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Sapperton Tunnel
under the A419, Coates to Daneway, Gloucestershire, UK
Sapperton Tunnel
associated engineer
Robert Whitworth
Josiah Clowes
date  1783 - 28th April 1789
UK era  Georgian  |  category  Tunnel  |  reference  SO965006
ICE reference number  HEW 629
photo  PHEW
Sapperton Tunnel carries the Thames & Severn Canal under the Cotswold Hills and the A419 Stroud to Cirencester road in Gloucestershire. When built it was the longest canal tunnel in Britain — a remarkable achievement in an age before electricity or motive power. The picture shows its east portal, near Coates. It has been disused for many years owing to roof collapses, though both portals have been restored.
On 17th April 1783, the Thames & Severn Canal Act was passed and canal construction began two months later. Its route runs from the Stroudwater Navigation at Wallbridge, Stroud, to meet the River Thames at Inglesham. The above ground section was put into use on 19th February 1785, and the whole canal was opened officially on 26th November 1789.
Sapperton Tunnel is the canal's major structure and starts some 12km south east of Wallbridge. Work on the tunnel commenced in about October 1783 and took five and a half years to complete.
Robert Whitworth (1734-99) was a consultant to the Thames & Severn Canal Company, advising on the canal's layout, and he surveyed its initial route. However, the company defined his position clearly, stating that he "was not at any time the only surveyor and engineer for the construction of the canal". Though Whitworth is attributed with designing Sapperton Tunnel, it's more likely that Josiah Clowes (1735-94), the canal’s "surveyor, engineer and head carpenter", was responsible. Clowes, who was also resident engineer, established his reputation as a canal builder on this project through his work in solving the difficulties encountered during its construction.
The tunnel is 3.5km long and is located at the canal's summit, below the highest parts of the Cotswold Hills. It has no towpath. Boats were moved along by the crew, who lay on their backs and propelled it with their feet by pushing against the tunnel roof, a technique known as 'legging'. It is much wider than earlier canal tunnels, which were built for narrow boats, and could accommodate the larger Thames barges. It has a horseshoe cross-section, 4.6m wide and 4.7m high, with the crown of the roof 3.1m above water level.
The south east portal (SO966006), at Coates, is of Cotswold stone in the Classical style, with niches and pedimented columns flanking the tunnel mouth. The canal enters this portal through a 9m deep cutting with masonry and brick side walls about 400m long, beyond which only the canal's north bank has a retaining wall. The north west portal (SO944034), at Daneway, is of plain limestone masonry. Stone walls about 30m long retain the canal banks at the entrance. Originally battlements surmounted the spandrel walls of both portals.
To construct the tunnel, 26 access shafts, 2.4m in diameter, were sunk, though one was abandoned before completion. The deepest shaft is 74.4m. Headings (small bore tunnels, just large enough for a standing man) were dug in both directions from each shaft and in one direction from the portals. The headings were joined together and the whole alignment checked. Then the tunnel was excavated to full bore by hand, with gunpowder used for rock blasting. Excavation arisings were removed through the shafts (with horse-driven winches, or gins) and portals, and deposited nearby.
The tunnel runs through a variety of strata, which dictated the construction method. Where it passes through Great Oolite (solid limestone) — 1km at the Coates end — the excavated roof is unlined, while the water channel has masonry side walls and an outer lining of clay. In beds of unstable clay — 461m at the Daneway end and 983m in the mid-section — the tunnel has a full lining of three courses of brickwork. Where the rock is Inferior Oolite (friable limestone) — 1km between the two stretches of clay — the tunnel is of brick, and the water channel has an outer clay lining.
Poor workmanship was a problem during construction. In some areas, only one course of brick was used in the arch of the tunnel roof lining. In the longer clay section, the brick arch was not always in contact with the overburden. Over time clumps of earth fell into the gap, pressing on the sides of the arch and distorting it from the springing. Clay heave also raised the invert of the water channel, reducing the draft from 1.5m to 600mm in some areas.
In the Inferior Oolite, rock falls resulted in holes in the tunnel roof up to 6m by 9m high in places. The rock contains many water-permeable fissures, which allows springs to form. The tunnel and adjoining lengths of canal suffered considerable leakage. In wet weather, water in the fissures forced its way through the clay lining, but in times of drought the canal's water drained away through the surrounding rock. Adequate water for canal traffic was an ongoing issue, as it was built without supply reservoirs.
Eventually, on 20th April 1789, the first vessel passed through the tunnel and it opened fully on 28th April. Despite a slight bend in the middle, it is straight enough to be able to see through from one end to the other. At the time of its construction, and until 1811, the tunnel was the longest in Britain. It has been surpassed by only two other canal tunnels (Standedge and Strood) and 10 railway tunnels.
In 1790, the problems with underground springs and water ingress were apparently resolved by recommendations from Robert Mylne (1733-1811). By 1791, it was necessary to line the affected sections with oak planking, with conduits to divert the water. It was a solution that perpetrated further difficulties when the timber decayed.
Over the years, the canal was drained frequently for repairs. This allowed the clay lining to dry out and crack, so that on refilling the water penetrated the clay causing it to swell and resulting in more distortion of the tunnel profile. By the mid 19th century, it was reported that the tunnel's width was reduced to 3.6m in some places.
To try to keep the tunnel operational, where uplift to the invert was worst, the damaged sections had 300mm diameter horizontal timber cross beams installed in the base of the channel, preventing the tunnel walls from moving inwards. Black circles on the side walls mark their positions. However, many of the beams have now snapped under the force of continuing clay movement.
In around 1904, to combat the ingress of water, vulnerable sections of the tunnel were lined with concrete, with outlets provided for the springs to discharge into the canal above the water line. Water supply issues persisted, with inadequate water in summer 1906. From July to October the same year, the canal was closed for repairs.
On 11th May 1911, the last barge passed through the tunnel. And by 1916, roof falls had made is impassable to navigation. The Thames & Severn Canal was abandoned in stages from 1927 to 1933.
In June 1952, the Coates Portal was Grade II* listed and the Daneway Portal Grade II listed. By 1972, when the Stroudwater & Thames & Severn Canal Trust (now the Cotswolds Canal Trust) was founded, both portals had lost their battlements.
In 1976, a survey of the tunnel revealed that two thirds of its length was generally satisfactory, about one fifth required repairs and one sixth of it needed complete reconstruction. The brick-lined parts of the tunnel were most at risk. A tunnel collapse completely blocks the canal at 281-381m from the Daneway Portal. It is also impassable at a series of cave-ins and roof falls in the central section, between 1.5km and 1.6km from the Coates Portal.
During 1976-7, the canal trust had the Coates Portal renovated using reconstructed Cotswold stone. Its original appearance was restored, except for the battlements. In 1996, similar work was carried out to refurbish the Daneway Portal, including reinstating the battlements.
A further survey in November 2008 found that at least 450m of tunnel, in the two main areas of roof collapses identified in 1976, requires reconstruction and relining. The central section, in particular, is deteriorating. Deposition of material into the canal, about a mile east of the tunnel, is blocking the channel and allowing water levels in the tunnel to rise — made worse by wet weather. Water levels in the tunnel also fluctuate according to groundwater levels. The water then reaches previously dry beds of clay, with damaging effects on the tunnel's brick lining.
The Cotswold Canals Trust organised boat trips into the tunnel for about 300m from the Coates Portal, water levels permitting. However, they were discontinued after bats (a protected species in Britain) were found roosting in the south east end, and have yet to be resumed (2014).
Resident engineer: Josiah Clowes
Contractors (1785 onwards): Charles Jones, Ralph Sheppard, Jeremiah Lee & Thomas Barber, Richard Jones, George & Pyner Bray
Research: ECPK
"Sapperton Tunnel Report: Informal survey conducted on 30 November 2008" in The Trow, No.144, Cotswold Canals Trust, spring 2009
reference sources   CEH W&WBDCE1

Sapperton Tunnel