Engineers were this week carrying out final preparations for the start of repairs to flood-hit Workington’s Calva Bridge.
The Grade II listed, three span 47.5m long masonry bridge was so severely damaged in last November’s floods that it was initially condemned.
It is now to be rebuilt, but there remains a 50/50 chance that the repair work could cause the bridge to collapse.
The main task is to underpin and repair the bridge’s central pier using mass concrete and masonry. The pier was partially washed away in the floods causing the bridge deck to drop by up to 230mm.
Repairs are now considered possible after an extensive structural analysis was carried out by Scotland Transerv, a Balfour Beatty/Mouchel joint venture.
It is actually responsible for managing roads in north west Scotland but did the work on behalf of Connect Roads, the Balfour Beatty-led highway maintenance firm that is responsible for the bridge.
The analysis concluded that the bridge can be repaired sufficiently to take full highway loading.
Earlier works discovered
Key to the change of approach was the discovery of remedial works carried out by the Highways Agency in 2003 to prevent water leaking into the structure from the deck.
The Agency had initially been responsible for maintaining the bridge until it was taken out of the national trunk road network.
The 2003 work had stripped the bridge down to its skeleton structure exposing two masonry diaphragm walls which follow the shape of the arches. Nine more diaphraghm walls running across the structure were also exposed. The Agency had filled the voids between these walls, forming a concrete saddle beneath the deck.
This discovery meant that the structure had a lot more strength than was previously thought.
“We discovered video footage that showed a concrete saddle being installed on the bridge deck”
“We discovered video footage that showed a concrete saddle being installed on the bridge deck to prevent water leakage,” said Scotland Transerv bridges manager Calum Galloway.
“This opened up the possibility of salvaging the bridge.”
The floods caused the foundations of one of the piers to be eroded on its upstream side, causing the 1,000t dead loads imposed on it to be unevenly distributed. As result the bridge is twisted out of shape. The roadway on the upstream side has sunk by 230mm while the deck on the downstream side has droppped by 90mm.
This made structural analysis tricky. “I’ve never assessed a bridge with only two thirds of a middle pier,” added Galloway.
“It’s a combination of design and analysis.”
Engineers carried out a remote topographic survey of the bridge and this data was fed into software which determined that the bridge could withstand normal traffic loading but not abnormal loads.The analysis is conservative because the concrete fill, which is believed to add strength to the structure, is technically considered to be dead load with no strengthening properties.
To repair the bridge, the upstream pier must be underpinned and repaired.
Connect Roads is using maintenance contractor Balfour Beatty to do the work.
It will first construct a causeway next to upstream edge of the bridge along two thirds of its length using local sand and gravel, and incorporating large diameter pipes to maintain river flow.
The contractor will then excavate bases for 1m deep precast concrete temporary piers either side of the existing damaged pier. Precast concrete blocks will be placed on top of the excavated bases.
Three steel beams, each with two 300t jacks attached, will be placed on the precast units and then slid into position underneath the damaged, upstream part of the pier.
The six jacks will raise the structure to carry a combined load of 300t, reducing the dead load on the remaining good part of the pier from 1,000t to 700t.
“This is most delicate part of the operation,” said Balfour Beatty contracts director Keith Bowman.
“We could trigger a reaction that might cause the bridge to fail.”
If the bridge remains stable, engineers will start removing masonry and debris underneath the damaged part of the pier to excavate a suitable base. They will then form a cofferdam around the damaged part of the pier using 1t hessian bags filled with sand and gravel before dewatering it.
A new mass concrete core will be poured into the damaged section of the pier up to the underside of the steel cross beams. Steel plates and more jacks will be installed above the concrete foundation before then the load is transferred to them.
The original jacks can then be removed, leaving the new jacks in place. These will eventually be cast into the repaired pier when the new mass concrete section is connected to the damaged pier.Shuttering will then be built up around the jacks and more concrete poured to fill the remaining void.
The repaired pier will be faced with 100mm ashlar stone to match existing facade.