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Subsidence from old mine workings

publication date: May 21, 2007
author/source: Derek Morgan
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Buying and selling houses in the South West is sometimes seen as more complex (and even more expensive) than elsewhere in the UK. This has nothing to do with agent’s fees, but the very nature of the ground on which a house stands.

Although other parts of the UK have their share of mining problems, the South West,particularly Cornwall, was one of the most intensely mined areas in Europe during the heights of tin and copper mining in the 18th, 19th and 20th centuries. This has left homeowners with the legacy of problems such as mineshafts, mundic and arsenic to name but three.

Before I expand on the problems, it is important to re-assure people that for the vast majority of houses in the South West the following problems will have no direct effect, except by way of costs for additional searches. Cornwall is not a honeycomb of holes with houses built on arsenic-laden ground (despite a rather keen TV reporter once trying to get me to say just that), but the risk is real and needs to be considered by any prudent buyer or wise seller.

Subsidence from old mine workings

Subsidence is probably the most obvious mining-related problem for properties in the South West. Over the years there have been several high-profile incidences of subsidence, such as the ones in Redruth, Gunnislake and at Bethel in St Austell. Most are caused by the collapse of very old caps on mineshafts, often just made of timber, which decay over time and eventually give way. Because a mineshaft may have been abandoned over 150 years previously and never even recorded, there are incidences where a house has been built over a shaft without the builders or owners ever knowing. Such subsidence often affects no more than one or two properties, but if the mine working is under the foundations, the result can be catastrophic.

The first step in assessing the risk of subsidence from historical mining is to obtain an archival Mining Report or Mine Search Report. These are reviews of the historical data held in the public domain and sometimes, private archives. The resultant reports are only two to five pages long and are usually produced in five days or so due to the time needed to examine the old maps and plans.

At around £50 each, Archival Mining Reports are ridiculously cheap due to the competitive nature of the market place. Buying a house in the mining regions of the South West without instructing a Mine Search Report is a risky business, as about 20-30% of archival reports will highlight a potential problem.

If an Archival Report identifies a risk, such as a recorded mineshaft beneath a house, the next stage is to instruct a Site Investigation. These can be expensive, sometimes disruptive, and always an unwelcome delay to both parties. The most common form of investigation for an existing dwelling is a drilling investigation. This involves drilling a series of holes underneath the house to prove that it stands on solid bedrock without any mineshafts or shallow mine workings. About 70% of the time houses get a clean bill of health, but every year enough significant problems are found to warrant the expense. With the average house now selling for over £180k, a £2k drilling investigation lasting just two days is money well spent.

There are a number of companies offering Archival Mining Reports and Site Investigations, but check them out first because this is an un-regulated industry. The basic checks would be to make sure they have suitable PI insurance and relevant qualifications. The expert witness cases that I get asked to work on are a reflection of this: when archival reports and site investigations for the same house, from different companies, have differing conclusions.

When mining problems are found beneath a house it does not always mean that the house is ‘un-sellable’ or that the sale will fall through. The recently publicised sale at auction of a terraced house in Albany Road, Redruth for £32k, shows that even the worst affected properties still have value in the current climate. The house has severe mining problems hence the drastically reduced value.

Specialist companies exist which fix mineshafts and other forms of mine workings. Mineshafts have even been fixed under houses without the need for demolition. Of course there will be a significant expense, possibly over £10k, and considerable delays that can last for weeks, but at the end of it the seller should have a sound property with the paperwork to prove it and no further investigations will be necessary.


Another ‘Cornish’ problem, but one which also affects Devon and other parts of the UK, is ‘mundic’. Simplistically this is either mass concrete or concrete blocks where the aggregate used to make them was mining waste. The waste rock from mining can contain deleterious components such as pyrite which can breakdown over time in the presence of moisture and cause the concrete to ultimately crumble and fail. The standard test is to remove cores from the walls of the property and have them examined in a laboratory to see if they contain the deleterious aggregate and to assess the risk.

Traditionally a Chartered Surveyor recommends mundic testing during the valuation stage and for most areas this is usually for concrete construction carried out prior to 1950. There are a number of specialist firms offering this service but the testing regime can be expensive and time consuming in some cases, though most testing should take a couple of weeks and only cost a few hundred pounds.

The industry is guided by the Mundic Block Steering Committee, which continually looks at improvements for mundic detection, analysis and reporting in association with RICS and the CML.

Contaminated Land

The mining industry produced a large quantity of waste material over the centuries and in some areas this has resulted in high levels of lead, arsenic, cadmium and other related contaminants. The risks posed can affect not just the occupiers but also the developers. A housing estate in Camborne, Cornwall was closed half-way through the build by the HSE after it was found that high levels of arsenic in the ground had not been properly assessed. This can be a problem for buyers where houses are sold ‘off plan’.

The introduction of Part IIA of the Environmental Protection Act 1990 in April 2000 changed many things revolving around environmental risk and has resulted in a lot more caution and a desire for greater information regarding what is in the soil. At present testing existing garden soils is not a major issue and only comes about if a property is identified as having once been part of a mine site or some other industrial use.

For most people an assessment of the risk from contaminated land will come via an on-line environmental report, but more detailed reports called Phase 1 Reports, which include a walkover survey, are often a more informed choice. Again in some cases this can lead to more expense and delays if a risk is identified, but most buyers would rather know if there are any hidden risks with a property.

When a theoretical risk is identified buyers often request testing to find out if the risk is real, and if so what it is and what they need to do about it. The testing usually costs several hundred pounds and takes two to three weeks to fully assess the results.

The occasional properties that are found to have high levels of certain contaminants can still be a viable and safe investment, provided the right tests have been carried out and a suitable remediation scheme has been put in place.

More detailed advice on any of the above issues or organisations is available from the author.

Derek Morgan has worked as a consultant for 13 years in Cornwall and Devon and is the Managing Director of Associated Property Solutions Ltd, which provides environmental and subsidence desktop reports and site investigations for people buying, selling or developing property in the South West. He is also a member of the Mundic Block Steering Committee. Tel: 01209 219 627