What voids, subsidence and sinkholes are, and how they are caused
Voids are gaps below the surface in the land. Subsidence is the downward movement of the ground beneath the road surface as a result of the soil being compressed by the weight of materials above (including vehicles, buildings and trees). Subsidence is a common problem in roads throughout the UK. Highways in the UK are subject to massive amounts of vehicle traffic each year. The accumulation of all of these vehicles travelling at high speeds can result in subsidence, as the constant vibrations from cars, vans, lorries and busses can cause movement in the soil below.
Voids, sinkholes and subsidence can often cause problems without any warning, their causes may not be immediately obvious and their extent not known until extensive investigations have taken place. Voiding and subsidence can occur due to a number of reasons. Natural voids can be caused by gaps in settlement, regular shrinking and swelling of clay and silt in soil, washout (such as due to leaking water pipes or rainwater), tunnelling animals such as badgers, moles and rabbits. Voids can also be man-made, examples being basements, culverts, underground tanks, tunnels and vaults, pipes, or chimney flues. Voids can also occur due to bubbles in poured concrete which over time crack and decay.
The collapse of a void under a carriageway is likely to cause structural damage and can result in accidents. The cause of each void is entirely dependent on the specific location, but voids can be a result of third party apparatus due to water leaks.
A sinkhole is a type of void that has no natural external surface drainage. Basically, this means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface over time dissolving the rocks beneath. This creates underground spaces and caverns.
Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur. Fortunately, sinkholes are not common in the UK. However, we need to remain vigilant to spot early signs of sinkhole formation to prevent avoidable damage to our roads and footpaths.
How we handle voids, subsidence and sinkholes
When we are presented with a void, subsidence or a sinkhole in the carriageway, our emergency team will attend immediately to make the area safe and secure.
We then have to arrange for a team to start working on this defect and begin with trying to identify the cause. The investigation can take some time to ascertain what has happened before we can decide on the correct way to proceed.
A void detection survey using Ground Penetrating Radar (GPR) is a low risk method to scan an area of ground and determine the location and extents of underground voids, sinkholes and subsidence.
Why the work takes so long
To proceed with any repairs, we need to have extensive discussions with any relevant third parties, then organise a team and traffic management to work at the location and the materials required.
This is all carried out on a reactive basis and not organised in advance during the planning as we would be able to do with our planned maintenance work.
We have received a large volume of void reports to deal with and unfortunately only have a small resource at our disposal for such issues. It can therefore take a while to get to each location, and we prioritise the busiest roads in the county first.
Why the dates on our roadworks map may not be accurate
Our roadworks map displays works dates based on permits which have been submitted to carry out work.
Once a void, subsidence or sinkhole is known to us, our permitting team will apply for an emergency road closure permit. At this point in time, it is difficult until the cause has been identified to provide an accurate estimated end date, and as such, unfortunately the map may not initially show an accurate end date. If the issue is sent onto the appropriate utility company, we are reliant on their timescales to resolve the issue.
Reporting a void, sinkhole or subsidence in a road
If you think there is a void, sinkhole or subsidence in a road, please call us straight away on 0300 200 1003 as this is likely to be an emergency. If you are in doubt as to whether it is an emergency or not, please call us.
For holes in the road which are not an emergency, please report it to us online under our pothole reporting form.