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These three river crossings are situated 20 miles east of London, and carry traffic across the Thames on the A282 trunk road that links the London Orbital M25 motorway between Junction 31 in Essex and Junction 1a in Kent.
History of River Crossing
Travel across the Thames between Tilbury in Essex and Gravesend in Kent via ferry began before The Domesday Book was written in 1086. However, a proper pier to serve all river craft, including the ferry, wasn't built until 1834 - previously, passengers had to travel to and from river craft in a waterman's boat. Passenger traffic doubled within a year of its construction and a million passengers passed through within four years.
By 1954, the pier was used exclusively for the ferry and a service continued to run until 1965 when the first tunnel under the river was established and demand for the ferry dropped.
The first tunnel was completed in 1963 at a cost of £13 million; construction had taken five years due to difficult tunnelling conditions through the chalk. Traffic flowed in both directions between the A2 in Kent and the A13 in Essex.
By 1972 traffic had more than doubled, and construction of a second tunnel began to the west of the first. Again it was hampered by the difficult conditions, cost £45 million, and took eight years to complete. The twin tunnels then carried two lanes of traffic each, travelling in the same direction. Only four years later it became evident that the new M25, sections of which were opening by this time, was going to cause traffic problems in the future. A traffic report forecast that the tunnels would be unacceptably overloaded by 1990.
In March 1986, the Department of Transport invited bids from the private sector to build a new crossing. The M25 would need to carry double the amount of traffic that the tunnels were currently capable of handling across the river. Various bids were tendered, including schemes for more tunnels, but the company Dartford River Crossing Ltd won the contract with the design for a suspension bridge. The company then took over running the tunnels as well as the bridge.
The lights in the tunnels are fed electricity by both counties bordering the crossing. A light supplied by Essex is followed by a light supplied by Kent and so on along the length of the tunnel. This is so that if the power supply is interrupted from one side, there will still be enough light to illuminate the tunnels.
Vehicle exhaust emissions are constantly monitored inside the tunnels, as ventilation is extremely important. Fresh air is circulated by fans situated in buildings at ground level above the crown of the tunnel on both sides of the river. This is fed into the tunnels by air ducts underneath the ground running between the ventilation shafts. Exhaust fans are in the same buildings, extracting the polluted air. Jet fans are in place along the ceilings to provide a high, forward air flow in case of fire. These will prevent drivers behind the fire from becoming affected.
Water seeping into the tunnels and rainwater running in from the bridge approaches is dealt with by a pumping system that drains the water, treats it, and then diverts it into the River Thames.
The tunnels are jet-cleaned weekly during the winter months and at regular intervals throughout the rest of the year.
Queen Elizabeth II Bridge
The Queen Elizabeth II river crossing at Dartford (commonly called the Dartford Bridge) was the largest cable-supported bridge in Europe when it was built. Work began in August 1988, and took three years to build at a cost of £86 million - it was completed on time and within budget.
Opened by Her Majesty Queen Elizabeth II on 30 October, 1991, the bridge carries four lanes of traffic from Essex into Kent southbound. Northbound traffic travels through the tunnels that run underneath the Thames.
The bridge is supported by four 84m steel pylons, situated above 53m high concrete piers. This gives the bridge a total height of 137 metres. 112 galvanised steel cables that have a combined weight of 1500 tonnes suspend the road deck (the motorway across the bridge) from the top of the pylons. The central span is 450 metres long, and is suspended 65 metres above the Thames, allowing even the tallest liners to pass underneath. The approach viaducts on the Essex side measure 1052m, and 1008m on the Kent side, giving the total length of 2872m. It has an expected life span of 120 years.
Anemometers measuring wind speed and direction are positioned mid-span and on the pylons; during windy weather the speed of traffic is gradually reduced to a safer level and the traffic is positioned in the middle of the bridge by closing one or more of the lanes. As the wind picks up, high-sided vehicles are diverted through the eastern tunnel, until at very high winds the entire bridge is closed and all traffic flows through the tunnel.
Normal motorway lighting keeps the bridge lit for traffic, and aircraft warning lights are located on the pylons. Navigation lights and radar reflectors line the shipping navigation channel.
Travel across the bridge is accompanied by a toll. This is so that the company that built the bridge can recoup their costs, and the three crossings would then be passed back to the Government, totally debt free, as detailed in the Dartford-Thurrock Crossing Act 1988. It was estimated that this would take between 14 and 20 years, but according to the Dartford River Crossing Company, the debt will be cleared by the end of December 2002. However, the tolls could be continued indefinitely by the Government, with some revisions, to slow the rate that the traffic grows.
The tolls are collected on the Kent side of the crossings; there are 14 booths northbound and 13 southbound. The booths have automatic cash machines that allow baskets to be used to collect change thrown in by the driver. Electro-magnetic sensors work out what the coins are, and when the total is reached the barrier opens. These can only be used for cars. Manual toll terminals are used by all other vehicles, and are staffed.
A microwave transponder tag called a 'DART-tag' is also available. This is an automatic vehicle identification device about the size of a road fund licence disc that attaches to the front windscreen of the vehicle. The tag is a way of paying for the toll in advance, and travelling through the booths more quickly. An antenna interrogates the tag when the driver approaches, and if it is in credit the barrier automatically opens. It is slightly cheaper to use the tag, and during the rush hour one booth is reserved exclusively for vehicles using it.
A Personal Postscript
This Researcher would like to thank her father, David, for his help and assistance, and mainly his memories, in writing this entry. Thanks Dad!