Balfour Beatty began work on the £355m A63 Castle Street scheme in Hull last year and has now reached a key phase of works – excavation.
The Highways England project involves lowering the A63 at the Mytongate junction into an underpass. Large and deep excavation work has begun to create the new 400-metre section of dual carriageway where east-west traffic will travel below north-south traffic.
Highways England project manager Fran Oliver said: “We’ve already carried out a large amount of work within the A63 Castle Street scheme since last year, including the completion of Murdoch’s Connection footbridge and the demolition of the Arco and Myton Centre buildings. On top of that we’re reaching the conclusion of extensive on site archaeology work at the 18th century Trinity Burial Ground.
“As a major scheme full of complex work in a busy city centre, it presents several unique engineering challenges, and this latest and integral part of constructing the underpass is arguably the most complex.”
Despite substantial in-house expertise, Balfour Beatty has subcontracted the ground engineering to German specialist Züblin Ground Engineering.
To construct the underpass walls, 200 panels will be installed, each 3.4 metres wide, 20 metres deep and are reinforced with concrete.
900mm-diameter piles will be sunk up to 30 metres deep. Without them, the high water table and tidal conditions could cause the base of the underpass to float due to buoyancy.
To help create the underpass base, a layer of high-pressure cementitious grout will be installed below the finish road level to seal out any water from below and also to grip the piles to add tension into the system preventing the underpass from floating.
The Mytongate bridge, which will go over the underpass, will be made from 52 beams, up to 11 metres long, each weighing six tonnes, with a concrete deck poured on top. Once the wall panels are completed Mytongate bridge will act as a prop supplemented by a further 15 mega props. Excavation work will see 40,000 cubic metres of material removed.
Work also continues nearby on construction of the new 37-metre Porter Street footbridge.