Hole closes section of motorway
The Highway Agency is carrying out urgent safety investigations and repairs after a 15 foot-deep hole appeared on a central reservation on the M2.
The hole, between junctions 5 and 6 of the M2 near Sittingbourne in Kent, measured around 16ft (5m) by 6ft (2m) and was 15ft (4.5m) deep.
A spokesman said: "Our engineers are on site carrying out an investigation and this includes the cause of the hole."
Drivers were warned of delays of 15 minutes eastbound and 30 minutes westbound in the area while traffic heading to and from Dover should use the M20 instead.
A 10-mile section of the busy motorway has been closed in both directions. Lanes would reopen "as soon as it is safe to do so," the spokesman said.
Professor Stephen Glaister, director of the RAC foundation, said: "We already have the most congested road network in Europe and our motorways carry a third of all traffic. This will have a major impact on a main route in and out of the country.
"The surprise is that something on this scale hasn't happened before yet everyday our highways are literally being undermined by the rain. When the deluge finally stops we will be left with a huge repair bill on roads across the country."
Most areas in the UK susceptible to sinkholes are relatively small or are in upland rural locations. They have also been known to occur in areas including the Mendips, parts of Wales, and the northern Pennines including the Yorkshire Dales.
Old coalmine shafts in Yorkshire have been known as a cause of holes which could "theoretically" be the case in Kent but is it "more likely to be related" to the probable chalk or limestone make-up of the area along with the rainfall, according to Sarah Fray, director of Engineering and Technical Services at the Institution of Structural Engineers.
She described the M2 hole as "unpredictable" adding that "it could potentially not be visible when there was construction and water has been know to be a mobilising influence" to cause an underground cavity to collapse.
She said: "Chalk and limestone is alkaline and rainwater is acidic - if you put acidic water on it, it dissolves over a period of time which can happen over a millenia".
Areas of chalk can be susceptible to sink holes especially where it is covered by younger clay and sand deposits. Ms Fray said: "Then when you have a lot of rain, this can mobilise the soil".
Dr Vanessa Banks, of the British Geological Survey, said: "This process occurs over thousands of years. During this period the growing void in the chalk remains bridged by the overlying deposits until infiltrating water is sufficient to physically remove the finest particles into the underlying void, thereby destabilising the 'soil bridge' and ultimately resulting in total collapse of the 'soil bridge' into the void.
"If surface water drainage was finding its way into such a feature, as is likely in a motorway setting, this would increase the likelihood of collapse. "
Professor Amir Allani, professor of civil engineering at Greenwich University, said: "It may be due to the current rain and flooding underground has created the erosion which, in time, has facilitated the layers above the weak strata to collapse."
He noted that when the ground is weak "the lorries above that exert pressure to the road" adding: "When you do not have support of the ground anything can disturb that cavity."
Earlier this month a sinkhole measuring 30ft (9m) deep swallowed up a car after opening up on the driveway of a house in High Wycombe.Nobody was inside the VW Lupo when the ground on Main Road in Walter's Ash gave way. Firefighters said the hole was about 15ft (4.5m) diameter.