Geotextile encased columns
A continuously, radially, woven geotextile sock made from a variety of polymers. These socks form encased stone columns when filled with compacted sand, gravels or crushed rock for use in very soft soil where conventional ground treatments cannot be utilised.
Geotextile encased columns are used as a ground reinforcement and improvement technique, in a similar manner to the use of stone or vibro-concrete columns and piles. They can be used in very soft soils with very low compressive strengths, such as clay, peat and sludge.
Appropriate selection of the geotextile fibre enables the columns to be used in chemically aggressive grounds, where for example uncased stone or concrete columns would have a limited life. Finally, geotextile encased columns have intrinsic flexibility and can support dynamic loads (e.g. from trains so that they are suitable for supporting railway embankments) without damage.
The advantages of using this technique include avoiding the need for excavating and disposing of unsuitable or even contaminated soil and minimising settlements taking place both during construction and in the longer term: these benefits result in cost savings and shorter construction times.
Geotextile encased columns are installed by vibrating a metal casing into the ground to the required depth, i.e. to the load bearing layers beneath the soft soil. The soil becomes locally compressed as it is displaced to accommodate the casing: this in itself provides some local strengthening and improvement effect.
The metal casing has two flaps sealing an opening at the bottom whilst the casing is driven into the soil. A textile sock is then installed in the casing and a funnel connected at the top for filling the sock with sand, gravel or rock. The casing is then vibrated out, thus causing the in-fill to compact and induce radial stresses into the geotextile containment. This process allows for speedy on-site installation of the geotextile encased columns.
The spacings and diameters of the columns, usually placed in a regular pattern, will depend on the soil properties and the superimposed load of the structure, e.g. the height of the embankment. Additionally, reinforcing geogrids can be placed above the geotextile encased columns to provide further strength and load spreading capability.
Over 60,000 GECs were used on the estuary of the river Elbe in Hamburg for the construction of a dike to protect low land reclaimed from the sea for the extension of the A380 Airbus factory. The columns were inserted into a very soft, alluvial soil to form the foundations for the dike. This solution saved one year of construction time, thousand of tonnes of sheet piling and millions of tonnes of sand and fuel .
A section of the Burlington Northern Santa Fe railway adjacent to the shoreline of San Francisco bay, had become unusable after the stockpiling of crushed concrete on land nearby had caused the foundations of the existing railway embankment to heave, thus causing misalignment of the track. The embankment was built on the very soft mud of the bay, which were underlined by stiff clay and sand layers. Geotextile encased columns were used to provide a more stable foundation to the replacement embankment, providing considerable economic and construction time savings over the alternative solutions .
- BS EN 14475:2006 Execution of special geotechnical works – reinforced fill
- BS 6031:2009 Code of practice for earthworks
- BS 8006-1:2010 Code of practice for strengthened/reinforced soils and other fills
- CIRIA Publication C573 Guide to ground treatment, London 2002
- CIRIA Publication SP123 Soil Reinforcement with Geosynthetics
- TRL Contractor Report CR341 Highway embankment over soft compressible alluvial deposits: guidelines for design and construction, Crowthorne, 1994
