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Plastic fantastic

Plastic piles are protecting a north England town from flooding where previous techniques had fallen short. Helen Cross explains how.

A major flood alleviation scheme in Gainsborough, Lincolnshire has been given a boost by plastic sheet piles.

Cofra UK supplied and installed these in a subcontract for Birse Civils. The scheme spans 3.2km of the River Trent flood defences, from the north end of Gainsborough to a railway bridge crossing the river in the south.

The aim of the scheme is to improve protection to housing and other assets along the river where tidal water moving upstream from the Humber Estuary meets with water flowing down the River Trent.

Client the Environment Agency (EA) is supported by consultants WS Atkins and Faithful+Gould, who both have a presence on site.
The defences include made ground embankments up to 4.5m high that sit on alluvial deposits comprising soft to firm sandy silty clay and river terrace gravels.

When the flood defences were assessed against current guidelines, it was found that areas required strengthening, raising and improved impermeability. The project has been divided into four sections and the scope of works for these are:

- Section one – a vertical plastic sheet pile wall will be placed in one existing earth bund to act as a barrier in times of flood. This earth bund will also be raised by 120mm.

- Section two – the current flood walls will be strengthened by the installation of two rows of ground anchors at 2.4m intervals.

- Section three – as per section two except that one 30m section of wall will be replaced with a new steel sheet pile wall restrained by one ground anchor.

- Section four – as section one, a vertical plastic sheet pile wall to be placed in another earth bund that will also be raised in two locations by 120mm and 320mm.

Cofra became involved with the project post tender stage last summer after the EA had done trials with other suppliers and contractors which had not been entirely successful. Keen to be involved, Cofra undertook trials in its plant yard, where ground conditions were known to be more difficult than in Gainsborough. This was to prove that the plastic sheet piles could be installed to the required depth.

These provide a low permeability barrier within the bund and due to the low weight of the equipment, could be installed while ensuring the integrity of the existing flood embankment. The works comprised installation of Cofra's Geoflex 210 sheet piles to a maximum 4.5m depth and a minimum 3m depth, over 1.7km.

Cofra began work in late October for a 15-week programme. The works comprised Section one and Section four, detailed in the table below.
Cofra adapted its plant and equipment to ensure the total load imposed on the existing earth structure was less than 2t/m2. This was achieved because Geoflex piling equipment is lightweight and navvy mats (portable working platforms) could be used to spread the load on sensitive structures. Cofra believes such weight limits would not have been achieved by conventional steel sheet piles.
Geoflex and steel sheet piles react to obstruction in the same way. Neither are able to be installed where large obstructions such as insitu concrete or demolition waste are present.

When obstructions were located, the affected piles were marked and the issue referred to the main contractor and design team while Cofra continued further along the line. The design team then confirmed remedial action and where required Cofra returned to the location to install further piles around the obstruction.

Cofra switched a vibrating hammer between two machines depending on loading restrictions. These were a 12t excavator in conjunction with the navvy mats, and a 7t Komatsu where the earth bund had restrictive access and width. Both these methods achieved between 20 and 40 linear metres of wall per day.

Cofra completed the installation of the remainder of the piles on the about £250,000 job in February.

Helen Cross is project and product manager with Cofra UK.

SECTION 4 FLOOD DEFENCE SECTION PILE LENGTH LINEAR METRES DAYS PROGRAMMED
10-R-44 3.5m 75m 4
4m 20m 1
4.5m 215m 11

10-R-42 3.5m 100m 5
10-R-41 3.5m 70m 4
4m 65m 4

SECTION 1
9-R-07 3m 590m 30
9-R-05 3m 320m 16
3.5m 270m 14


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