aligned obliquely over a length of more than 32 m across the
passage. Once the cofferdams are in place, the water within
will be drained out, and the steel rings and brickwork will be
removed from the top down, Miller says.
Although this plan will involve the outright closure of the
Royal Victoria Dock for roughly six months, this was considered the safest option in order to avoid a potentially catastrophic “sudden inundation event” in which the existing
tunnels might be breached while still underwater, Miller says.
In that worst-case scenario, millions of gallons of dock water
would pour into the tunnels, and the resulting rapid loss of
water in the docks could cause a progressive collapse of the
fragile dock walls. The dock walls will be supported by a customized system of props and carefully monitored as the water
is removed, Miller adds, and other nearby structures also will
be monitored for movement.
Because the groundwater level on the northern and
southern sides of the passage is quite high, the project will
also install a so-called tube-à-manchete system over a length
of roughly 100 m on either side of the passage, Miller says.
This will involve drilling a series of perforated plastic tubes
The intermittent arches that line the approach ramps to the
Connaught Tunnel will be cleaned, repointed, and structurally
repaired where necessary. The vertical alignment will be
lowered to accommodate the size of modern trains.
into the ground and then pumping in an environmentally benign grout to consolidate and seal the surrounding
The high water table and the greater weights of modern
trains mean that the marshy soil beneath the surface portions
of the Connaught Tunnel also will have to be strengthened,
Miller notes. This is being accomplished by the installation
of some 3,000 concrete columns in the ground at either end
of the tunnel; the columns will be installed to depths of as
much as 9 m in order to reach the more competent geology
of the river terrace deposits, she notes. The work on the Connaught Tunnel is expected to be completed in 2014, although
completion of the overall Crossrail project is not expected until 2018. —ROBERT L. REID
SEPTEMBER 2012 Civil Engineering