THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
SPECIAL WORKS
Culvert supported internally whilst working overhead.
Once a section is exposed the culvert is grouted on the outer surface before moving to the next section.
This strengthening workmethod required the agreement of the Environment Agency, the Somerset Lead Local Flood Authority and South Somerset District Council (for listed building consent). An ecology survey was also carried out. Further to this, an archaeologist was contracted to inspect the fill removed from the arch, as it is suspected this may be the first substantial uncovering of the structure since the extensions were added.
■ The plan to rehabilitate the culvert was quite complex and included:
■ Creating a bypass system for the stream flows during the course of the works
Exposing the culvert’s ‘external’ faces and crown.
using pumps and hoses that pass through the culvert.
■ Supporting the existing structure from within the culvert with packers and
beams which were installed by hand.
■ Removing the existing road surface down to the external form of the culvert
arch.
■ Grouting the structure from the outside of the arch to stabilise the arch and re-
establish the overall structural integrity of the culvert to enable the weight limit to be removed on the road above.
■ Backfilling and relaying of the roadway to modern standards.
■ Entering the culvert using confined space entry techniques to repoint the
internal face of the culvert arch with Lime mortar, prepared to what is believed to a type similar to that used originally, to bring the culvert backto original finish.
Gentle picking removes the ground cover which is then removed using the suction excavator.
FOLLOW US
■ Re-establishing the culvert portals to their original state.
■ Reopening the flow of the stream through the culvert.
Put like that it does not sound too difficult! However, the state of repair of the original arch was such that any major ground disturbance may have caused the whole structure to collapse. So, a method of working had to be found that would not create this situation.
Specialist advice was sought about a suitable lime grout for this repair work. Usually, lime mortar and grout require a very long time to cure, in this instance suppliers have developed a pre-bagged mix, utilising additives, that allow the initial cure time to be hours, rather than days, while maintaining the essential qualities of lime mortar - flexibility, strength and malleability.
A DELICATE TOUCH
With both internal and external works required for the bridge/culvert strengthening works, given the structural state of repair of the culvert, accessing the external face of the arch required some delicate engineering.
The potential for high vibration to destabilise the culvert structure was high, particularly given the shallow cover, so the use of machine- mounted hydraulic picks, bucket excavators or hand-held jackhammers was ruled out. So, an alternative was required that would not destabilise the structure during the works. After investigating options, it was decided that the use of a lightweight, hydraulic pick, with spoil removal being achieved using a vacuum excavator was the most likely option to be effective.
The contract for the main rehabilitation works was awarded to South Wales-based civil engineering contractor Walters Group through its Avonmouth division. Walters’ responsibility covered the road closure that would be necessary (some 8 weeks) and the reconstruction works for the culvert and bridge
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