The
largest of three boxes installed at Lewisham railway station, the
twin cell tunnel was required to enable a new extension of the
Docklands Light Railway to pass under the existing tracks and
platforms.
Of particular interest on this project were
the following factors. The ground through which the tunnel had to
be driven was very weak comprising loose silt and sand overlying
soft clay. The tunnel had to be installed at a depth of 1.7m
below the rail sleepers and at a severe skew angle to the
existing tracks and platforms. The tunnel passed under the
existing platform foundations and through the foundations of the
existing outer platform walls. Moreover, there were clear signs
of past movement of the existing embankment side slope and of the
platform structures on the jacking pit side. An abandoned culvert
was present in the bottom right hand corner over the full length
of the proposed drive.
A composite cellular shield was required to
properly support the tunnel face, comprising a fabricated steel
upper portion and a concrete lower portion. The steel portion
could be embedded in the silts and sands whilst the relatively
wide walls of the lower portion could be used to support the soft
clay.
The patented ‘anti-drag system’
(ADS) was used at the top and bottom of the box and clay slurry
lubricant was used at the sides.
As the jacking pit was shallow and the box
very wide, anchorage for the top ADS was difficult. The ADS was
anchored by a series of tendons which passed around pulleys
mounted on the back of the box and which were then anchored to
the jacking base. Jacks were used at the tendon anchorages to
compensate for stretch in the tendons as the ADS load
progressively increased to around 800t.
Differential sideways loading on the box arising
from the severe skew was countered by side guides and, during the
early stages of tunnelling, by the application of eccentric
jacking thrust.
The tunnel was installed to within 25mm on both line
and level and required a maximum jacking thrust of 4,500t. The
maximum total surface settlement was 75mm and the maximum surface
movement in the direction of jacking was 70mm. Track alignment
was maintained by manual fettling as necessary.
Minor reinstatement work was required to the
platforms after tunnelling and was carried out during routinely
available night-time possessions.
The railway station and tracks remained fully operational throughout the installation period.
Client: Mowlem Civil Engineering Ltd
System design: John Ropkins Ltd
Contractor: Edmund Nuttall Ltd
Scope of work: Design and construct
Contract period: 14 months
Completion date: February 1998
© John Ropkins Ltd