Source: United Kingdom – Executive Government & Departments
Scientists comment on the sinkhole that has appeared in Surrey.
Dr 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.”
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