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High Holborn Hot Spot

A cassette raft foundation is being used in Holborn to overcome Crossrail.

Where London's Southampton Row meets High Holborn is a busy junction, both above and below ground. Below the junction, is a knot of used and disused tunnels, as well as space assigned for the proposed Crossrail route. And above ground, contractor Bovis Lend Lease is building a new office block at 1 Southampton Row.

Bovis has demolished the interior of the existing neoclassical building, while retaining the façade. It is now preparing to cast the concrete basement slab to allow the new seven-storey scheme to sit on top of this snarl of tunnels.

There are two main constraints on this £40M office and retail project. The first is that the façade had to be retained in order for the project to gain planning permission. This has added to problems of working on a tight inner-city site. So consultant Ramboll Whitbybird specified an external retention scheme to maximise the space inside for other construction activities. The other problem is the location of the future Crossrail tunnels.

"There is a lot of talk about how to tunnel through London, but there is also a knock-on effect on new buildings on top of the tunnels," says for Ramboll Whitbybird project director Simon Smith.

When Crossrail is up and running in 2017, the east and west tunnels will run directly under the site, and their exclusion zones will cover almost the entire building footprint. This eliminated the possibility of piling deep into the ground, forcing the designers to think outside the box when it came to supporting the six storeys of new structure.

And, of course, foundation design is further complicated by the requirement for column-free space exerting greater loads on fewer points.

Ramboll Whitbybird proposed two possible column layouts: one with 12 internal columns and one with only four internal columns.

"The four-column option has the 'wow' factor, but left lots of the point loads on the edge of the raft," explains Smith.

Point loads on the edge of raft foundations force the foundation to work harder to distribute the load into the soil. The raft needs to be
stiffened to allow the load to spread further.

A heavily reinforced 1.5m-thick raft was proposed in the original feasibility study. But results of the project's site investigation dealt a devastating blow to the designers. It showed that neither the foundations for the façade nor the groundwater levels were as had been assumed.




The façade piers rested on pad foundations and not the more typical strip footing. Pad foundations are difficult to underpin and must be jet grouted. This involves the in-situ mixing of soils with cement grout, using a special rotary drilling tool to increase the strength of the soil. And this would impact on the construction programme.

But this was the least of Ramboll Whitbybird's worries. The groundwater was higher than originally thought – nearly 1m above planned formation level. So the site would have had to be sealed down to clay level.

"Both jet grouting and a secant basement wall would have been added on to the programme and this all happened around the same time as the cost review," Smith recalls. "Everything was against us."

Some quick thinking was required and the cassette raft solution, which had been deemed "too mad" earlier in the project proved to be the best solution.

Using this form of construction halved the thickness of the basement slab to 750mm, enabling formation level to be higher, above groundwater level and façade footing.

A cassette or cellular raft is made up of a substantial basement and ground floor slabs, connected by a grid of concrete cross walls that run through the basement storey.

"It's a stiffer structure which will mean less differential settlement. Therefore, when Crossrail do tunnel below, the building will be more resistant. Also, there are high-point loads around the perimeter of the building, due to the column-free layout. The stiff structure helps to distribute these loads," Smith explains.

The development is due for completion in April 2009.

History of the cassette raft

The concept was used after the Second World War when materials were scarce. It comprises two slabs linked by a grid of walls. However, the complicated analysis procedures involved meant it was difficult to allow for openings in the basement's interior walls. There is now software available that makes analysis easier. Sofistix, a 3D finite element programme, was used to model ground conditions and soil structure interaction.

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