Geotextile Encased Columns
Vijfwal Houten, NL
Royal BAM Group and Van Wyk
In the South part of Houten in the Netherlands, landscape embankments (Bastions) were placed at the end of the housing projects. They were planned as a connection between the residential area and the natural landscape around. The landscape hills (Bastion West and East) had to be built with cohesive soils which occurred in the project areas. For these embankments on extremely soft soil, settlement calculations were performed and settlements of 1.6 m to 1.9 m for Bastion West and 0.5 m to 0.8 m for Bastion East were expected to occur. This created a problem because of the extended estimated consolidation time. In addition, the considerable settlements endangered an adjacent brick wall founded on concrete piles. One other concern was the global stability of the embankments, especially due to the excavation of a dewatering canal around the Bastions at the toe of the embankment. Several options for the construction of the Bastions were analysed, however the use of Geosynthetic Encased Columns (GEC) was found to be the best solution with regards to the reduction of the settlements as well as to the global stability of the embankments
The construction works started with the installation of a working platform with a 1.0 m thick sand layer. From this working platform the GECs were installed. This method used a displacement pipe with a diameter of 0.8 m. After this the geotextile encasement was installed inside and filled with sand. As the displacement pipe was pulled out with the flaps open, the column fill was compacted by optimised vibration. The Royal BAM Group operated at both Bastions with the same equipment changing only the length of the steel pipe. Using this equipment up to 40 GECs per day were installed. After the foundation work was finished the contractor van Wyk progressed with the construction. Since all GECs were installed from the same level the length of some columns had to be adjusted. These columns were uncovered and shortened according to the final design. Furthermore, the canals around the Bastions were excavated. After the ground surface was shaped the horizontal geotextile was installed. Finally the Bastion embankments were built on the prepared platform. After a period of consolidation of two months the works were planned to progress with the construction of the roads and facilities at the Bastions.
The use of this foundation system seems to be suitable particularly if the subsoil in place is too weak (cu <15 kN/ m2) to be treated with regular stone or gravel columns and the structure does not require completely stiff and settlement-free pile elements. The GECs are an appropriate supplement to conventional foundations systems.