Misterton is a village with a population of around 900 that is located about 1 mile (1.6 km) southeast of Crewkerne, Somerset, UK. It lays on the main A356 route which runs between the A303 to the north of Crewkerne and the A37 into Dorchester, Dorset.
Through the centre of the village there runs a small stream of unknown name which is a tributary to the nearby River Parrett. The stream is bridged on Middle Street, with a culvert to carry flows beneath the main road. The culvert, which is believed to have been first built in the 1700s, almost 300 years ago, is a flat ‘D’ design which was constructed using hand-shaped local Ham Stone with lime mortar joints, with dimensions of 2.4 m wide and a maximum internal height of 675 mm. It was originally built to a length of about 5.5 m, probably wide enough to take the horse drawn wagons and carts of the day. Later extensions were added at each end using a similar construction method to the original structure. The downstream (northern) end was extended by approximately 1 m. The upstream (southern) end was extended by about 1.8 m, where a slight bend of 25° to the centre line of the rest of the bridge was added. The total bridge length is currently 8.29 m. In 1933 a 100 mm thick concrete invert (on the stream bed) was constructed. The structure currently holds Grade 2 listed status having been granted this in 1987. Not only this, but the road is also a main route for local HGV vehicles to a local haulage firm that deals with much of the farm produce in the area. The depth of cover from the road surface to the top of the culvert arch is just 145 mm.
As with many structures of this age the culvert has been found to be in need of serious renovation and a local weight limit of 18 t has recently been enforced on the road for some months, whilst Somerset Highways engineers designed and planned the required renovation works. The difficulty here is the Grade 2 listed status. This requires that the structure is restored as far as possible to its original form using the techniques and materials of its original construction. This of course eliminates most if not all modern techniques that might have been considered for the rehabilitation operation.
The responsibility for the bridge and culvert works fell to Somerset County Council Highways Division.
Investigations almost 20 years ago showed structural decline in the culvert, so bore holes were drilled into the Ham Stone structure from the road surface into which support bars were grouted. These have been uncovered during the recent rehabilitation works.
Road traffic incidents since this work have also affected the structural integrity and further investigations have also shown that some of the key stones in the Ham Stone arch have been displaced, which is in part why the recent weight limit has been imposed on the road above.
This strengthening work method 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 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 back to original finish.
- 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 jack hammers 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 structures. The sub-contract for the surface removal and Vacuum Excavation was appointed to Vac UK Ltd, a Kilkern Company.
On site Vac UK Ltd operated its RSP-manufactured, twin fan suction excavator which is based on a Mercedes tridem AROCS 8×4 chassis. The twin fan system creates a suction power of 44,000 m3/h and a max vacuum pressure of 55,000 Pa.
The opening works were completed in relatively small sections, about 500 mm wide, with the outer culvert wall being exposed using the small hydraulic pick powered from the vacuum excavator and the spoils being removed as the road surface was broken down. This was possible because the weight of the vacuum unit did not have to be placed on the bridge structure directly but just to one end. This meant little or no direct excess loading on the bridge/culvert structure. Once the road surface was removed and the depth of the excavation around the crown and down the arch sides progressed, spoil removal became easier as the soil type changed from hard road and substrate to softer soil type materials. Once a section of arch was exposed over about 2 to 3 m, the section was cleaned and grouted to provide the extra support needed. This process was repeated until one side of the bridge was completely supported. Excavation then moved to the other side of the road and the process repeated.
Once the full width of the culvert arch was fully grouted the road surface was back filled and replaced to the required standards, the operation switched to the inside of the culvert.
Removing all but the necessary internal support, a man- entry system was established within confined space working regulations. The inner surface of the culvert was then cleaned and repointed using the specially formulated lime mortar to provide a finish that closely resembled the look and form of the culvert as it would have been originally constructed. All supports were then removed, and the culvert and bridge returned to normal operations.
With the repair works completed the bridge will be able to carry all normal highway traffic loads up to 44 tonnes.
Commenting on the project for Somerset Highways Nick Jacklin, Senior Engineer – Structures said: “This was never going to be the easiest of bridge/culvert renovations given the Listed status of the structure. However, working together and utilising some of the newest vacuum excavation technology the project has been fully successful and we expect the bridge and its culvert to now be operation for many decades to come. Our congratulations must go to the Walters crew and the Vac UK team for enabling this to be the case.”
For Vac UK Ltd, Joanne Ashford commented: “We were particularly pleased to be able to offer a solution for the uncovering of this culvert given its age and structural state. To
be able to use such a large machine on such a delicate works programme has really shown us that vacuum excavation can be utilised in a variety of circumstances where many may not expect to find it operating.”
For Walters Group site manager, Craig Ellis said: “I must congratulate our team on the success of this particularly delicate operation. The utilisation of the vacuum excavation system really aided the success in exposing the fragile outer surface of the culvert whilst minimising any potential for further deterioration inside the culvert. This operation just goes to show how authorities, contractors and subcontractors can work closely together to achieve a very successful solution to the most challenging of projects. I would also like to thank the local population for putting up with us for the weeks necessary for the works. They have been patient and understanding even though they have been required to travel some extra distance to allow these works to be concluded successfully. I am sure they will feel the benefits now the road is fully open to all traffic.”