Foundation of Constructions on Very Soft Soils with Geotextile Encased Columns - State of the Art

Abstract

Foundation systems with geotextile encased columns (GEC) are used for soil improvement and primarily for road embankment foundations in Germany, Sweden and the Netherlands since almost 10 years (Raithel et al, 2004), but latterly they are also used in dike construction. In this paper the essential main features of the calculation of the bearing and deformation behaviour are described. Further the know-how gained by using the different installation methods and measurement results of the foundation system ‘geotextile encased columns’ are discussed. Also a comparison of the gained settlement reduction between encased and non-encased columns (i.e. granualar piles) will be shown. Bearing System GEC and calculation model Bearing System GEC With the foundation system GEC gravel-sand-columns are installed into a bearing layer to relieve the load on the soft soils. Different installation methods are thereby used. Due to the geotextile casing in combination with the surrounding soft soils the column has a radial support, whereas the casing is strained by ring tensile forces (Raithel et al, 2004). Due to the supporting effects of the casing, a special range of application, in opposite to conventional column foundations (i.e. granualar piles), is in very soft soils (cu < 15 kN/m²) like peat or very soft silt/clay as well as sludge. As opposed to conventional stone column foundations, geotextile encased sand or gravel columns can be used as a ground improvement method for very soft soils. By a non -encased column, the horizontal support of the soft soil must be equal to the horizontal pressure in the column. By a GEC, the horizontal support of the soft soil can be much lower, due to the radial supporting effect of the geotextile casing. The horizontal support depends also on the vertical pressure over the soft soil, which can be much smaller. As a result a stress concentration on the column head and a lower vertical pressure over the soft soil and therefore a large settlement reduction is o btained. To withstand the high ring tension forces, the geotextile casings are manufactured seamlessly. The columns act simultaneously as ve rtical drains, but the main effect is the load transfer to a deeper bearing layer. The GEC are arranged in a regular column grid. Based on the unit cell concept, a single column in a virtual infinite