SPECIAL WORKS
Renovating a Grade 2 Listed culvert in Somerset
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 2.4mwide 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 mmthickconcrete 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
46 | August 2021 |
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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.
STEMMING DETERIORATION
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 workhave 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.
Cracking of the road surface over the culvert highlighted the deterioration below.
Displaced blocks and detritus
inside the culvert also highlighted the need for renovation.
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