London's £181M, 4.5km Docklands Light Railway will extend the existing line under the River Thames to Greenwich and on to Lewisham in south east London. It was one of the early flagship Private Finance Initiative schemes in the UK. The 24.5 year concession to finance, design, construct and maintain the extension was awarded to preferred bidder City Greenwich Lewisham Rail Link plc (CGL Rail) in May 1996.
CGL Rail appointed Light Rail Group Contractors (a joint venture of UK's John Mowlem
& Co and Japan's Mitsui-Nishimatsu tunnelling JV) to design, construct and commission the infrastructure. Work started in October 1996, with Mowlem responsible for all but the 1.1km twin tunnel link between the Isle of Dogs and Greenwich, which is in the hands of the Mitsui-Nishimatsu JV. Design for the link is being undertaken by UK consultant WS Atkins.
The first of the 1.1km long twin bored tunnels under the Thames between Island Gardens on the Isle of Dogs and Greenwich on the south bank was finished at the end of October last year, and the second started a month later. A 6m diameter slurry shield TBM is driving the 5.2m internal diameter concrete-lined tunnels through Thames Gravel, the Woolwich & Reading Beds and Thanet Sand.
Keller Ground Engineering, subcontractor to the joint venture, is responsible for the design and construction of the ground treatment works along the tunnel route on both sides of the river, which includes two new stations at Cutty Sark and Island Gardens.
As the tunnels approach the southern portal in Greenwich, they pass under a railway cutting and a nearby shopping centre, Royal Hill Court. Out of the tunnel, the DLR will run through a short section of cut and cover before rising to the new station adjacent to the existing mainline station.
The tunnel alignments here are sometimes only 4m below the cutting and the pad foundations of the three storey Royal Hill Court. The problem is further complicated by the presence of a large diameter sewer, the Deptford discharge culvert, that runs alongside the cutting. Maximum settlement of sensitive structures from the tunnelling (advancing at rates of up to 24m every day) was predicted at 50mm, though more importantly with angular distortions of up to 1:200.
Originally the contract called for permeation grouting throughout the route, but after a trial at nearby Millwall Park (see box) it was shown that a combination of compensation, jet and permeation grouting was suitable for the contract.
Keller project director Charles McAnally believes this could be the first time in the UK that compensation grouting has been used on such a large scale in gravels.
Keller has also used the technique in similar ground in Europe, most recently on the Lisbon Metro in Portugal. 'Lisbon acted almost as a testing ground for the method and we gained useful information for the DLR from it,' says McAnally.
Initial conditioning of the ground is carried out in blocks from arrays of 'Soilfrac' (steel tube-a-manchette) pipes well in advance of the tunnelling. This grouting primes or 'tightens-up' the ground and in practice produces a small amount of heave (up to 5mm). As the TBM approaches, movements are closely monitored and the compensation grouting used to control settlement of the critical structures within the specified limit of 15mm. As the ground is already conditioned, any further grouting has immediate effect. After the TBM, additional grouting is used to stabilise localised zones of settlement.
The Soilfrac pipes are arranged in two layers and were installed within a tight drilling regime, being threaded between the mass of foundations and underground services. 'The pipes had to be installed with a depth tolerance of plus or minus 0.5m over a distance of nearly 50m, which is technically very difficult,' says McAnally. And under the railway cutting, a layer of boulders that had not been picked up by site investigation meant that drilling parameters had to be adjusted and tightly controlled to ensure correct positioning.
Expected settlement rates were up to 6mm-8mm per hour but using the two layer system and a range of specially designed rapid setting cementitious grouts, Keller has effectively halved the settlement experienced during the tunnel drive. Of the remaining half, 80% is primary, with 2mm to 4mm of settlement occurring per hour, and 10%-15% is secondary, occurring within two to three weeks. The remaining 5% to 10% of settlement tails off over a period of time.
Even then, 2-4mm settlement per hour is almost instantaneous due to the rapid progress of the TBM and it is still very difficult to deal with. 'On JLE, we experienced similar settlements over 24 hours over a much shorter tunnelling distance,' says McAnally. Angular distortion must also be maintained at 1:1000, but when the first tunnel went through, Keller actually achieved better than this in places.
Monitoring of the settlement of sensitive structures has led to the use of sophisticated instrumentation, including real-time Geodetic surveying, used to monitor 300 points along the 160m section of railway cutting. Radio telemetry is used to transmit data back to the site offices.
Three survey stations have been set up around the cutting, which must be positioned with precise levelling and resection before every geodetic survey as they are within the area of potential movement. A servo-controlled Geotronics geodometer, which was programmed with the original positions of the targets, automatically locates them and records any movement with an accuracy of 1mm. Some 60,000 geodetic measurements were taken over the first tunnel pass.
The site has also served as a major testing ground in the UK for new generation electronic liquid settlement cells positioned within the Royal Hill Court shopping centre.
Stations along this section of the route are in deep buried boxes, formed in advance of tunnelling. Keller constructed the eight break in and out blocks for the TBM using a combination of jet and permeation grouting.
Specification for the treatment required that the strength of the blocks was 3N/mm2 after 28 days. McAnally says that in the case of the six jet grouted blocks this is a very challenging target in the treatment of such variable soils. 'We have treated soft alluvium, Thames Gravels, Woolwich & Reading Beds (stiff clays containing with bands of gravel) and Thanet Beds (a dense fine medium sand).'
He believes that jet grouting has not been used before in the UK to treat the latter two formations.
At the time of writing, five of the six treatment blocks have been penetrated by the TBM and all have performed well. 'This is probably the most varied and technically challenging jet grouting operation we have undertaken in the UK to date,' reflects McAnally.
Permeation grouting was used at the break-in at Greenwich because access is tight. The method is suitable for medium sands and gravels and can be carried out more economically using cheaper silicate grout. The downside is that any bands of fine sand and silt cannot be effectively treated with permeation grout and strength gains are low.
After tunnelling is finished, Keller will monitor the survey points along the route for up to six months carrying out further grouting if needed, although McAnally expects very little movement.