Brynglas: A story of innovative solutions
Nov 2011
Peter Kenyon, TunnelTalk
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As engineers prepare a full damage assessment following July's fire in the westbound section of the 365m-long Brynglas highway tunnel near Newport in Wales, TunnelTalk reporter Peter Kenyon searches the 1960s archives and discovers that the tunnel's history is one of innovative solutions to what the then Secretary of State for Wales, Cledwyn Hughes MP, admitted were unexpected difficulties.
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A truck fire caused serious damage to one of the Brynglas tunnel tubes
- It may have been 1962 when ground was finally broken on the Brynglas twin tube tunnel that would take the M4 Newport bypass under the populated Crindau ridge, but the origins of what were the first tunnels built on the British motorway network date back to 1946.
- The original location and designs called for a single tube structure , but those plans were shelved until 1961 when engineers were asked again to look at the problem of bypassing Newport and the double challenge of passing through the Crindau ridge and across the River Usk.
- Although the location was to remain the same as that proposed in 1946, the original design had to be redrawn to comply with radical changes to construction standards over the intervening 15 years. Design and construction now fell to consultants Sir Owen Williams and Partners (which was taken over by Amey in 2006).
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Engineers at the tunnel face (Circa 1965)
- Twin bored tunnels were designed because, even by 1961, it was obvious that increasing car ownership was causing traffic congestion. According to an official British Parliamentary record in Hansard dated 8 May, 1963, the estimated total cost of the project, including the twin platform Usk Bridge, was £1,625,000 (the equivalent to £26 million at today's prices). A later record from the same source reveals that the tender sum received for the work wasin fact £1,506,100, and that the scheduled completion date would be May 1965.
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Construction methods
This proved highly optimistic, and almost from the moment ground was broken in 1962, problems arose. Chief among these was the fact that the Crindau ridge through which the tunnels were to pass, had become urbanized with several new homes directly above the proposed alignment. - Trial borings were carried out in the Devonian Marls and sandstone formations, but these failed to identify the serious nature of the geological faults that would present a host of problems once construction started.
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Sir Owen Williams' firm took on design
- By mid 1966, and already a year behind schedule, questions were being asked in the House of Commons which raised the wisdom of tunnelling in the first place.
- In answer to a written Parliamentary question by Roy Hughes MP on 29 July, 1966, the then Secretary of State for Wales, Cledwyn Hughes MP, explained: "Several alternatives were fully investigated before the decision [to opt for a tunnel solution] was taken, and the unexpected difficulties and cost involved in constructing the tunnels through the hill do not outweigh the disadvantages of the alternatives."
- He added: "A large number of trial borings were taken before construction work began but the geological faults are of a type which would not be revealed by conventional boring methods. More extensive investigations were impossible because of the built-up nature of the hill."
- Work started with a pilot heading in the westbound tunnel and in 1963 excavation of the main tunnel started on a topheading and bench process. The ground was supported with stiff steel ribs at 0.6m centres. But continuing difficulties caused large overbreak.
- Only slow progress was made on all four faces during the next 12 months and, after the failure of some temporary I-section steel ribs caused a serious rockfall, work was stopped to allow time for a rethink. It was then, reportedly, that the consultant came up with an idea that, as far as staff knew at the time, had never been used before, an idea about which some were very sceptical could be successful.
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Secretary of State for Wales (1965) Cledwyn Hughes MP
- Short shields of heavy steel construction would provide full support for the rock at the face and afford safe working platforms at various heights for workers completing the drilling and excavation processes.
- And so it was that a crown shield was designed with three horizontal platforms that allowed full face excavation, and the original design of concrete lining to be cast immediately behind. Two shields of 1.5m long were installed in mid-1965, with hydraulic rams to support the face and nine shove rams, five of which could thrust against the lining when this had reached sufficient strength. The remaining four rams transmitted thrust forces along horizontal steel box columns to a point 9m back where the thrust could be taken on matured concrete through a transverse beam lodged in special pockets cast into the concrete. Trailing tapered bars supported the roof between the shield and the concrete. When falls occurred the rock was shotcreted to prevent deterioration of the surface before concreting. Two pilot tunnels were constructed into the cross section each main tunnel, one at each side of the profile to provide support and guiderails for the shields.
- Two further shields were installed in mid 1966 and the primary lining of the tunnels was complete at the end of 1966. The installation of the shields permitted steady progress and protection for the construction of the tunnels, while at the same time limiting falls. The reinforced concrete invert was constructed as a separate operation. By using the shields, 1m to 5.5m of tunnel was constructed/week/face in 0.9m rounds.
- But despite the project's overall success, the fact that the hill above was a residential area dogged the excavation from start to finish, and beyond.
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East portal of the Brynglas Tunnel today
- The Parliamentary record concerning Brynglas is full of questions concerning the subsidence that affected a number of houses situated above the tunnels, and demanding to know what recompense was going to be paid to the owners.
- In one case a resident had refused to accept the District Valuer's valuation of his property and the bill to accommodate him and his family in a hotel while a deal was negotiated had already reached £1,850 (about £27,500 in today's money).
- Such questions continued until 1969, when Hansard Parliamentary records reveal that the contractor purchased eight houses that were structurally damaged by the tunnelling works.
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Engineers assess fire damage to M4 tunnel - TunnelTalk, Aug 2011
Truck blaze damages UK traffic tunnel - TunnelTalk, July 2011
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