Digging Deep
The excavation for a tunnel to link Regent Street's former Regent Palace Hotel and Café Royal sites held more than a few surprises for contractors.
A 7.5m deep tunnel below Glasshouse Street will allow goods and materials to be ferried between the two sites, currently being redeveloped into offices, shops, a tourist information centre and a luxury hotel. By servicing both sites, via the tunnel, from a single service yard on the Regent Palace Hotel site, Glasshouse Street will be freed up for the extensive public realm improvements planned by landowner the Crown Estate.
Prior to excavation work beginning Sir Robert McAlpine Technical Services devised a temporary works system that would also support a bridge over the deep trench - permitting construction vehicles to travel along Glasshouse Street whilst work carried on below - and successfully applied for a road closure from Westminster Council.
In late 2008 workers began digging slit trenches to get a glimpse of the services below the road. Having already carried out an electronic scan, as well as reviewing the written records provided by utilities companies, Sir Robert McAlpine was not expecting too many surprises.
But the 1.5m deep trenches revealed a large number of additional pipes and cables, clashing with the proposed positioning of seven of eight king posts. As well as extensive unrecorded fibre optic cabling, two mystery services had the contractors scratching their heads. Sir Robert McAlpine's Gareth Healy says: "We believe two were old gas mains that fed gas-powered street lights in the early 1900s. Plus, there was a 1.5 inch thick cast iron pipe from around 1871 that we're pretty sure was used as part of the old London hydraulic power system. I actually found that one on Google Images, by typing in ‘cast iron pipe with oval joints'. An item came up about the way the Victorians had used these high pressure pipes to pipe water to power lifts in buildings." During the late 1800s, water power was transmitted through a vast network of hydraulic mains to thousands of hotels, shops, offices, mansion blocks, docks and factories.
The discovery forced a major redesign. "Because of the extent of the services, there was 
very little flexibility in where the king posts could be located," Healy says. After a concentrated effort from the site team and SRM's technical services department, the king posts were successfully moved into new positions, with one fitting into such a tight gap the post had to be reduced in size and reinforced to compensate. But the installation of beams between the king posts to hang the utility services off while excavation carried on below revealed a second clash, this time with the gas main supplying the Café Royal. Works were delayed while the team waited for the Café Royal to move out before applying to disconnect the offending main. "Normally to get a gas main disconnected takes four to six weeks,"Healy says. "We did it in 16 days. Then we started excavation."
The work continued without incident, until the diggers revealed an old Victorian sewer. Helpfully, this obstacle was clearly marked on plans and the contractors were prepared, having diverted the sewer prior to any work beginning. After breaking through the top to check the diversion had been successful, the brick sewer was demolished. "Quite fortunately no services were in the sewer, or we would have been in a right pickle," Healy says. It has become common for utilities companies to use the additional space in the huge Victorian sewers - typically large enough for a person to stand in - to locate their wires and cables.
"Then we exposed the underside of the Café Royal retaining wall," Healy recalls. "Once again, we weren't fully aware this was there as we didn't have their construction drawings. This brought a major design change to the process." The design team were sent back to the drawing board once more to create an underpinning system to support the café building. Old brick walls, the sides of long-forgotten vaults underneath the Regent Palace Hotel, were also discovered, and had to be hammered out.
Having overcome a considerable number of unforeseen obstacles, the complex excavation was completed in May.
Stanhope development director Paul Hargreaves says: "The tunnel excavation has been hugely challenging, and the fact the team has been able to come up with a workable solution is a real testament to their hard work and resourcefulness."
