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Put a sock in it

INNOVATION IN GEOTECHNICS

Geotextile encased sand columns are preventing settlement of a high-speed railway embankment in the Netherlands. Max Nods reports.

Construction of the Antwerp to Amsterdam high speed railway requires a 200m long, 6m high embankment across an old landfill near Breda in the south west of the Netherlands.

The Westrik landfill was created by the excavation of sand to build the nearby A16 motorway.

The pit was used for municipal, construction and industrial waste from 1947 until 1959.

Environmental investigations carried out between 1983 and 1997 revealed that the 5m to 7m deep waste is underlain by sand with occasional clay layers and is contaminated with oil, PACs and heavy metals. The ground generally has a neutral pH but is occasionally highly alkaline.

Design and construct consortium HSL-Combinatie Brabant Zuid was concerned that any organic waste material could break down, causing settlement that was difficult to predict. Two solutions were considered: a 'zero settlement' concrete slab supported by 10m long piles or excavation and removal of waste.

Excavation was rejected because of time constraints and the problem of disposing of the waste. This left the piled slab.

However, piles had to be resistant to the chemicals in the landfill for the structure's 100-year design life and obstructions in the waste could cause problems during installation.

Enter the Möbius GEC (geotextile encased columns) system.

Consisting of sand piles encased in a geotextile 'sock', the rapid displacement-based system compresses soft soil and increases its shear strength.

It can be used in very soft soils such as peat, clay and sludge with strengths of less than 15 kN/m 2and had already proved itself in controlling settlements under the abutments of a bridge over the Merwede canal on the Rotterdam to Germany freight line and on the massive reclamation contract for the new Airbus facility in Hamburg (GE May 2002).

Installation begins by vibrating a steel casing into the ground to the load bearing layers. The casing has two flaps at the bottom which are forced closed during driving, displacing the ground.

Huesker Synthetic's round weave Ringtrac geotextile is connected to a funnel which is put in the top of the casing. The 800mm diameter tube is filled with sand through the funnel and the casing vibrated out, causing initial compaction of the sand and stressing the geotextile. Columns are usually placed on a 1.7m to 2.4m triangular grid.

The geotextile limits the movement of the sand, generating a relatively large horizontal stress in the column. This makes it stiffer than the surrounding soil and allows it to transfer embankment load to the load bearing layers, dramatically reducing settlements and also deformations due to dynamic loads such as train movements.

Ringtrac generates high ring stresses at low strain levels, has low creep tendency and is highly durable. In 'normal' soil conditions high-strength polyester yarns are used, but at Westrik, a special polyvinyl alcohol yarn with ring strengths of between 200kN/m and 300kN/m was used as it could cope with high alkali levels. Creep tendency of both materials is similarly low, with strain at breaking point about 6%.

Embankments have to be a minimum height, related to column spacing, to achieve the optimal arching effect. Fortrac geogrid is laid in a load transfer platform over the piles to increase arching, give extra horizontal stability and help transfer horizontal railway loads. Fortrac R300/30-30 with a tensile strength of 300kN/m was used at Westrik, with embankment fill placed directly on top.

To prevent large post-construction settlements, the system is designed so most occurs as the embankment is built. Surcharge is used to accelerate movements.

Sand columns were installed at Westrik in June and July this year. About 1350 columns were installed for the main embankment and 825 installed for two 1m high side embankments for a junction with another railway line. One machine installed between 40 and 80 columns a day, depending on ground conditions.

Monitoring to date indicates settlement well within the predicted and specified range (a maximum of 30mm over the design life of the structure). Settlements under parts of the embankment not founded on the sand columns are three to four times larger.

Max Nods is marketing manager of Huesker Synthetic (Germany).

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