Geotextile Encased Columns (GEC): Load Capacity, Geotextile Selection and Pre-Design Graphs
Embankments over soft subsoils supported by piles or stone columns have several advantages over the classical unsupported embankment foundation when compared in terms of bearing capacity, serviceability, and duration of construction. In extremely soft soil conditions, lateral support can be problematic for stone columns. An alternative system, which can both provide the required lateral support and increase bearing capacity, is the "Geosynthetic Encased Columns" (GECs). This system includes a high-modulus, creep resistant geotextile encasement called Ringtrac that confines the compacted sand or gravel column thereby providing constructability and bearing capacity even in extremely soft soil. The following describes the design principles, technologies and procedures developed over the past ten years and the importance of the geotextile encasement's tensile modulus. Also presented are graphs showing the influence of different factors on settlement. They could be also used in preliminary designs for orientation and proper encasement selection.
The system Geotextile Encased Columns (GEC) for foundation of embankments on soft soil was developed in Germany during the last 10 years, including construction technologies, more precise and verified design procedures and corresponding high modular low-creep encasement geosynthetics. It is an efficient alternative to traditional piles or compacted stone columns. Due to the confining encasement of the compacted sand or gravel columns in the GEC-system it can be applied even in extremely soft soils with e.g. Su < 2 kPa, which is in fact more a suspension than a soil.
In the meantime, the system has reached the stage of maturity. A wide range of high tensile-stiffuess, low-creep encasements are available (Huesker 1997 - 2003). This allows an optimized design for any specific project varying the tensile moduli and the percentage area of GECs in the embankment base. Due to the use of low-creep polymers the post-construction creep-strain related settlements can be reduced to negligible Ievels. Many successful, highly optimized GEC projects have been designed and constructed in Europe involving various soil and construction techniques.
Simple design graphs can be employed as shown herein for preliminary estimates and for identification of tendencies including the influence of different design factors. The final design should include not only precise calculations of vertical settlement and bearing capacity based on sound geotechnical investigation and data, but also an analysis of global stability or (sometimes) numerical analyses.
Usually, a horizontal reinforeerneut has to be installed on top of GECs at the base of the embankment.
The general behavior of embankments on GECs is similar, but not identical to that of embankments on traditional stiff piles. The entire reinforced GEC system is as shown stmgly strain dependent, and the behavior of GECs is "softer" in relation to traditional piles. It is an interactive, ductile, self-regulating system.