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Mercury Bridge - The Repairs - A summary by PJ Welch - Mouchel North Yorkshire

Following collapse of the upstream end of one of the piers, causing partial failure of each of two abutting arch spans. North Yorkshire County Council instigated an operation to restore the bridge, a Listed Building, to its original form.

The County Council's Consulting Engineers Mouchel were to manage the project and to design the permanent works to rebuild the bridge. In order to prevent further collapse immediately, Morrison Construction were engaged to install temporary support works, which were conceived and designed by Morrison Construction together with Butterley Engineering, with assistance from Pell Frischmann on foundation design.

As an immediate prelude to this, a substantial bund was formed upstream of the damaged pier, to protect it from further scour, supplemented by a large volume of stone installed to fill the scoured voids in the river bed.

The main difficulty in supporting the remaining arches lay in precluding the hazard of having men work beneath them: the form of support had also to be such as not to obstruct flood flows through the bridge, nor to inhibit access for subsequent building of a new pier and arch repairs.

This was done by forming cylindrical caisson supports deeply seated in the river bed or foundations supported on piles at each 'corner' of each of the two affected spans. Onto these were placed extremely deep and stiff steel beams, running close and parallel to each side of each pier: these were installed by a process involving a remotely positioned crane, the beams having a launch nose so as to allow them to be presented and entered horizontally through the arches.

To support the arches (both in the event of further pier settlement and for new construction, to replace that which had collapsed), a series of triangular steel support frames had next to be installed, seated on the large through beams and tied transversely across the span, to prevent 'spread' of the feet under load. These frames were again positioned outside of the bridge structure and pushed through into position without exposing the workforce to the risk of further collapse of the as-yet-unsupported arch.

Timber chocking next ensured a close and continuous support for the remaining masonry arch rings. The bridge was now secure against further collapse and it was possible to work beneath it, so as restore the foundation integrity and to rebuild the missing section of pier.

The geology of the site is such that a permanent foundation of sufficient strength could not realistically be formed at an ideally modest depth and piling was required, both for the replacement section of pier and to consolidate the remaining length of pier. This was done using small diameter Fondedile piling which, drilled from a high level through the core of the remaining pier, is invisible on completion and thereby meets the imperative to retain the appearance of this Listed Building.

Given the propensity of the River Swale for flooding, the ground level beneath the undamaged end spans was lowered to compensate for the partial obstruction of the centre spans by temporary works.

At this stage, it was judged important by the County Council that vehicular access, at least for light vehicles, be restored at the earliest time and this was accomplished by the installation of a slightly adapted proprietary steel temporary bridge, already held by the County Council for emergency use. Sited away from the damaged side of the bridge, and over the remaining length of pier, whose foundations were now secure, this allowed a single lane traffic regime to operate, subject to a 7.5 tonne vehicle weight limit. Pedestrians were also accommodated.

A temporary cofferdam was designed in steel and placed to enclose the position of the intended pier rebuild and, despite repeated inundations by subsequent river floods, it allowed the formation of a stainless steel reinforced concrete pier base on the heads of the supporting Fondedile piles.

A new masonry pier section with reinforced core was next built onto the foundation and grafted onto the end of the remaining section of the original pier. The stone selected will be a good match for the original. Onto this and onto supporting framework resting on the triangular steel support frames, rebuilding of the broken arch sections is now proceeding, in solid masonry.

The geometry of this element is extraordinarily complex because of the imperfect geometry of the existing arches, with which it has to harmonise. On its completion, a weak concrete infill will be installed to support the road and the spandrel and parapet walls will be rebuilt as the original.

Prior to the execution of the works, a careful evaluation of the environmental impact was undertaken and during the work, great care has been taken to control any potential pollution of the waterway. On completion of the rebuild, care will be taken to restore the disturbed environs of the bridge to a safe, stable and aesthetically acceptable state.

A great deal of liaison and co-operation has been required to execute this scheme, involving the County Council, Morrison Construction, Dunhouse Quarries, Eslington Stone, Mouchel, Pell Frischmann, Fondedile Foundations, Richmondshire District Council, the Environment Agency, landowners and English Heritage.

The project managers wish to express their appreciation of the help and co-operation of all parties.

PJ Welch
Structural Engineering Manager
Mouchel North Yorkshire

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