|All change at St Pancras|
|Stuart Armitage describes the engineering challenge of relocating a 350-tonne brick water container to a new site alongside St Pancras Canal Basin|
|On a cold evening in the late autumn last year a meticulously planned exercise came to fruition as the first 140-torme section of the St Pancras Waterpoint was lifted by crane to a specialised trailer ready for transportation to a new location. Faced with the prospect of this historic Grade II listed structure being demolished under the Channel Tunnel Rail Link Act, English Heritage had agreed that relocating it was the only practical way to ensure its survival.
The Waterpoint is an ornate, predominantly brick structure, which houses a 63,000 litre capacity water tank, originally used to store water to supply steam locomotives on the Midland Railway. Designed by the office of Sir George Gilbert Scott, architect of the gothic station buildings, the Waterpoint incorporates many finely detailed brick and stone features similar to those of the terminus complex. In particular, a series of gothic arches is incorporated into the front and back elevations of the nine metre wide by five metre long rectangular building. The arches rise from the first floor level to the base of the tank, spanning over recessed panels of extremely finely pointed, herringbone brickwork.
English Heritage does not generally endorse the relocation of listed buildings, as this causes a building to be dislocated from its historical context. But with relocation a necessity in this case, a suitable reception site was identified some 600 metres to the north, alongside St Pancras Canal Basin. English Heritage approached Heritage of London Trust Operations to undertake the Waterpoint relocation and subsequently the Morton Partnership was appointed as engineer. By decree set in the Channel Tunnel Rail Link Act, no work at St Pancras could begin until after 2 July 200 1, and the Waterpoint had to be moved by 31 December 2001 or it would be demolished. These constraints defined the six-month window in which the relocation project had to be carried out.
|The upper section lift from the transport trailer at the reception site
|The Morton Partnership assessed the impact of relocating the building brick-by-brick. We concluded that this would lead to an unacceptable level of damage because of the high strength of the original mortar relative to the strength of the brick and stone. Accordingly it was decided to cut the 350-torme building into sections and transport these to the new site. Specialist contractor Abbey Pynford developed the proposal to a full working design. At the same time, I became involved as project engineer responsible for structural design at the reception site and contract administration.
I had to identify and satisfy all of the statutory requirements of the local authority, liaise with English Heritage on the methodology for dismantling and
reassembling the Waterpoint, and liaise with Rail Link Engineering (RLE), the consortium managing the CTRL project. Work started on site at the beginning of July 2001. A very strict safety agreement with Railtrack permitted a site compound and operations adjacent to the mainline railway lines.
The agreed procedure for moving the Waterpoint involved making horizontal cuts through the six-millimetre bed joints at two levels, using a guided 600-millimetre long diamond-edged chainsaw to split the building into three sections. The upper section contained the iron water tank and surrounding brickwork; the middle section comprised the first floor and feature arches; and the lower section comprised the relatively plain brick plinth. It was decided that, due to the amount of damage to the plinth and because it was in part formed by the adjoining viaduct wall of the St Pancras Cellars. it would not be moved. Instead a replica of it would be constructed at the reception site.
|The guided diamond-edged 6mm masonry saw cutting through the joints in the 450mm thick brickwork|
|At each of the two cuts a reinforced-concrete ring lifting beam would be cast. A heavy lifting specialist, Jim Parkinson, would lift the upper and middle sections of the building on to trailers and transport them to the new site. There, a second 800-torme crane would reassemble the building on the replica plinth. The plinth's purpose-made pressed bricks were commissioned from a manufacturer in Leicestershire, located close to where the original bricks were produced.|
|The steel lifting frame being manoeuvred for the upper section lift at the original site|
|The lower-level, cast-in-place, heavily reinforced concrete lifting beam|
|The lower section on the transport trailer|
|I had turned my attention to designing the foundation slab at the reception site, which would span between the two massive brick viaduct walls adjacent to the canal basin. The design principle is that the load is transferred from the slab down through the walls to foundation level. Calculations for foundation loading had to be agreed with RLE, as the new location of the Waterpoint is directly above the crown of the proposed Thameslink tunnel. Bearing capacity calculations were carried out for the viaduct walls.|
|Top: the upper section lift at the reception site, viewed from St Pancras Canal Basin
Bottom: the upper section lift at the reception site
| The alignment of the new Thameslink tunnel also meant that the location of the piles to support the crane at the reception site had to be carefully considered. Similarly, at the original site the piles had to be located so as not to interfere with the new foundations for the station deck for the CTRL, which will be constructed directly over the original site of the Waterpoint. Once the heavily reinforced foundation slab was completed, the brick plinth was constructed. To ensure perfect alignment of the plinth, it was decided to construct it as two skins of brickwork. A core was built prior to the move to receive the middle and upper sections, and after the move face-work was constructed flush to the face of the imported brickwork above.
The lifting and transportation operation was completed over a period of three days in November 2001. Works to complete the plinth and repair and refurbish the Waterpoint will start soon. It is proposed that the finished building will serve primarily as a viewing tower, with a new internal staircase giving access to the tank where a timber deck will be constructed.
The new site of the Waterpoint on the viaduct will provide views of the CTRL terminus, King's Cross and St Pancras Stations and Camley Street Natural Park to the south, and over the canal basin to the north. The building will also provide for displays and many other educational or recreational uses. It will continue to provide an important feature on the Camden skyline.
|Stuart Armitage is anengineer with the Morton Partnership.|