In October 2008 the normally tranquil Sheinton brook became a raging torrent after exceptionally heavy rainfall swept across Shropshire.
At Sheinton, the force of the water undermined one of the supporting piers for the three span brick-built Grade II listed bridge across the brook. It collapsed while engineers were reinforcing the damaged foundations. They had moved to a safe area minutes before after a contractor noticed cracking along the structure.
The collapse caused significant disruption to local residents who faced lengthy diversions to commute to and from the county town of Shrewsbury. Shropshire Council acted quickly and a temporary road bridge and approaches were built in the fields alongside the old bridge.
So extensive was the damage to the old bridge that rebuilding it was out of the question; it had to be replaced.
“Building an exact replica from scratch would have been expensive and would have involved substantial in- channel works within this site of special scientific interest,” says Shropshire Council structures team leader John Williams.
“The Environment Agency does not look favourably on the use of in-channel piers because of their potential to trap river-borne debris. It was therefore decided to opt for a single span arch spanning the entire width of the river.”
“The Environment Agency does not look favourably on the use of in-channel piers because of their potential to trap river-borne debris”
Shropshire Council wanted a new bridge which retained as much as possible of the spirit of the old crossing, which meant creating an arched structure.
Consultant Mouchel singled out a radical new method of precast concrete arch construction developed by Northern Irish firm Macrete. The system, called FlexiArch, consists of a series of tapered pre-cast concrete voussoirs that rest against each other to form a self-supporting arch similar to a traditional brick or stone arch. But, unlike traditional masonry arches, which require temporary support during their construction, the FlexiArch is delivered to site as 1m-wide strips in a flat-pack. Full span strips are then craned from the delivery lorry onto pre-constructed springing units to form the arch barrel.
The FlexiArch system enables a conventional arch to be built without the need for expensive, in-channel centring.
Dew Construction was appointed as the principal contractor for the project. It faced a series of challenges.
“The Macrete system had previously been used only on small arches. This one spans more than 13m and it is the largest precast arch of its kind ever built in the UK,” says contracts manager Piers Myatt.
“This arch spans more than 13m and it is the largest precast arch of its kind ever built in the UK”
Work began on site in May 2010 with the construction of mass concrete abutments, founded in mudstone at a depth of about 2m below the river bed. Precast concrete springing units, supplied by Macrete, were then dowelled into place on top.
Macrete then delivered the FlexiArch units which Dew craned into place. “The next stage was to install the spandrel walls” explains Myatt.
These are also precast concrete and sit on the arch barrel secured with tie-bars passing through from one elevation to the other.
Lifting and placing such large FlexiArch sections had never before been attempted on a live site, though similar structures have been successfully installed at Macrete’s yard.
The void created between the arch and the spandrels was filled with 130m³ of mass concrete.
Once the concreting work has been completed, the bridge will be clad with brick and stone masonry to give an appearance similar to the old bridge. It is scheduled to open in October.