A large culvert was required to be
constructed through a 12m high embankment supporting a four track
main line railway, to provide a vital link in a seven mile long
flood alleviation channel on the River Thames, downstream from
Maidenhead.
Ground conditions at the site comprised
mixed clayey granular fill overlying water bearing sands and
gravels overlying weathered chalk, with a water table within 1.5m
of ground level.
The intended method of ensuring a stable
tunnel face was to grout the embankment and use a cellular
shield. However, after carrying out grouting trials it was
concluded that the security of the embankment could not be
guaranteed. The most certain method of stabilising the ground was
to freeze it.
The normal method of freezing ground
requires the installation of vertical freeze pipes.
Unfortunately, this was not practicable across the railway
embankment. Instead, an array of horizontal freeze pipes was
installed along the culvert alignment, between the south side
headwall and the north side reception pit.
Many drill steerage problems were encountered,
primarily due to the granular nature of the ground and the length
of the holes. A total of 180, 90mm diameter freeze and
instrumentation holes were drilled. It took three months to
freeze the ground, by which time a single concrete box had been
constructed on a jacking base in a dewatered open-cut excavation.
Freezing the sands and gravels dramatically improved their
strength and stability characteristics, permitting a
free-standing vertical face to be worked in an open shield.
The
patented ‘anti-drag system’ (ADS) was used at the top
and bottom of the box and anchored to the jacking base by means
of steel tendons. To prevent the box bottom and bottom ADS from
becoming frozen to the ground, heating cables were cast into the
box floor.
Once the ground was frozen, the jacking pit headwall
was removed and tunnelling commenced. Because the frozen sands
and gravels had a strength similar to medium strength concrete it
was necessary to use roadheader cutting equipment to excavate the
tunnel face.
A speed restriction of 60mph was imposed on the
railway during the jacking operation which took 28 days of round
the clock tunnelling to complete. The maximum jacking thrust was
6,000t .
The maximum total surface settlement during jacking
was 70mm. There was no movement in the direction of jacking. The
culvert was installed to a horizontal alignment within 30mm of
true, whilst vertical alignment was 150mm high at the front and
20mm high at the rear. The high front alignment derived from
early difficulties in trimming the extremely hard frozen ground
at the base of the shield, and was within the designed tolerance.
The concrete shield was used to form part of the
stop log structures incorporated at each end of the box, which
enable the culvert to be drained for maintenance purposes.
This was the largest box jacking project at the time, and was the first time jacked box tunnelling had been undertaken in frozen ground.
Client: Railtrack
System design: John Ropkins Ltd
Contractor: Edmund Nuttall Ltd
Scope of work: Design and construct
Contract period: 27 months
Completion date: December 1999
© John Ropkins Ltd