Cofra has started with the application of Geotextile Encased Columns when former Mobius sections, in the past the market leader in the installation of GEC, were acquired by our parent company Boskalis. Due to our strong relationship with Huesker Cofra has become one of their preferred installers to install the GEC columns on every location in the world.

What it is

GEC (geotextile encased columns) are granular piles, most of the time consisting of sand, encased with a high strength geotextile. The purpose of this geotextile is to ensure the piles integrity and provide confinement in very weak soils up to an undrained shear strength of 15kPa. Above 15kPa the soil has enough confining pressure to assure the integrity of the pile and sand or stone can be placed without a geotextile. The piles are most often 80cm in diameter.

How it works

The key component in the Geotextile encased column is the casing of the pile in the form of the geotextile. This geotextile provides support to the infilled material and creates a stiff flexible casing that is tensioned by the outwards directed horizontal stresses. The system acts as a drain and as a pile. The pile transfers most of the load down to the bearing strata and creates an equilibrium with the surrounding soil, limiting the load on the soft material and thus the settlements and residual settlements. The system is often used in conjunction with a basal reinforcement on top of the piles to improve the spreading of the load.

Why it's used

The system is used on location where very soft soil exist and a solution with PVD and surcharge is not capable to provide the required stability or residual settlement within the given construction period. Applying GEC will result in the following effects:

  • Reduction of residual settlement by 50 – 75% and can be mitigated by temporary surcharge
  • Up to 90% of the consolidation takes place during construction
  • Able to be used in extremely soft soil (e.g., undrained shear strength < 15 kPa)
  • Loadable immediately after installations

Application area

Geotextile encased columns are used in the following markets:

  • Infrastructure: The GEC technique is used for the fast construction of embankments for roads, railways, runways and aprons for airports and port areas.
  • Dredging and land reclamation: GEC can be used to avoid the use of sand keys
  • Construction: The GEC technique is used for the fast construction of embankments for construction sites.

The following soil types are suitable for the use of GEC:

How it is installed

A GEC pile installed by vibrating an 800mm steel casing into the soil and embedding it about 0.5m into the bearing strata. Once at depth the geotextile, custom made to the appropriate length, is lowered into the casing and filled with granular material. In the meantime the installation set is installing or retracting the casing of another pile. Once a pile is full with sand the casing is retracted with vibrations to densify the sand and the GEC pile remains in place.

The piles are often used to provide the bearing capacity for the machines.

 

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