select search filters
briefings
roundups & rapid reactions
Fiona fox's blog
Scientists comment on the sinkhole that has appeared in Surrey.
Dr Liang Cui, Associate Professor, School of Sustainability, Civil and Environmental, University of Surrey’s, said:
“There are two main types of sinkholes. One forms when fine particles are washed out of the soil, creating cavities which then grow over time. Variations in the groundwater table caused by heavy rainfall or pressurised water from a burst pipe can cause rapid sinkhole formation. The other type is caused by dissolution of soluble components in rocks, such as limestone, chalk or gypsum. This is typically a very slow process, but acidic water can accelerate this process. Once the underground cavity grows to a certain size, the overlying ground can collapse. This can range from gradual ground depression to a sudden, catastrophic collapse.
“Loose, granular soil or weakly bonded is more prone to erosion and sinkhole formation. A careful site investigation, such as borehole testing, can reveal these vulnerable ground conditions by examining the ground profile, soil types and properties, and any existing cavities. Soil stabilisation work can be carried out to reduce the risks of sinkholes; for example, inject grout or other materials into the ground to bind soil particles.”
Chris Barker, Associate Director and geotechnical specialist, Arup, said:
What is a sinkhole?
“A sinkhole is a depression or hole in the ground surface caused by collapse of the underlying soil or rock.
How do they happen and how might they progress?
“Sinkholes can form by natural geological processes as well as human activities e,g, underground mining, leaking pipes and drains. The main trigger for sinkholes forming is water. Water can gradually dissolve some soluble rocks and can also cause erosion of sandy soils.
What makes an area more at risk and which areas in the UK are particularly vulnerable?
“Areas prone to sinkhole formation occur throughout the UK. Areas more at risk of sinkholes are those with historic underground mining as well as areas with soluble rocks like gypsum and chalk. Also, locations with leaking or damaged water and sewer pipes can cause erosion of sandy soils and formation of a sinkhole.
What were the conditions at the Godstone site that made it especially at risk?
“Inspections and investigations underway should determine the ground conditions at the Godstone site and inform the specific sinkhole mechanism.
Are they predictable, and what can be done to protect a vulnerable area from sinkholes?
“Sinkholes are generally difficult to predict but areas prone to sinkhole formation are known throughout the UK.
What is done after a sinkhole occurs to restabilize a site and prevent further sinkhole events?
“Depending on the size, depth and location of the sinkhole, several techniques may be used to restore stability. Excavation of collapsed soil, injecting grout or placing other suitable stabilizing materials into the ground to stabilize the surrounding soil can be undertaken. Identifying and addressing the underlying cause of the sinkhole is important to prevent future occurrences. This could also involve repairing broken or leaking pipes, improving drainage systems, or addressing other factors contributing to the formation of a sinkhole.
Some coverage said the council is carrying out borehole testing at the Godstone site, what is borehole testing, and what will it tell experts?
“Borehole testing is a common method of investigating the ground. Understanding the layers of soil and rock types and depths as well as groundwater is important to determine the sinkhole mechanism and the most suitable re-stabilisation.”
Dr Andrew Farrant, Regional Geologist of Southeast England, British Geological Society, said:
“Godstone is located on the Folkestone Formation, a sequence of medium- and coarse-grained, well-sorted cross-bedded sands and weakly cemented sandstones of Lower Cretaceous age (part of the Lower Greensand Group). These sands are weakly cemented and are prone to erosion and flushing by running water. Normally this would not be a problem, unless there is a sudden change in the hydrogeology, for example by digging a large hole below the local water table or due to a sudden ingress of water, from say a burst pipe, or heavy rainfall/flooding. It is probable that a burst water main has flushed out weak sandstone bedrock, creating a void which has then collapsed.
“However, there are records of historic sand mines in Godstone (data from the Chelsea Spelaeological Society Records [1] and from historic maps [2]. These are mostly relatively small pillar and stall mines sometimes dug for storage, but mostly for extracting sand [3]. Historic maps published in 1872 suggests there is an entrance to an old sand mine very close to the site of the sinkhole [4]. Two old mines further south have been explored and surveyed by the Chelsea Spelaeological Society; both have entrances on the east side of the road south of the present collapse. The nearest and largest has an entrance in an old builder’s yard/garage at [TQ 3504 5184] and extends c. 100 m to the east under the allotments (CSS Records, Vol 3, p. 26-29). The area immediately west of the present collapse was a historic sand quarry, but this has been infilled and built on, so it is unlikely this contributed to the collapse. Another short mine was found off this now backfilled quarry, leading under what was then allotments to the south. It is possible other unrecorded sand mines occur in this area.
“More extensive mines for extracting sandstone (for hearthstone or ‘firestones’) occur further north in the Upper Greensand Formation, but these are all north of the M25 in a different rock unit, and unrelated to this event. Many other sinkholes in southeast England are due to the collapse of old chalk mines (‘deneholes’), but this is not the case here.
“So, the Godstone collapse is a not true sinkhole caused by dissolution of soluble rocks. What is not clear at this stage is the actual cause. We cannot definitely say what caused this event without further information. It is probable that the sinkhole is due to a burst water main flushing out weak unconsolidated sands from beneath the road over a period of time, creating a cavity which then collapsed when it got big enough, but it could also be linked to the collapse of an old unrecorded sand mine. It is possible that a leaking pipe has weakened the rock and led to the failure of a putative mine (assuming it exists and extended under the road); equally it is possible than gradual failure of a mine roof caused the water main to fail, triggering further subsidence.
“By their very nature, collapse features caused by burst pipes or collapsed infrastructure are mostly likely to occur in built up areas or beneath roads, and could happen in other areas with similar geology and aging infrastructure. Many recent sinkholes have been caused by collapses of old sewers or culverts, such as the sinkhole near Merthyr Tydfil last December [5]. There have been other recent instances of sinkholes due to flushing out of sands and weak sandstones, for example the sinkhole that closed Storrington high Street last October [6]. This was also on the Folkestone Formation.”
Dr Phil Collins, Deputy Dean, College of Engineering, Design & Physical Sciences and Reader in Geology & Geotechnical Engineering, Brunel University of London, said:
“The term ‘sinkhole’ is often reserved for naturally formed features, though the fundamental processes and end results are similar. In this case, the cause of the visible holes appears to be the washout of soil from beneath the road as a result of a burst water mains. This burst would have pumped a large volume of water into the soil in a short period of time, overcoming its strength and turning it to a slurry which was then washed away. It isn’t clear where the soil went yet, but it is likely to have gone into the sewers, which may cause problems elsewhere.
“One of the contributing factors is the local geology, which is sand, and the former land use which is a sand quarry that was located immediately to the west of the sinkhole. Sand can be strong when compacted as the particles are packed together, but weak when it becomes wet, and particularly if there is a lot of water under pressure.
“The area near the sinkhole has been developed over the last few decades and some of the properties look quite new. Normally, the ground would have been investigated before any construction. This might have involved shallow trial pits or boreholes. The local geology would possibly have been seen as fairly stable and low risk, and the buildings are not large so the amount of investigation could have been limited. Geotechnical engineers looking at the site would have been primarily concerned with soil strength and drainage and, perhaps quite reasonably, are unlikely to have considered the ‘lateral blast’ effect of a burst high pressure water main.
“What is currently unclear is why the mains pipe ruptured. One possibility is soil movement that caused the pipe to deform. Interestingly, an ‘Entrance to Subterranean Sand Pit’ is marked very close to the position of the sinkhole on the 1868-9 Ordnance Survey map and, while this has been infilled, its may have left a legacy of deeper, weaker soil. There are some clay-rich layers in the local sands that could heave and shrink over time. An alternative is that there was a structural failure in the pipe or a nearby sewer.
“The pipe rupture does highlight the challenge that comes with having buried infrastructure as they are often buried at depths in excess of a metre. The depth is to reduce the impact of deep freeze during a very cold winter and the place the pipe below the soil most exposed to seasonal changes in moisture content. Unfortunately, this makes them hard to monitor and repairs can be difficult, including the replacement of soil in the excavated hole once the repair is complete.
“Hopefully, in this case, the extent of the damage is limited to the areas that have already collapsed and the foundations of the adjacent properties have not been affected. Repairing the collapse hole might not be straightforward as the ground is currently saturated and will need to drain. There will need to be ground investigations to see if the hole extends further under the road and buildings. Ultimately, it may be filled with processed sand that would need to be artificially compacted or with concrete.”
Declared interests
Dr Andrew Farrant None
Chris Barker “I am currently a co-author to the update of the CIRIA C574 Engineering in Chalk guide. The update has been partly funded by industry.”
For all other experts, no reply to our request for COI was received.