The way the cabling has been laid has put the highways at risk because the trunking is too close to the surface, making it vulnerable to damage by heavy vehicles.
By not digging deeper, the cable TV companies have saved themselves at least pounds 20 a metre. Contractors are laying 16,000km (10,000 miles) a year and are halfway through the 100,000km programme.
The testing of the plastic duct surrounding the cable is also inadequate. Experts say it is of the sort used for a cardboard tube.
Documents obtained by this newspaper reveal mounting concern among county councils that they could be left with enormous bills for repairing cracked and collapsed highways for years to come. A letter from the National Street Works Highways Group, which represents county councils affected by cable TV laying, to its members, speaks of "a potentially huge liability in the future".
The letter, which is dated 21 November 1996, says that "a number of authorities have challenged the depth at which cable companies are placing their apparatus".
Councils have been unable to determine who is to blame for the fiasco. The same letter said the only reassurance councils had obtained was that the specification "must be acceptable because it is practice throughout the country".
Laying cables for television networks does not require planning permission and is not directly overseen by the Government. The depth of the cable is the responsibility of the National Joint Utilities Group, a powerful body which comprises representatives from the major utility carriers, including the cable companies.
In the US, television cables are buried between 1 and 1.2 metres below ground. But in this country the standard depth is 450mm under roads and 250mm under pavements. This compares with a usual British Telecom and Mercury depth of 450mm.
But, say experts, 250mm is not deep enough. Peter Goode of Nottinghamshire County Council, who chairs the National Street Works Highways Group, said "any loading will have a significant effect - my colleagues have a lot of concern".
So concerned was another county council, Staffordshire, about trenches collapsing that it commissioned consultants to study the problem. The study, by Euro Construction Corporation, and drawing on research by the University of Ulster, found that just one lorry passing over a duct is enough to cause failure.
The building industry journal Construction News says "contractors have suspected for some time that cable TV duct is being laid at a depth which is too shallow, and the duct itself was not designed for use at this depth".
John Riordan, managing director of McNicholas Engineering (Southern) is reported as saying that 250mm is "not adequate".
Ray Acheson, who was Mercury's national construction manager before leaving to found Euro Construction Corporation, said his Ulster-based consultancy looked at three possible reasons why trenches were failing: wrong materials, poor workmanship and "one we did not think likely at the start, that the ducts are not fit for the purpose". Instead of remaining circular in the ground, the plastic ducts extend to form a shape like a rugby ball.
The ducts used in Britain must comply with British Standard BSEN50086 but, said Mr Acheson, that was not high enough. "Our tests showed we can get a cardboard tube like the sort used for kitchen foil to pass any test the cable duct has to pass."
Then, said Mr Acheson, his firm's study examined the depth of cover and "we were horrified". Together, the duct and the shallowness of the trench, said Mr Acheson, "means pavements will continue to move and subside. Pavements and roads are going to have more work on them. Tracks will sink, become dangerous and need fixing."
By not digging to at least 450mm, said Mr Acheson, the cable TV companies have saved themselves pounds 20 to pounds 25 a metre. Under their deal with the cable TV companies, councils have a two-year warranty for any repairs to pavements and roads caused by cable laying. After that, the cost must be met by the councils.
The Cable Communications Association said the problem was not their concern. Calls to the National Joint Utilities Group were not answered.Reuse content