Client: Essex Highways
This project included the removal of temporary supports to install a permanent reinforced concrete pedestrian underpass under the existing road bridge which carries the B1389 Newland Street/Colchester Road.
Originally constructed in 1842, Catholic Bridge consists of eight cast iron beams with brick jack arches. Strengthening works were undertaken with the installation of trestle supports during the Second World War and a second set of supports were installed in the 1990’s.
In 2014, three of the cast iron beams were identified as unsafe during a support inspection and as a result, temporary supports were installed to keep the bridge open to traffic.
Throughout this project Jackson used real time monitoring on the existing cast iron beams to ensure the stability of the live bridge 24/7.
To allow space to construct the proposed underpass, the two sets of existing temporary supports needed to be removed, however, the existing structure still required additional support. This was achieved by installing portal frames running parallel with the cast iron beams but situated central to each jack arch.
The position of the concrete foundations required for the new support system clashed with existing foundations. To allow the new foundations to be constructed, further temporary supports were installed immediately under the cast iron beams. These were supported on the abutment walls and by the original trestle supports. This then allowed, during a weekend road closure, removal of the second trestle support and construction of the new concrete foundations. These had to be reinstalled before the road was reopened to traffic on Monday morning.
Due to uncertainty regarding the stability of the existing abutments the excavation for the foundations was spilt into three sections along the abutments. This required careful planning as access was restricted. Three reinforcement cages were preassembled and fixed within three sided shutters which were through bolted to facilitate lifting. These were to be the first three sections of concrete to be placed following excavation during the weekend.
Jackson also reduced the weekend shift by 25% through the use of a vacuum excavator (Vac-Ex). The Vac-Ex was used to excavate under the bridge in preparation for new concrete foundations to support a new temporary steel portal frame. This method provided a safer, lower risk alternative to traditional excavation techniques, as there is no need for additional excavator movements and dumpers. Doing so, the team were able to save 12 contract hours and work more efficiently in constricted areas.
Once the excavation was complete, the concrete foundations for the temporary steel portal frame were poured using volumetric mixers. The concrete was allowed to cure for 12 hours before replacing the second set of temporary supports and the road opened to traffic on Monday morning.
Having completed the foundations the portal frames were then installed by threading the steelwork into the existing temporary supports. When all of the frames, support beams and stools were complete, nine portal frames were synchronously jacked during a further road closure. The jacking ensured that the portal frames now carried the load. This allowed the team to remove the existing temporary works and create space to install the permanent reinforced concrete pedestrian underpass.
On completion of the main section of the underpass structure under the footprint of the original bridge shuttering was erected to the remaining void between the two structures. This enable the annulus to be filled with foamed concrete placing it in four layers. To ensure the void was completely filled the concrete was poured through ‘letter boxes’ in the shutters and using sacrificial pipework. The crown of the jack arch barrels were filled with grout placed from the road above.
The ‘ends’ were then closed off with reinforced concrete wingwalls which extended to support the parapet walls of the original structure. The wingwalls were then faced with brickwork sympathetic to the existing facing bricks.
Anti-vandal lighting units have been fitted and all new surfaces coated with anti-graffiti paint.