Piling on the pressure

What happens when your building threatens the operation of one of the world’s busiest underground networks, the closure of which would cost millions of pounds a day? That was one of the questions facing main contractor Bouygues UK when it took on the task of building a half-submerged extension to Westminster City School on a small site in central London.

Creating the school’s new sports hall and teaching facilities as part of Westminster council’s £152m Building Schools for the Future (BSF) programme involved digging five metres into the ground – the only option on a restricted site where views of the school’s Victorian block are protected by planners. The subterranean solution meant driving sheet pile foundations just metres away from the District Line tube tunnel and working under the scrutiny of London Underground contractor Metronet.

Meanwhile, the proximity of two further site boundaries formed by the existing school and the neighbouring Victoria hostel for homeless people, meant noise and disturbance had to be kept to a minimum. Bouygues’ solution was to mobilise an innovative Japanese silent sheet piling machine that drives piles side on rather than from above.

The complex groundworks on the site also included the use of heavy-duty hydraulic propping, normally reserved for port and marine environments, to brace the sheet piles against the forces generated by the adjacent buildings. A combination of propping towers and giant vertical props was required to support construction of a massive high-strength beam supporting the two-storey block above ground.

Bouygues’ ingenuity in solving the site’s problems helped it win the Westminster BSF programme, as part of an initial bid for three non-sample schools, including St Marylebone School for Girls and Pimlico Academy.

‘I’ve never seen a site like this, where there are problems on every side – the existing school, a service road we couldn’t put any weight on because it had the district line underneath, a congested main road,’ says Huw Riley, design engineer at Bouygues UK, who worked on the bid with architect Anshen & Allen. ‘But then building underground is much more fun than building on the surface and this innovative concept is what won us the job,’ he adds.

When it opens this month, Westminster City School will be the first of nine secondary schools upgraded under Westminster’s BSF programme. The design-and-build project comprises two new-build blocks facing Westminster’s busy Palace Street plus the refurbishment of the 6,000m2 Victorian block and another extension built 50 years ago.

The school wanted a full-size 16m x 25m sports hall, but didn’t want to give up an existing playground to the rear of the site. Planners also required that the existing 150-year-old Victorian building should remain visible from the main road, which precluded building the sports hall above ground at the front of the site.

Groundworks began in March 2008 with the demolition and site clearance of two small Victorian buildings facing Palace Street. In June, piling subcontractor Giken Europe, a subsidiary of Japan’s Giken, began driving 340 sheet piles down to 7.5m around the entire building perimeter. These act as retaining walls along three of the site’s four boundaries: Palace Street, the tube tunnel to the south, and the existing school building.

On the fourth, northern boundary, the sheet piling bordering the Victoria hostel, a 173-bed facility for homeless people, required extra reinforcement and propping to withstand the pressures exerted by the hostel’s deep basement.

To construct the retaining walls, Giken Europe first pre-augered holes along the line of the sheet piles to loosen the soil in preparation for pile driving. Two silent sheet piling machines were then used simultaneously, a Giken hydraulic Silent Piler inserted sheet piles along Palace Street and the boundary with the Tube tunnel, while a Giken Zero Piler machine drove piles along the party wall with the hostel and on the interior side of a temporary walkway between the existing school and the newbuild.

Both machines operate by sitting astride previously driven piles and slowly ‘walking’ along them while driving new piles into the ground. Clamping legs grip three previously driven piles to create a reaction force for pressing down on the next pile. To drive the first piles the machines must sit on a massive 25-tonne counterweight, using that weight as ballast to drive the first piles in.

Piles are fed into the machines by cranes: the Silent Piler uses a mobile crane which crawls along the back of piles behind the machine, feeding piles in from an attached carriage. Just one operative is required to control the machine and crane via a remote control. The Zero Piler was fed by a 65-tonne crawler crane.

Any movement,anything at all and the entire project would have had to be stopped

Huw Riley, Bouygues

While conventional silent piling machines require about three metres of clearance on either side and drive the piles straight down from above, the Zero Piler is designed to work alongside existing buildings and it sits on the piles at a right angle. Both pilers are vibration free and completely silent, although the engines that power them generate some noise – approximately 70dB at a distance of six metres, according to Giken project manager Duncan Collins.

The silent, low-vibration Giken piling technique was vital to prevent disturbance to the District Line tube. A Metronet engineer was on site throughout excavation and piling work and dictated how Bouygues should operate near the tunnel. A system of lasers was also set up inside the tunnel to record any movement during excavation and piling work, which was monitored at London Transport’s head office.

‘Any movement, anything at all and the entire project would have had to be stopped. If you collapsed that tunnel, the insurance would be incredible. It costs about £1m an hour to stop the London Underground,’ says Riley.

After about six weeks of piling, two 14-tonne diggers began excavating the entire building footprint down to 2.5m. As the likelihood of piling collapse increases below 2.5m, a cross-bracing frame had to be installed to allow the excavation to continue to the full 5m depth required for the sports hall.

Calum Kidd, an engineer at Bouygues UK, explains: ‘We installed a Groundforce hydraulic excavation support system 1.5m below the tops of the sheet piles. It comprises several large props craned in individually – the largest was 25m long. They are normally used for marine excavation support, but here we needed something special to reduce the risk

of piling failure. Once propped against the piles, the braces are pressurised using a generator, where a gauge allows you to monitor the exact pressure they are exerting.’

With the bracing in place the excavation could continue, although the14t diggers had to be swapped for two smaller 5t diggers able to crawl beneath the propping. These fed material to a single 20t digger positioned in the centre.

In total 630 lorries – 30 to 50 a day – were required to remove the excavated material from the site over a four-week period. Scheduling them in was a major concern as a side access road was restricted to use by local shops, which meant using the congested Palace Street for deliveries and muck away.

‘Everything had to be delivered and taken away on a just-in-time basis as there was simply no room for on-site storage,’ explains Bouygues site manager Frank Girard. ‘We had gone for an insitu concrete frame because we prefer working with concrete, but that required room for storing material that we didn’t have.’ The alternative was scheduling in trucks fitted with concrete pumps to fill the foundation and walls. Roughly 3,000m3 of concrete and 400 tonnes of steel were required for the frame, with as much as 13 linear metres of wall poured a day.

But before pouring began, the foundations were waterproofed. ‘Waterproofing was important as the school is in the middle of a flood zone. Believe it or not, if it floods the official evacuation plan is go to Buckingham Palace and stand on the roof!’ quips Riley.

First, three coats of Flexcrete Cemprotec Clutch Filler, a water-based, epoxy and cementitious modified polymer mortar, was applied along the joints between piles, then a layer of Cemprotec E-Floor epoxy and cementitious modified polymer topping was applied directly to the piles up to a height of 500mm and on top of a hardcore layer across the excavated area. Next, a strengthened concrete mix designed to set in just 14 days was used to pour the ground slab. Once set, the concrete acted as permanent bracing for the sheet piling, and the hydraulic frame could be removed.

The Westminster City School extension is due to open on schedule and in time for the new school year in September. The fact that the team has managed to stick to programme despite the many logistical issues presented by the site is ascribed by Girard to Bouygues’s experience with concrete, which meant it didn’t have to subcontract any of the work and allowed it to build in a more ‘seat of the pants’ style than some might be comfortable with.

‘With standard details like a wall or a slab, we sometimes do the rebar without reference to drawings and based on our previous experiences. When drawings come in, sometime just a few hours before we close the shuttering and begin the pour, we make adjustments.’ It’s a risky strategy, but one that’s paid off..