SMITHAM STATION UNDERPASS JACKED BOX
The new A23 carriageway is designed to pass under Tattenham Corner Branch Line at Smitham Station. The station and the railway are to remain operational throughout the project, so an engineering solution was required that avoided impact on both.

Hochtief and their designers, Scott Wilson Rail in conjunction with Tony Gee and Parners, proposed a construction method whereby the road underpass for the new carriageway would be constructed "offline" just south of station access road, and then jacked into position.

A method for achieving this goal is outlined here.

PREPARE ROAD AND STATION FOR JACKING
The existing Station Access Road, south of Smitham Station, will be temporarily diverted for a period of around 6 weeks to construct a piled concrete slab that will support the existing road when the underpass is jacked beneath. The slab will also anchor 600mm diameter horizontal steel tubes that will be augered in from the Windermere Road side to provide temporary support to the station platforms and track when the underpass is jacked beneath.
Here we see a cross section through Smitham station along the line of the new road, with the piles and horizontal tubes shown in red and the concrete slab in purple. Once the slab is complete the road will be returned to its original location. Pedestrian access from the existing Station access road will be maintained throughout.

The station subway will be replaced with a temporary footbridge between platforms during Summer 2004. A temporary access from Windermere Road to the London-bound platform will be provided prior to subway closure.

BRIDGE IN CONSTRUCTION LOCATION The above diagram shows in pink the concrete underpass box in its construction location. The box will have a span of 22m and will be 37m long. In addition to the box itself, a jacking slab will be constructed beneath the bridge (shown in green). This provides a low-friction sliding surface, and sufficient weight to push against once the bridge is ready to be moved.

JACKING UNDER THE ROAD AND STATION
Once the box has been constructed, the jacking equipment will be brought to site and a trial slide undertaken. The diagram below indicates how the jacking operation will look. A tunneling “shield” at the front of the box will advance as the ground is excavated at the shield face. The jacking is expected to take about four weeks.

JACKING UNDER PLATFORM 1

JACKING UNDER THE TRACKS

JACKING UNDER PLATFORM 2 TO FINAL POSITION

BRIDGE AND COMPLETED ROAD


Once the box is in position, the jacking slab will be covered with a layer of fill material to prepare for the new road. The A23 carriageway, with footways and cycleways either side, will pass beneath the station and Station Access Road between piled retaining walls, clad with architectural brickwork.