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No room to move

The deep basement of a new cancer treatment centre is being built on a tight site in London. Jessica Rowson reports.

Relatively speaking, the London Clinic's basement is probably one of the deepest ever constructed in the capital. The new cancer treatment centre is being squeezed in between three roads and Georgian buildings on a site which is only 1,200m2 in plan.

But at 16m deep, its volume is a cavernous 1,7400m3. However, the design team's biggest challenge has been to fit in all the state of the art medical equipment in the space available on each floor.

One of the major pieces of kit to be installed will be a Cyberknife Robotic Radiosurgery system and two 100 tonne LINAC linear accelerators. Both are pieces of equipment which treat cancer patients with doses of radiation. To prevent radiation reaching other people in the building,
the equipment must be shielded. This has traditionally been done using concrete or lead.

Instead, Alan Baxter Associates decided to use Ledite blocks, a radiation shielding technology launched in America in 1993. Ledite blocks are about twice as dense as concrete and take up less space compared to concrete shielding.

"The walls would have been between 1.5m and 2m thick if you used normal concrete," explains Alan Baxter partner Jim Gardiner. This is compared with the 1.3m thick Ledite lining.

"It's quite a restricted site and if we had used concrete we wouldn't have been able to get so many machines in. Also if the machines become redundant [and need replacing] the blocks can be removed."

LINACs weigh a massive 100t each and are very sensitive to movement. The Cyberknife operates to a fraction of a millimetre. Because of this, Alan Baxter decided to pile the basement's 1.5m thick raft to make it more resistant to ground movement, despite indications that ground movements would have been minimal.

"If the machines are just a fraction out they don't work," explains Gardiner. "We could have designed the slab as a raft, but because of the equipment – even though we were talking about incredibly small movements – we decided to pile it and put clay board underneath."

The LINAC machines with their associated shielding have a large footprint and it was a requirement that three be fitted on the lowest floor with potential for a future fourth. The only way to make everything fit, even with the thinner Ledite shielding, was to push the basement walls out to the site boundary.

"It's a big job and not a huge site," says Gardiner.

There was an existing masonry and concrete basement on the site, but as space was so tight, the existing basement wall had to be demolished at the same time as the new basement wall was installed, taking care to ensure that the surrounding roads and structures were undisturbed.

Contractor McGee achieved this by installing kingposts in the ground at approx 3m intervals allowing the small strips of wall in between to be cut away one at a time. Panels between the posts could then be installed. Doing this allowed the new basement walls to be pushed out by about 400mm, leading to a gain of 40m2 in total.

Below the kingpost retaining wall, the new basement walls are piled. Secant construction, where the piles interlock to form a waterproof barrier, was used through the first 6m of fill and gravel just below ground where there were concerns about water ingress.

Contiguous piling, where the piles are next to each other, was constructed through the underlying London Clay below. Clay is less permeable and therefore a lower grade of basement wall was acceptable.

The basement was built from the bottom up. The piled walls were propped as the basement was excavated and the props were replaced by floor plates as the basement construction worked its way back up. The 600mm diameter piles used in the walls were sized on the deflection criteria and span between the wall props, which was 5.5m in the lowest storey.

A tunnel is being built across the road at Devonshire Place, to connect the main hospital building with the new cancer centre.

"There are operating theatres on one side of the road and cancer treatment and imaging on the other," explains Gardiner. "The tunnel will avoid patients having to go outside between the two."

McGee hopes to break through into the basement of the original building next month after which the road above will be rebuilt. The London Clinic basement is due to be completed by the end of February although work above ground will continue for another two years.

Keeping the metro moving

The site already constrained by streets on three sides, has one further complication: the busy Marylebone Road that runs along the north side of the site also has the Metropolitan line running underneath.

The cut and cover Metropolitan line tunnel and the shallow brick arch which provides its roof is near the road surface, with just 670mm cover at its shallowest point.

"Ground movements were a huge issue," explains Gardiner. "As there was not much load on the arch, the relatively small movement of the retaining wall could have led to a brick falling out onto a train."

Geotechnical Consulting Group (GCG) provided specialist advice on the impact of building so close to the tunnel.

"The main problem was that we were dealing with a 160 year old brick tunnel", explains Professor Hugh St John, GCG director. "The worry is the arch would stretch causing cracking. The arch has not got a lot of weight and if the two sides move out, it starts to drop."

GCG modelled the effect of the excavation on the tunnel while the new tunnel and basement retaining walls were monitored throughout construction to check for movement. "The wall movements are pretty much as expected," says St John.

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