Depressions or sudden collapses of the ground are known interchangeably as sinkholes, sinks, swallow holes, swallets, shakes, shakeholes and doline. They can develop progressively as growing depressions or catastrophically as small to massive collapse holes up to 600m diameter. They occur all over the world and naturally in Northern and Southern England associated with the occurrence of Limestone, Chalk and Gypsum bedrocks.
The recent collapse of the ground and the appearance of a 9m diameter sinkhole on the 3rd February 2014 in Main Road in Walter’s Ash, High Wycombe, Buckinghamshire highlights once again the natural hazards posed by the ground. In this case a hole suddenly opened-up swallowing the Conran family’s VW Lupo car.
Sudden collapse of ground to form sinkhole in area underlain by Karst Limestone.
High Wycombe is an area of Southern England underlain by chalk which has properties very different to other types of rock. Areas underlain by chalk may contain natural voids or caves. These form over many thousands of years as groundwater circulates through and dissolves the chalk. High Wycombe is also an area that has been mined historically for chalk and, occasionally, mining areas can have shafts which were not capped off correctly, leading to the collapse of the loose infill of these features creating voids. While these may remain intact below ground for hundreds of years they can be prone to sudden collapse.
Such collapse are impossible to predict and can be time dependant or triggered by some disturbance or change for example caused by road vibration, extreme rainfall, the fracture of a sewer or water pipe or a recent change in land use.
Geoinvestigate have carried out several sinkhole investigations – a particularly notable job being the borehole investigation and stabilisation of sinkholes/shakeholes which unexpectedly appeared in the cricket pitch, tennis courts and beneath the chemistry block of a school in Kirby Lonsdale near Ingleton, Yorkshire – an area underlain in parts by Karst limestone. A Trial pit investigation 10 years before by another company unfamiliar with this type of problem had found voids in the clay soils which cover the site but had not realised what they were. The holes were filled and stabilized with cementitious grout over a 2 week period during the school holidays.
Sinkholes are commonly formed where the underlying rocks are carbonate rocks either limestone or chalk. Sinkholes may be formed gradually or suddenly, and are found worldwide. A sinkhole is caused by some form of collapse of the surface layer. They can be caused by the chemical dissolution of carbonate or gypsiferous rocks or suffosion processes in sandstone which results in the leaching away or outflow of small mineral particles and dissolved substances by water filtering through a mass of rocks.
Sinkholes can also arise from human activity such as the occasional collapse of abandoned mines and underground salt cavern storage facilities. More commonly, sinkholes occur in urban areas as a result of water main breaks or sewer collapses when old pipes give way. They can also occur from the over pumping and extraction of groundwater causing the lowering of the water table and the removal of subsurface fluids.
A catastrophic massive collapse of the ground occurred on the Barrow to Ulverston railway line on Thursday 22 September 1892 near Lindale-in-Furness. A 10m diameter hole opened up under a loco which fell into the abyss and disappeared for ever. Today the loco still lies buried beneath the railway, though the depth remains a source of speculation. The collapse is believed to have been connected with the occurrence of iron ore mine workings beneath the railway line and it was said that before the collapse miners below ground could hear the shunting of steam trains at the surface. Recently celebrity pop figure and famous railway enthusiast and owner Pete Waterman raised the possibility of sinking a declined adit to recover the Victorian Locomotive from Lindale.
In England, sinkholes occur in the Carboniferous Limestone from Kirkby Lonsdale to The Peak District in Derbyshire. They also occur in the Permian Gypsum deposits from Harrogate to Ripon and Darlington and in the Chalk belt from extending from Luton, High Wycombe via Swindon to Basingstoke, Guildford and Maidstone. Areas with marked circulating groundwater flow patterns such as Ripon are more prone to sinkhole formation than others. Sinkhole formation can be dramatic, because the surface land usually stays intact until there is not enough support. Then, a sudden collapse of the land surface can occur.
Unless sinkholes show themselves at surface by the tell-tale formation of a depression or building cracks or subsidence they can be very difficult or impossible to detect and their collapse is almost impossible to predict. Geoinvestigate provides several Site investigation methods to locate sinkholes.
Depending on the nature of the site Geoinvestigate uses reconnaissance surveys, site inspections, desk studies, aerial photographic interpretation, drilling or probing on a grid pattern to locate voids or lose ground or Ground Penetrating Radar (GPR) surveys. Locating sinkholes and subsidence without digging, probing, or drilling can be accomplished using non-destructive field surveys. The two main methods for detecting sinkholes are aerial photography and geophysical procedures. Even applying these techniques there is no guarantee that sinkholes will be found.
If you are a developer, home owner, house builder, consulting engineer or project manager and you are worried or require advice about sinkholes and how they might affect your property following the collapse in High Wycombe contact Geoinvestigate at our Site investigation office in Reading or our Middlesbrough or North West offices.