High-speed trenching and a geosynthetics encapsulation system is sealing off a lime pit in north London. Damon Schünmann reports.
Lime-pits, reservoirs and sites of special scientific interest (SSSI) are not a good combination.With all three in close proximity, the rebuilding of the JVC headquarters near Staples Corner, north London, poses something of a segregation problem.
The lime-pit dates back to the 1930s and borders an SSSI, as well as being a short hop from the Brent reservoir.Main contractor, Kajima Construction Europe (UK), had to carefully consider these factors to satisfy the Environment Agency.
One solution to the planning conditions for the site was the installation of a hydraulic barrier to contain any possible leakage from the old lime works.
Designer Buro Happold approached geosynthetics subcontractor Cofra to provide a containment method and Cofra's Geolock encapsulation system met all of the EA's requirements. It consists of a series of interlocking high-density polyethylene (HDPE) membranes which are now being installed at the site within a bentonite-filled trench.
Card Geotechnics is acting as consultant to Cofra, providing design and compliance testing of the containment barrier. Following a supplementary site investigation to determine the location of the London Clay interface, Card prepared detailed designs for the barrier. It is providing a site inspection service to ensure construction complies with the design.
Cofra's managing director, Mike Drew, claims the method of excavation is a European first. He says using a trencher to create the 290mm wide and 8m deep channel is dramatically increasing the speed at which the 360m perimeter channel can be completed.
He explains that the trencher's chainsaw, powered by a 650hp engine, can cut at about 80m an hour, compared with the meagre 30m a day achievable with a long-arm excavator.
Although the trencher can rapidly slice the ground open, site limitations mean it could not be the sole means of digging, and an excavator is needed for less straightforward areas. 'In places we were within 2m of the edge of the site and the other side of the boundary is the highly sensitive SSSI, ' Drew says.
Working close to such a delicate area, or in areas where construction has already begun, the far slower excavator comes into its own.
'One of the major problems on the site was the restrictions of working space which has called for some very close liaison with other sub-contractors, ' Drew says. 'The proximity to completed work means we can't use the faster method because of bentonite splash.'
The trench itself is filled with Castle Protomix bentonite, which comes premixed with a small amount of cement and a polymer.
The cement means the slurry will permanently set once it is insitu rather than returning to a liquid state should it become agitated at a later date, and the polymer promotes the rapid absorption of water.While this gives support to the sides of the trench, the sodium-activated bentonite also acts as a secondary level of barrier protection.
Once the pour has finished, an HDPE membrane is inserted into the trench.This must penetrate the underlying London Clay by at least 2m and to make sure this is achieved, the base of the sheet is folded over so a mandrill can be inserted into it. The mandrill initially drives the membrane down under the weight of gravity, but a vibrator is then used to ensure the desired depth is reached.
Hydrotite is used to maintain the integrity of the barrier between each 2.5m wide HDPE sheet. This is an expanding hydrophilic water seal which is threaded between the connecting male and female panel joints. More commonly used as a tunnel joint sealant, the rubber swells when it comes into contact with water.
The requirement to install the barrier had another advantage. With the new EU landfill directive becoming law this month, the financial onus is on developers to treat or seal off contaminated ground rather than using 'dig and dump'methods.
As Kajima project manager Spencer King says: 'We were able to use on-site material as back-fill because the site was contained.'
Although the cost of the Cofra solution was £200,000, Kajima was able to make a significant saving. An estimated 22,000m 3of material would have otherwise had to be removed from the site at cost of about £2M. King says the containment barrier made the project feasible.
Because of the nature of the site and the proximity to the SSSI and the reservoir, a number of Environment Agency requirements had to be satisfied. King says: 'Because we used a proven containment system, it helped to prove our other systems to the agency.'
Nevertheless, the use of the trench on a brownfield site has meant extra diligence is needed when digging and pouring.'All precautions have to be taken to ensure we know where existing or redundant drains are, as the bentonite will migrate through them. A contingency plan has to be ready should this occur, ' Drew says.