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Geotechnical: How to avoid that sinking feeling

Clive Edmunds

Last month alone, at least eleven sinkholes appeared in the ground, causing huge shock and anxiety for nearby residents.

Collapse at Oatridge Gardens, Hemel Hempstead on 15 February 2014

Source: Reproduced by kind permission of Peter Brett Associates LLP

Collapse at Oatridge Gardens, Hemel Hempstead on 15 February 2014

The British Geological Survey is warning that there are likely to be more sinkholes to come. To put this into context, in a typical year, geologists would expect to see only one or two sinkholes a month in urban areas.For developers, sinkholes can jeopardise brand reputation and may lead to considerable delays and extra over costs. So it’s essential to do the geology homework.

Let’s first take a look at why there has been this recent flurry of holes in the ground. Although sinkholes open up for all kinds of reasons, by far the most common cause is a sudden influx of water. Since 2000, there has been an exponential increase in rainfall which may be an indicator of climate change. Intensive rain storms and wet periods, resulting in 50% to 100% more rainfall than average in an area, have become more common.

Between 2006 and 2007, the south saw an increase in rainfall of over 12%, while between 2008 and 2010 the increase in rainfall accelerated by 39%. Incredibly the South has just experienced its wettest January since records began when an amazing 246% increase in rainfall occurred.

All this rainwater has to go somewhere, so it soaks through the soil, increasing the loading over soil arches above voids in the ground, wetting and softening the soil and eroding fines. Where the voids are present within pre-existing solution features and they collapse, the hole at the surface is called a sink hole. If the void is associated with a man-made feature for exampkle mine working then it is called a crown hole.

What are the implications of sink holes and crown holes for property? They can cause extensive structural damage when they suddenly appear in urban areas. Sometimes properties have to be demolished, in particular when repair costs exceed insured value. It is important to recognise the geohazard risk in advance of development since it can be expensive to mitigate using special foundations and drainage control.

In a recent talk to the Royal Institute of Chartered Surveyors, I pointed out that some areas of the country are far more susceptible to sink holes than others. This is particularly the case in the South where sands, gravels and clays overlie chalk. Elsewhere, land underlain by limestone, gypsum and salt can also be prone to sinkhole development in certain circumstances. Our national PBA Natural Cavities Database, which has over 32,000 records, shows that chalk counties like Kent, Essex, Berkshire, Buckinghamshire and Hertfordshire are particular hotspots for cavities. Limestone is also vulnerable to sink holes in areas like South Wales and the Peak District. Places like Ripon, underlain by gypsum, are subject to periodic sinkhole occurrence as well.

Developers should also watch out for those areas with old mine workings. Our land instability archives reveal that the majority of the recent large collapses in the South were in fact crown holes which have appeared over old mines. Our national PBA Mining Cavities Database has around 16,000 records and they reveal that all types of mining- from tin to iron ore, slate and chalk can leave behind this legacy. These mines are often very old and can pre-date historical Ordnance Survey maps so developers need to be wary and seek specialist help. Standard desk studies may not highlight the risk.

Currently, we are investigating the causes and extent of one of the largest sinkholes I have ever seen, on the site of suspected old chalk mine workings. The hole opened up a recently in Hemel Hempstead Hertfordshire. The collapse affected 50 properties causing many families to be evacuated. It has taken an amazing 200m3 of foamed concrete to fill the hole – which is equivalent to almost 20 truckloads. A second void was located by ground investigations nearby and has also been backfilled with yet more concrete.

There are plenty of other examples too. In Walter’s Ash, Buckinghamshire, where a hole swallowed up a car, the area is known to be riddled with old brick works with chalk mines below. We had previously dealt with three other collapses nearby.

A sinkhole in Upper Basildon, Berkshire, which left a car teetering on the edge, was also the site of an old brick works and probable past chalk mining. A large collapse has also occurred in Nettlebed, Oxfordshire, again within a known chalk mine working area associated with old brick works.

These examples bring home just how essential it is to find out what lies beneath a site.

  • Dr Clive Edmonds is a partner at Peter Brett Associates

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