A pedestrian and cyclist subway was
required under a four track main line railway at Didcot,
Oxfordshire, to provide access between a new housing development
and Didcot town centre.
It was essential that neither the two
‘Intercity 125’ tracks nor the two local tracks should
be closed or diverted. A ‘jacked box tunnel’ solution
was therefore adopted.
The subway was to be installed
approximately 1.7m below the railway sleepers. Ground conditions
at the site comprised a loosely compacted silt-stone fill
overlying soft clay. The ground water table was just below the
box.
A single concrete box was constructed on a
jacking base in a jacking pit to one side of the railway. A steel
cellular shield was rigidly attached to the front of the box and
two jacking rigs to the rear. The patented ‘anti-drag
system’ (ADS) was used to control ground drag at the top of
the box during tunnelling.The tunnel face was hand mined and the
spoil removed by conveyor.
During tunnelling and subsequent grouting, speed
restrictions of 40mph and 20mph were imposed on the high speed
and local tracks respectively. After entering the shield through
the jacking pit headwall, the tunnelling operation took just five
days of continuous working to complete and required a maximum
jacking force of 800t.
During
tunnelling the tracks and embankment were regularly monitored and
rail misalignment was corrected twice by a small track
maintenance gang.
The subway box was installed to a horizontal
alignment within 25mm of true, whilst vertical alignment was 55mm
low at the front and 15mm low at the rear.
Accumulative ground surface settlement was limited to a maximum of 75mm, this occurring above the centre of the box. Accumulative horizontal movement of the ground surface in the direction of jacking was limited to a maximum of 25mm. These movements developed slowly and in a predictable manner.
On completion of tunnelling, the ADS ropes were
removed and all perimeter voids were grouted. Reinforced
brickwork retaining walls were then constructed in-situ to
complete the subway.
This project was nominated by British Rail (Western) for an Institution of Civil Engineers merit award.
Client: British Rail (Western)
Designer: John Ropkins
Contractor: Jacked Structures Ltd
Scope of work: Design and construct
Contract period: 9.5 months
Completion date: June 1989
© John Ropkins Ltd