Two
existing under-bridges below mainline railways were required to
be strengthened and renovated whilst avoiding disruption to the
train services. To achieve this, new concrete box under-bridges
were constructed off line and then inserted into the voids of the
existing structures.
The patented box tracking system was used
to move the structures into position. This technique uses wire
ropes as the slide surface and as the means of propulsion.
In each case, prior to structural work
being undertaken, the existing brickwork to the bridge abutments
was cleaned, repaired where necessary and repointed. One metre
wide strips were excavated between the abutments and then filled
with mass concrete to provide a one metre deep foundation for the
tracked box in its final position.
A twelve metre long approach strip adjacent
to the subway entrance was excavated and filled with hardcore.
Blinding concrete was then laid on the hardcore and continued
into the subway opening to provide an accurate casting base and
slide path.
Groups of wire ropes were laid down along
the length of the slide path and casting base.
The reinforced concrete box was then constructed,
incorporating a steel soffit plate laid on top of the wire ropes
and a semi-circular steel dolly built into the rear of the box
base. Prior to the commencement of tracking, horizontal shear
keys were cut into the existing brickwork of the abutments and
the voids under the existing deck troughs filled with concrete.
Steel guide angles were positioned on the slide path and the box
was waterproofed.
The wire ropes were wrapped around the dolly and
gripped to a steel anchor beam laid loose inside the box. Two
1300mm stroke 200t jacks provided the driving thrust, reacting
against brackets fixed to the box base. The jacks pushed the
anchor beam forward which pulled the ropes around the dolly,
forcing the box to slide forward on the ropes.
At the end of each 1300mm stroke the jacks, anchor
beam and ropes were reset and the operation repeated. It took
only two hours to track the box into its final position. Gaps
50mm to 75mm wide were left around the sides and top of the box
and these were later grouted.
Finally, parapets were constructed on either side of
the under-bridge and the external ends faced with brickwork.
As box tracking does not require any external reaction arrangements, it is a very cost effective means of moving a box structure into position.The technique can be used for moving very much larger structures than those featured here.
Client: British Rail (South-West)
Designer: John Ropkins
Contractor: Edmund Nuttall Ltd
Scope of work: Design and construct
Contract period: 3 months
Completion date: March 1996
© John Ropkins Ltd