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Demolition of a 100-metre three-span road bridge over an active railway.
The bridge was demolished over four weeks to make way for the northern section of HS2’s phase one route between London and the West Midlands.
Principal Contractor – Balfour Beatty Vinci JV
Demolition Contractor – John F Hunt Regeneration
Client – HS2
Demolition Methodology
Through use of the available archive drawings and site investigation Andun carried out a bridge stability assessment and designed a demolition sequence to reverse the original construction of the structure and remove the spans in reverse order of construction.
This mitigated the risk of stress reversals in the structure which have the potential to lead to instability.
A key constraint of this project was the need to remove span 2, which was situated over a live railway.
The methodology for the demolition was to remove the first two spans in two stages.
The first stage was to mechanically remove the deck and parapets, before using a mobile crane to lift the remaining steelwork away.
The final stage, once the first two spans were removed, was the third span.
As this span was located away from the railway, it was removed by mechanical demolition methods using an excavator breaking out the concrete.
The steel beams were removed using a controlled collapse mechanism onto some pre-prepared debris stock piles.
Temporary Works Design Elements
In addition to the demolition engineering and Network Rail CREd Role, Andun provided a range of supporting temporary works designs.
Track Protection – The track protection was constructed of a layer of geotextile over the rails and some filcor blocks and timber mats to allow excavators and plant to work over the railway without damaging the track.
3D Sequence Drawings – This allowed the demolition contractor to demonstrate the demolition sequence to stakeholders including HS2 and Network Rail.
The images were also included in the demolition contractors method statement for clarity in sequencing.
Lifting arrangements – To assist in lifting the beams, specific cuts were detailed. Sit cuts in the webs and V cuts in the flanges to avoid unnecessary lateral movement and ensure stability.
Crane Positioning – Due to operating in the rail environment this required an additional factor on top of the outrigger load to comply with the CPA guidance for mobile cranes alongside railways controlled by Network Rail.
The mobile crane was positioned on hardstanding in a former yard and was used throughout the cut and lift sequencing.
Bridge Stability Assessment – Andun carried out a stability assessment on the bridge to ensure that at every stage of the demolition the structure remained stable.
This included the design of stability strapping to provide lateral support to the tall deck beams of the bridge. The stability strapping provided was a simple but effective solution using tonne bags and fabric load-rated ratchet straps.
Network Rail Interface – Andun acted as CREd for the project which including interdisciplinary check meetings, submission of Form 0022 & 003s as well as organising the design checks as necessary for the project.