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Betting on a winner

GEOTECHNICS OF TRANSPORT

Committees, trials and thorough testing of a bituminous membrane have ensured that the water levels on a sensitive Irish nature reserve are not threatened by road construction. Gareth Beazant reports.

County Kildare is probably best known as the home of the Irish thoroughbred horse industry. Both the Curragh plains and Curragh racecourse are National Heritage areas of environmental and scientific significance with recorded presence of rare grasses and fungi. In addition, this part of Kildare is underlain by a major aquifer which gives rise to a unique wetlands - the Pollardstown Fen national nature reserve.

So when Kildare County Council engineers were faced with the task of designing a 14km town bypass through this area, they undertook extensive studies of possible routes to consider the effects on the environment. An environmental impact statement was prepared and the decision taken to construct the motorway in a cutting to the south of the town.

About 3.5km of this cutting penetrated the aquifer requiring a construction solution which would ensure that the hydrogeological conditions were not significantly affected. With the road surface 1.75m below the water level in the surrounding ground, a system of 'tanking construction' was devised to maintain average summer water level in the ground.

A vital component of the tanking would be the geomembrane liner. After lengthy consultation, site trials and theoretical analyses, a Coletanche bituminous geomembrane was chosen, a division of the road giant Colas.

'The initial part of the tanking contract was a 500m trial section, to ascertain its effectiveness and to confirm its appropriateness for the remaining 3km, ' explains Gerry Stenson, general manager of Coletanche.

Coletanche NTP 3 - one of seven grades available - was used, consisting of a non-woven geotextile impregnated with bitumen.

'The original design specified three geosynthetic layers - a PVC or an LLDPE membrane, a protecting geotextile and a geosynthetic clay liner, ' says Stenson.

'Site trials and theoretical analyses proved that our NTP 3 liner could not only replace the originally specified three layers, but also enhance the effectiveness of the tanked construction system.' Base layers formed an underdrain system to control the water pressures around the geomembrane and the Coletanche was covered with a 1m layer of low permeability soils. The road would be constructed on top of this.

Because of the sensitivity of the project and the complex interaction of so many construction activities in a confined area, a very stringent construction quality assurance plan (CQA) was put in place. The CQA is supervised by an engineer who is independent of both the client and the contractor. The plan included independent laboratory testing of one of every six rolls delivered, in addition to Coletanche's own stringent testing in Galway (see box).

'A monitoring committee including local and central government officials and ecologists needed to be completely satisfied about environmental issues, ' says Stenson.

Installation of the 500m trial section commenced in October 2001. After this was tested, the results compiled and the committee satisfied, work proceeded apace on the complete 204,000m 2installation in February 2002.

'This is the first time that there has been such a significant use of a bituminous geomembrane on one site in Ireland, ' says Stenson.

A 100% weld testing regime was also an integral part of the CQA. All 90,000 linear metres of seams were tested by three different methods - vacuum box testing, hand-held Panametrics ultra sonic device and ultra sonic testing using the CAC device.

The CAC is unique to Coletanche, and was developed to test weld integrity on the capping of a low level nuclear waste facility in France in the mid-1990s. A portable computer interprets the readout from the ultrasonic device and a printout indicates the position of any flaws.

From these tests, even the slightest fault can be detected.

This means that repairs, where necessary, can be carried out almost immediately and backfilling is not delayed.

Off site testing of the welds is also carried out. A sample of weld from the flat and from the slopes is removed from each 5,000m 2ofinstallation and sent to an independent laboratory for destructive testing.

'The exhaustive amount of testing was understandable, ' says Stenson. 'And we believe that the results have confirmed Coletanche's qualities and will reduce the amount of testing requested in the future.' The 'belt and braces' approach to quality also requires all welders to be certified. As there is no system for welders of bituminous geomembranes in Ireland or the UK, the French Asqual certification system was used.

Each 5.15m wide roll is logged on a GPS system for a record of where every roll was installed and where every joint is positioned.

During the project, a team of five was installing an average of 1,200m 2per day including the 1:1.5 side slopes. On flat ground, this figure can easily be doubled. A hydraulic beam is attached to an excavator and the roll of Coletanche unwound into position by the operator. The team then welds the overlaps of the material to form a bond.

Being both manufacturer and installer is a huge benefit believes Coletanche. 'It gives a level of comfort to the customer as it eliminates the blame culture which can arise between a manufacturer and an installer - even if they are approved installers.'

Liner of duty

Toughness of Coletanche ES2 grade was behind its choice on another contract. Colas UK has been active over the last 12 months for British Waterways on canal lining schemes, installing the material, manufactured with an elastomeric bitumen, on the Anderton Embankment of the Trent & Mersey Canal in Cheshire, Thwaite Brow on the Lancaster Canal, and the Grand Union Canal at Milton Keynes. Coletanche was also used as the waterproof liner of the brick arches of the Grade ll listed Dove aqueduct, on the Trent & Mersey Canal near Burton-upon-Trent.

Another team from Colas UK has recently returned from the Falkland Islands, having completed a 38,000m2 landfill lining with Coletanche for the Ministry of Defence. Work was completed ahead of schedule.

Roll recipe

In the heart of the picturesque Galway Bay, the Coletanche factory process for the bituminous geomembrane starts with the delivery of bitumen that is pumped from a ship docked in Galway harbour to Cold Chon bulk storage facility.

Filler is then added to the bitumen and mixed for a set period of time. This fillerised bituminous binder provides the waterproofing, flexibility, chemical resistance and ageing characteristics of the geomembrane.

A non-woven polyester geotextile and a glass fleece are then impregnated with the bitumen. The geotextile provides the geomembrane with its mechanical characteristics such as tensile strength and puncture resistance. While the glass fleece contributes to the tensile strength of the membrane, its primary role is to provide stability during manufacture.

A terphane film is incorporated on the underside immediately after the impregnation process to provide an anti root barrier, leaving a 200m edge seam designated by a kraft paper, which is removed when the membrane is being unrolled on site. To complete the process, sand is applied to the upper face to prevent the roll sticking together during transportation. It also significantly improves the angle of friction with any overlying soil on the membrane. The roll is then coded and removed to the stockyard for loading and delivery which Coletanche claims is very fast.

Rolls shipped on a Friday night can be ready for installation on site in the UK by the following Monday morning, it says.

To comply with the relevant ISO 9002 and 14001 requirements, a stringent testing regime is carried out on every roll produced. A 0.35m wide section across the entire 5.15 m width of the roll is removed and brought to the in-house laboratory for testing. Tensile strength, puncture resistance, elongation, thickness, mass per unit area and cold flexibility are all checked and the results recorded. Furthermore a 1m long by 0.3m wide sample of each roll is placed in storage as a 'referee' sample to answer any potential queries that might arise in the future. Each sample must be kept for 12 years.

While the number of employees working at the factory varies according to demand there is a core of young, energetic and highly motivated individuals who are keen to continue the success that Coletanche has achieved to date.

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