Clyde Tunnel
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The Clyde Tunnel is a crossing beneath the River Clyde in Glasgow, Scotland. The tunnels connect the districts of Whiteinch and Govan in the west of the city.
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[edit] History
Efforts to improve the transport infrastructure of Glasgow post-World War II were hit by the problem of crossing the Clyde. Downstream of Jamaica Street in the city centre, it was perceived to be impossible to build a bridge due to the prevalence of shipping in what had been the second city of the British Empire still reliant on the sea. The solution — to build a tunnel beneath the river — was not a new one, with the Harbour Tunnel at Finnieston having been built in the 1890s. However, the Clyde Tunnel project was to be built in the boom of the car era and would be a much larger project. It was given the green light in 1948 but financial difficulties prevented work from beginning until 1957.
[edit] Construction
A tunnelling shield, based on Marc Isambard Brunel's design used for the Thames Tunnel a century earlier, was used to dig the two tunnels[1]. The tunnels were cut perfectly circular with the road deck sitting a third of the way up[2]. Under each road deck is the pedestrian/cycle tunnel and ventilation ducts. 16 miners operated the shield working shifts in a compressed air environment to ensure that the rock and the river above did not collapse into the tunneling area. The digging itself would be done only with great difficulty due to the geology of the area, hard rock sitting under a soft silt layer beneath the river.
The techniques for decompression after a period of working in a high pressure atmosphere had not been perfected at this time and – also owing to the prevalence of workers refusing to go through the decompression sequence given the length of time required (around an hour) – there were a number of cases of decompression sickness diagnosed as a result, resulting in two fatalities. Also, work on the tunnel was halted for a time after an explosion when compressed air escaped through the tunnel lining into the river, flushing outward in a fountain.
The first completed tunnel tube, for northbound traffic, was eventually opened by Queen Elizabeth II on July 3, 1963, with the southbound tunnel opening in March 1964. The total cost of the project was £10 million. Ironically, by this point, the downturn in seafaring trade and improvements in engineering technology had allowed the consideration of bridges downstream of the city centre, namely the Kingston Bridge and, much further downstream than the Tunnel, the Erskine Bridge.
[edit] The tunnels
The tunnels are each 762 metres (2,500 feet) long with a gradient approaching 6% or 1:16. The width of the river at this point is 123 m. Each road tunnel carries two lanes of traffic as part of the A739 road, and are completely separate except for a very small crossover passage between the road tunnels at the nadir (barely large enough for a single pedestrian). They are monitored and controlled from the two Portal towers, one at each end of the tunnel, where CCTV monitors the entirety of the tunnel. Beneath the tunnel decks run foot and cycle tunnels, and below those the tunnels' services.
[edit] Repair work
The estimate for traffic levels in the tunnel was between 9,000 and 13,000 vehicles per day during construction; on the first day of opening, 22,000 vehicles used the tunnel. Current estimates for traffic levels are around 65,000 vehicles per day. These unanticipated high volumes of traffic have increased the wear and tear rate for the tunnel, causing it to close repeatedly for repair and renovation work, which has disrupted traffic in the area for over a decade, and has arguably led to the increase in traffic causing similar problems on the Kingston Bridge. The latest set of repairs, costing as much as the initial outlay for construction, is for a second modern fireproof layer to allow the tunnel to meet European safety standards after St Gotthard and Mont Blanc tunnel fires, along with new air extraction systems and new lighting. For these works, which began in March 2005 and were scheduled to last until at least April 2006, one of the tunnels remains closed during off-peak hours (19:00 to 07:00 daily). This work, however has been beset with problems, and as of September 2006 is still to be completed. Signs informing road users of the temporary tunnel bore closures have had the completion date blanked out. The roadworks are coinciding with the Hold Your Breath project aiming to add an artistic experience to travel through the tunnel. The pedestrian tunnel is also having its years of graffiti removed as part of its beautification project, however it is an on-going problem, and often is replaced within weeks of being removed.
[edit] Breath Holding
A popular game amongst local children, and some adults, is attempting to hold one's breath for the duration of the journey through the Clyde Tunnel. This is possible due to the short length of the tunnel - a car travelling at the 30 miles per hour limit takes 57 seconds to pass through, and at off peak times traffic is often flowing at around 40 mph (a transit duration of 42 seconds). Success is hampered by snarlups (particularly at the interchange on the north end) slowing traffic.
