soil is self- regulating, that is if the column yields the load is transferred to the soil, which in turn increases its resistance towards the column causing a redistribution of forces back to the GEC until balance is reached. The results of the Raithel analysis (which Figure 2: GEC installation, displacement method Figure 1: Analysis model considers deformations and strains of the column fill, encasement and soft soil) include the maximum radial strain and strength of the geotextile, and settlement in the column top plane. The high permeability of the column system facilitates the dissipation of excess pore water pressure produced by the embankment overburden, a behavior similar to that of vertical drains, thus most of the settlement occurs during the construction phase. Creep settlements are also greatly improved. Studies suggest a reduction between 50% and 75% of the unimproved ground deformation, which is
