A challenging project for the Roger Bullivant teams.
Guildhall is situated on the River Ouse, the complex contains a collection of Grade I, II* and II listed buildings built around the 15th-century hall and riverside meeting room, with parts dating from 1445.
Naturally, this project faced many challenges from challenging ground conditions, piling engineering for the restoration of part of the building through to the continuous challenges regarding restricted access. The only access to the site at the first stage was via the front entrance through a small brickwork corridor from a restricted access road. Guildhall is located within St Helen’s Square, an urban courtyard surrounded by fashion, lifestyle, and eateries. Plant and construction methods were required to be carefully selected to make use of the restricted width alleys that give access to the site.
The River Ouse was utilised for deliveries, a tower crane was erected on-site to enable plant and materials to be lifted into the work area from a barge.
RB carried out the design and installation works for the crane base mini piling, underpinning piling, and SFA# mini piling to the proposed South Range (Grade II* listed) and North Annexe buildings. RB installed 220mm bottom driven mini piles to depths of 14m onto sandstone rock head to a swl of 165Kn compression, -47.5Kn tension, and 5Kn shear for the proposed crane base.
For the South Range new build section, RB installed 12No. 300mm sectional flight auger to the South Range new build section. The only access to the site was via the archway through Mansion House off Coney street. This was not suitable for concrete delivery and therefore provided a solution to install the piles using batched grout.
RB installed 56 No. 450mm sectional flight auger piles for the North Annexe new build section. A small concrete wagon and pump gained access via the enclosed alleyway off Lendal to complete this section of the work.
Due to historic movement RB was then required to underpin the existing North Annexe Tower to allow the new North Annexe to be constructed. RB originally proposed a vibration-free jack pile system integral with RC ground beams externally and internally connected with needle beams.
During the excavation works for the ground beams carried out by VINCI Construction UK, an old brick arch was discovered within the makeup of the main wall. A major redesign was carried out and involved stepping underpinning down in the arch and then returning to its original level beyond the arch. RB installed 16No. 220mm BDST piles for the tower crane.
The underpinning works were the most challenging, the discovery of the arch exacerbated the access problems as this resulted in further reduced levels. Working closely with VINCI, they provided scaffold access and a lifting gantry to gain access to the underpinning area. Upon curing of the underpinning system, vibration-free jack piles were installed using the dead weight of the structure as a reaction to install the piles. This was the preferred method of VINCI Construction UK, Arup, and York City Council due to the vibration-free operation.
A section of the jack piles installed, refused 6m into the ground on unknown obstructions. All piling works were delivered within the programme schedule allowing follow on works to proceed as planned.
The project ended in early November 2021 after 23 weeks of works.
VINCI Construction UK, Project Manager
City of York Council, Assistant Director of Regeneration and Assest Management
#SFA piling offers a similar solution to CFA piling, except the auger is made up of shorter sections of hollow stem continuous flight auger, added as the drilling progresses, thus allowing intervention in areas of limited height.