In April the contract C315 for refurbishment of the tunnel (formerly known as the Silvertown Tunnel) was awarded to Vinci Construction UK at a value of about GBP 35M (USD 56.9M), beating a JV of Hochtief Construction, J Murphy & Sons and BAM Nuttall; and another Volker Fitzpatrick and Barhale.
The Connaught was built as a 600m-long twin-bore, brick-lined railway tunnel. Some rail and ceramic insulators are still present from the third-rail electrified track for the North London Line to North Woolwich that closed in 2006 when the Docklands Light Railway Stratford Extension took over passenger duties in the area. In relation to current surface features it runs under the Connaught Passage waterway between the Royal Victoria and Royal Albert Docks.
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Site investigation
Capita Symonds engineering geologist Sophie Norgate has been working with Oli Mills from specialist contractor Soil Mechanics and Crossrail engineers on extensive site investigations, including cored boring in the brick and concrete invert of the existing tunnels, verification of the state of the existing lining and reference surveying before the main construction work begins. The tunnel is adjacent to a busy highway bridge over the dock area, and also London’s City Airport, which lies between two dock basins. It was agreed that the surface should be extensively monitored for movement. One other investigation task that remained was to check on the condition of apparent service tunnels alongside the main tunnels, revealed only on original construction sections. Others to be carried out by Vinci included checking for the presence of unexploded World War II bombs, and asbestos in the structure and ground fill.
A recent visit was led by Linda Miller, project manager on the site for Crossrail, an American tunnelling engineer with experience on Boston’s enormous ‘Big Dig’ highway project, and a former US Army helicopter pilot. The site manager for Vinci is Richard Wall-Morris.
Victorian railway
The original tunnel was constructed for steam rail in 1878 by cut-and-cover methods. The docks were constructed simultaneously, including the Connaught Passage over the tunnel. This led to problems in 1935 when it was found that the larger ships that were then using the old docks had started to scrape the top of the tunnel structure (following dredging), leading to fears of inundation through structural damage. In order to preserve the Connaught Tunnel the central, underwater, length of brickwork lining was replaced with a full circle of cast-iron bolted segment rings. Another structural hazard occurred in the blitz of WW II when a bomb penetrated the tunnel lining. This was repaired with concrete and the patch can still be seen.
Other features of the existing structure include refuge ‘manholes’, and two circular ventilation shafts at each end of the twin bores. At the surface the two shafts have brick structures around them.
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By GlobalDataThe new section needs to be larger to accommodate Crossrail trains and overhead power catenaries. In order to accomplish this the invert of the tunnel will be lowered and the weaker sections, including the cast-iron lining, replaced. About 100m of tunnel wall will be removed and replaced with pre-cast concrete lining in a ‘binocular’ section (see figure 1, left). The brick arches at both ends near the tunnel portals will be preserved and repaired as they have been found to be generally sound and of architectural merit.
In order to reinforce the central, underwater length of tunnel, a large concrete slab will be laid in the passage floor between the docks to protect and stabilise the tunnel structure. Vinci will first dredge a wide channel in the Connaught Passage just west of the Connaught Bridge Road to allow the placement of a onemetre- thick, reinforced-concrete slab of 1,000m3. This will necessitate the use of marine pontoons and divers to tremie the concrete into place.
Once the concrete slab is secure the cast-iron lining will be removed, one ring at a time, for installation of new support. During this stage of construction the remaining brick-lined tunnel will be first structurally grouted to ensure ground stability, and then filled with low-strength, foam concrete to maintain lining integrity. Excavation of twin bores to the new necessary sectional dimensions will take place through the foam concrete by smallscale methods. The actual excavation method has not yet been decided but Miller revealed that rotary cutterheads, probably mounted on mini-excavators, are likely to figure highly in the work rather than manual methods. The pre-cast concrete segmental rings will then be installed as excavation progress allows.
Water handling
When in use the railway tunnel has had to have continuous pumping of drainage water employing a Victorian octagonal pump house on the surface. Water will also feature during construction, although the final waterproofing solution has not been decided yet.
Wall-Morris told T&TI that current thinking was to connect a central sump and pumping chamber in the tunnel with the surface at the existing pumphouse that will be retained and upgraded. The connection will be made by employing pipe jacking or directional drilling. Drain water would then be removed via this route.
One current feature of the Connaught Tunnel that amused Miller is that one graffiti artist, in additional to the usual ‘tags,’ has adorned a refuge hole with the slogan “ tunnels.”
Any connection with the purchase of a can of white spray paint by the T&TI editorial staff is purely coincidental!
Project manager Linda Miller beneath brick arches near the west portal of the tunnel Figure 1 – Planned refurbishment of the twin bores to excavate the invert and enlarge the bore within temporary foam concrete. Note also the position of parallel service tunnels to be investigated. One of two air shafts viewed from the tunnel where it splits into two bores The Victorian pumping station adjacent to the Connaught Passage between the docks, Connaught Bridge and DLR
