It’s raining sinkholes!

Whether or not you believe that climate change is occurring, the last decade or so has shown some significant changes in rainfall patterns in the central and southern parts of the country. These changes have influenced ground instability resulting in the recent spate of “sinkholes” that occurred in February.

Looking back at the rainfall statistics (www.metoffice.gov.uk) it is apparent that since 2000 the mean annual rainfall figures in southern and central areas have increased. In the past there were some notable ground collapses in the early and mid 2000s when intensive rainstorms locally produced rainfall of over 50% to 100% higher than usual. Higher than average rainfall was also a feature of 2008 to 2010. By contrast 2011 was a much drier year than normal and this pattern continued into early 2012. During this time few ground collapses were taking place.

As 2012 progressed rainfall greatly increased from April onwards producing a wetter than average year. 2013 also started with average rainfall interspersed with drier months but got wetter during the autumn to winter period, continuing into the very wet months of January (>200% above average) and February (>270% above average) this year. The cumulative effect of the rainfall reflects the incidence rate of subsidence as follows:

·        - 7 subsidences in 12 months in 2012

·        - 17 subsidences in 12 months in 2013

·       - 13 subsidences in just 2 months in 2014

Size isn’t everything!

Fortunately in Britain ground collapse is rare and not as dramatic as examples from places like Florida, South Africa, Guatamala or China, but even so the holes can be reasonably large and dangerous. In late December a collapse over old mine workings at Foolow, Derbyshire produced a hole about 50m across and 40m deep. This occurred in a rural area but only endangered local sheep and walkers! By contrast a number of smaller holes that occurred further south in February caused alarm to residents and structural damage – fortunately none of it causing injury!

·         At Walter’s Ash, High Wycombe, a hole (circa 4.5m diameter, 9m deep) swallowed a VW Lupo car parked on the drive outside a house

·         At Upper Basildon, near Reading, a hole (circa 3m diameter, 3m deep) opened up beneath a car as it travelled along a driveway, but luckily the vehicle spanned across the hole allowing the family to get out safe

·         At Hemel Hempstead, another hole (circa 10m diameter, 6m deep) appeared below housing causing significant structural damage and evacuation of residents

·         At Nettlebed, Oxon, a fourth hole (circa 5m diameter, 6m deep) occurred within woodland, needing fencing off to secure it safely

·         At Gillingham, Kent, a large hole (4m diameter, 9m deep) formed within the grounds of a school

Since the collapses occurred some of the holes have been backfilled with foamed concrete to make them safe while ground investigations are carried out to determine their cause. Over 200m³ (or 20 concrete lorry loads) of foamed concrete were used to infill the holes at each of Hemel Hempstead and Gillingham.

What is a sinkhole?

Strictly speaking a sinkhole is a collapse of ground over a naturally formed void at depth. They occur where the ground below the surface has been dissolved away – they are typically found in areas underlain by chalk, limestone, gypsum and salt. In southern and eastern England, sinkholes are associated with areas where sand, gravel and clay layers overlie chalk. PBA is currently investigating and stabilising several sinkholes on chalk where structural damage has occurred in areas like Reading, Marlow, Maidenhead, Beaconsfield, the Chalfonts, Grays and Hertford. The recent collapse in Ripon, underlain by soluble gypsum, is another example of a sinkhole that caused significant structural damage.

However, many of the recently formed holes are suspected to have originated as a result of ground collapse over man-made voids in the ground i.e. mine workings. This type of collapse is referred to as a “crown hole”. There are large numbers of unrecorded historical mine workings across the country, liable to intermittent subsidence depending upon weather or leaking drains that reveal their presence each year. PBA is currently investigating possible mined ground and collapses at Gillingham, Nettlebed, Upper Basildon, Hemel Hempstead and Chalkhouse Green.

Solutions for sinkholes

After backfilling the sinkhole to prevent it enlarging either a series of exploratory holes are drilled into the ground at close centres around the feature or the surrounding ground can be surveyed using geophysics. Geophysical survey techniques that can be useful, subject to conditions, include microgravity, resistivity, conductivity and GPR. Depending upon the setting investigation techniques such as dynamic probing or rotary drilling may be used. The aim of the work is to be able to create a 3D ground model of the cause of the collapse at depth in order to design a suitable remedial stabilisation scheme. Remedial stabilisation techniques used from the surface can include grouting (using cement and/or resin), soil reinforcement (geogrid installation), piling, capping and excavation/replacement. Sometimes safe access into the collapsing void can be achieved and the infilling or reinforcement works can be carried out in situ.

What action should conveyancing solicitors take?

Given the potential impact on a property, ground instability risk is clearly something for which conveyancing solicitors should be checking as part of their due diligence process. Landmark’s environmental reports – both residential and commercial – all contain data from multiple sources (both manmade such as mining and underlying natural conditions) in order to provide a risk screening and next steps. For residential transactions, in the case of the premium SiteSolutions Residence report, a full ground stability risk assessment is included, whereas Homecheck Professional and Envirosearch will highlight risk and suggest the purchase of further, more detailed reports or potentially a surveyor visit should that be appropriate.

If you’d like more information on the risk reports, please contact Landmark Information Group on 0844 844 9966 or email helpdesk@landmark.co.uk

Blog prepared by Dr Clive Edmonds, Peter Brett Associates LLP, a leading specialist in the recording, investigation and stabilisation of geohazards and Landmark Information Group.