A particularly challenging project was the
construction of twin box tunnels beneath Silver Street railway
station, Edmonton, North London, forming part of a new vehicular
underpass for the re-aligned North Circular Road.
As working space was limited, it was not
possible to construct a large jacking pit capable of
accommodating the boxes. Instead, a relatively small jacking pit
was used and the boxes were cast at ground level as a series of
counter-cast interlocking segments. These were individually
lifted into the pit, mated and stressed together using
longitudinal tendons. Both tunnels comprised thirty segments each
weighing 160t.
Two cellular concrete shields were constructed in the jacking pit.Thrust walls were constructed at the rear of the pit and groups of jacks were arranged on jacking rigs to provide thrust at all four corners of the box. This allowed the jacking thrust to be adjusted, to provide the steerage necessary in the early stages of tunnelling when the length of box being jacked was limited.
Ground conditions at the site were very
difficult. A layer of water-bearing gravel overlaid
over-consolidated clay containing a layer of sand in which there
was a high artesian water pressure.
To stabilise the gravel a jet grout cut-off wall was
constructed around the perimeter of the proposed tunnels and the
gravel inside this curtain was fully grouted. Horizontal pressure
relief drains were installed to relieve the artesian water
pressure in the sand layer. Horizontal ribs were incorporated in
the external sides of the boxes to prevent them from becoming
trapped by the over-consolidated clay as they advanced through
the ground.
The north tunnel was jacked to completion
first. Because the south tunnel had to pass close to a heavily
loaded bridge foundation, it was jacked sideways off the north
tunnel during its installation, in order to provide support to
the ground underlying the bridge foundation. The patented
‘anti-drag system’ (ADS) was used at the top and bottom
of both tunnels and on the side of the south tunnel adjacent to
the bridge foundation.
Once the difficult operation of entering the shield through the jacking pit headwall had been achieved, each tunnel took approximately four weeks to install. The maximum jacking thrust required was 5,500t.
On completion of jacking the face was boarded, the
top ADS ropes were pulled out, the longitudinal tendons were
fully stressed and the perimeter voids were grouted.
In spite of the need for steerage in the early stages of tunnelling, the boxes were installed to an accuracy of 25mm on both line and level. The maximum settlement of the overlying platforms arising from the installation of both tunnels was within the 75mm maximum specified. The railway tracks carried a 20mph speed restriction and their alignment was manually adjusted on the completion of each tunnel. As the settlement curve was extremely shallow, the settlements were readily tolerated by the station structures. It was necessary, however, to re-lay the platform copings and surfacing.
Client: British Rail (Southeast)
Designer: John Ropkins
Contractor: Jacked Structures Ltd / J Murphy & Sons Ltd
Scope of work: Design and construct
Contract period: 18 months
Completion date: February 1995
© John Ropkins Ltd