An under-bridge was required below the
single track Upminster to Gray’s railway, to provide a main
road link between the Lakeside Shopping Centre and the Chafford
Hundred housing development.
High voltage overhead power lines along both sides of the track, overhead track electrification, and the need for British Rail to avoid closure of the railway, led to a ‘jacked box tunnel’ solution being adopted.
Ground conditions at the site comprised
weathered chalk overlaid by loose sand. The chalk contained
swallow holes that had been loosely filled with sand from above.
A single concrete box was constructed on a
jacking base in a jacking pit to one side of the railway. A
concrete shield was rigidly attached to the front of the box and
five jacking rigs were attached to the rear. Steel
‘spiles’ were incorporated in the roof of the shield to
control the flow of sand from the swallow holes as they became
exposed at the tunnel face.
The patented ‘anti-drag system’ (ADS) was
used at the top and bottom of the box. The top ADS controlled
disturbance of the overlying ground and the bottom ADS maintained
the box on an accurate vertical alignment. The top 2m of the face
was hand mined and machine excavators were used to mine the lower
7.5m of face. A speed restriction of 20mph was imposed during
tunnelling and subsequent grouting
The box was installed to an accuracy of
15mm on both line and level. Total ground surface settlement was
limited to a maximum of 100mm over the centre of the box, whilst
ground movement in the direction of jacking was limited to a
maximum of 15mm.
During tunnelling, the ground surface and track were
monitored for movement. Track alignment was maintained manually
by a small maintenance gang.
Most of the settlement was attributable to a loss of
sand from the swallow holes which, in spite of the measures
adopted, proved difficult to avoid completely. A maximum jacking
thrust of 3,500t was required and the jacking operation took just
9 days.
The contract included concrete portals at each end
of the under-bridge. The reception end portal was constructed
prior to tunnelling and the shield was jacked into it. An in-situ
connection was then made to complete the monolithic structure.
This was the largest box jacking project in the U.K. at the time and the techniques employed were so innovative that it was featured on the BBC television programme ‘Tomorrow’s World’.
Client: British Rail (Eastern)
Designer: John Ropkins
Contractor: Jacked Structures Ltd
Scope of work: Design and construct
Contract period: 18 months
Completion date: June 1991
© John Ropkins Ltd