Geosynthetic-encased columns (GEC): A proven case study over 30 years

Abstract

The construction of embankments on very soft soils is a challenge due to the low shear strength, low permeability, high compressibility, and high-water content of the subsoil. The surcharge load imposed by the embankment can result not only in a local or total loss of stability but also in unacceptable settlements and horizontal deformations or thrusts, which could endanger nearby structures. To overcome these problems, different construction methods are applicable. One particularly interesting and still innovative foundation system is the use of geotextile encased granular columns, which can be considered “State-of-the-Art” in Europe. It was developed and introduced in 1994, and since then many major projects have been realised using geotextile encased columns (GEC). The overall loads and stress concentrations above the column heads induce outwardly directed radial horizontal stresses in the columns. The particularity of the GEC system is that these stresses are counteracted not only by the inwardly acting pressure of the soft soil but also –most importantly – by the radial resistance of the tensile stiff geotextile casing. This paper briefly describes the GEC system and includes the description of a few notable projects using GEC as a foundation system: land reclamation at the Airbus site in Hamburg, Germany, and the Second Niger Bridge in Africa. The geotechnical challenges, design basis, and what can be considered the most important findings of each of these projects are also discussed in this paper.