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Geoinvestigate were asked to comment on the possible causes of the coal mining subsidence and recent building demolitions at West Allotment, Shiremoor. Subsidence and cracking of several homes was first noticed in 2015 and in 2016 families were moved out of several properties into temporary accommodation. The Bayfield Estate is a relatively new development and part of a larger development of several hundred houses by Bellway Homes on the east side of Newcastle near Shiremoor.
The Coal Authority have issued few press statements on the cause of the damage other than it is due to ‘historical coal mining subsidence’. The estate has been the focus of intense geotechnical and ground investigation by the Coal Authority over several months including drilling and monitoring.
Mining subsidence is so serious that 5 houses have been considered irreparable and have been demolished so far and a further 10 are earmarked for demolition. Subsequent to writing this article 20 houses are to be demolished.
According to a source building subsidence problems and cracking initially affected House Nos 12, 14, 15, 16 and 17. The‘epicentre’ of the problem and the greatest movement seems to have been around House Nos 14 and 15 on Bayfield Road. Mining subsidence and the demolitions are broadly concentrated in the same area of the estate.
Understandably local residents of the estate are worried whether their homes might be the next to have problems. They are also concerned about property blight and being unable to sell their houses with some already reporting difficulties. Residents have complained that they are not getting enough information from the authorities on what has happened and why the problems weren’t picked up many years ago when the development was at the planning and investigation stage. The Chronicle Newspaper has been reporting on the problems at the estate as they have unfolded over many months and has raised concern about the reluctance of residents to talk to the press about the situation. While this is perhaps understandable because residents are concerned about the negative impact of publicity on house prices in the area it is creating difficulties getting an overall picture of the extent and nature of the problems. Others have accused the authorities of a lack of transparency.
Geoinvestigate believes that the house demolitions may be due to the unexpected occurrence of coal mining subsidence in the area. We believe that this coal mining hazard was missed or not fully appreciated during the original Site investigation of the area in the early 2000s. Consequently the presence of unstable mine working wasn’t catered for in the design of the new buildings at the site and the new houses have been exposed to mining subsidence hazard.
The cost of putting right the problem and financial compensation is likely to run into many millions of pounds as each house is worth between 130K and 150K. The human cost has been significant with distressed families being asked to leave their homes and friends and move into temporary accommodation with an uncertain outcome. Neighbours fears about property blight and the difficulty of selling their homes even in areas of the estate currently unaffected by subsidence is understandable.
Geoinvestigate’s search uncovered that coal mining may have been carried out beneath the site. Correspondence between James Clerk Maxwell (the world renowned Cambridge scientist) in the 1870s to his uncle Robert Dundas Cay who owned Prospect Hill Farm (the previous name of the land on which Bayfield Estate was built) together with notes from the Maxwell archive indicate that the family were in 1874 keenly alive to mining subsidence, claims and compensation. In fact reference is made to a claim for ‘surface damage’ at the farm after Robert Cay took possession of it in 1855. Surface damage was a term used by Victorians to describe subsidence of the ground surface caused by mining. Similarly surface damage is referred to many times in the archives of the Cleveland ironstone mines in the late 1800s and early 1900s in the context of the cracking, sagging or collapse of roads, farmland and houses arising from mining subsidence.
The ‘surface damage’ that Cay and Maxwell discussed may include the circular mound or depression which can be clearly seen a little to the north of the Prospect Hill farm buildings on the OS map of 1861. While this feature might be a pond it could also be a large sinkhole collapse or a pond formed in a depression where the ground has sunk or foundered because of mining subsidence below it. This feature is about 10m in diameter and as the current street plan shows is close to the ‘epicentre’ of the subsidence problem at the Bayfield Estate and Nos 14 and 15 these being the first demolitions. It is possible therefore that House Nos 12, 14, 15 and 16 were built over or close to ground which had already collapsed or was highly unstable and likely to experience further subsidence especially if the weight of new buildings were added to it. This however is speculation.
Perhaps of most surprise is a 1788 borehole log which turned up during our search. The log of Boring No 1806 is presented below. Unfortunately the exact location of the borehole is unknown though it is possible that it was located on the south side of Prospect Hill because the borehole encountered ‘whin’ placing it in the vicinity of the Tynemouth Dyke which only occurs south of Prospect Hill. In addition the farm would have been a recognisable landmark at this earlier time with Prospecthill railway station only being established after 1845.
The existence of an exploration borehole of this early date is staggering reflecting the sophistication of 18th Century Georgian coal prospectors and drillers who would have used crude drill rigs and boring tools similar perhaps to the those shown below.
The reference on the log to ‘Box and old borehole’ is interesting because it suggests that the borehole was an earlier borehole which had been re-drilled or re-bored and deepened several years later in 1788. Box has several meanings in the lexicon of early mining terms including fine debris at the bottom of a mine cart – the latter also known as a Box or cart. Therefore Box may refer to the fine debris which had subsequently fallen into the borehole after it was abandoned. Re-drilling of the borehole would also explain the unusual absence of a description on the log of the ground above the ‘box and old borehole’ whereas a full rock description is given below it in the newer section of borehole.
In 1788 Boring No 1806 encountered a 1m thick unnamed coal seam at a depth of 23m. Other coal exploration Boreholes were sunk in the area in between 1750 and 1795 and there are references in mining and geological journals of the time to ancient mine workings being encountered in later 1700 workings in the area.
Had the mining risk at Bellway Homes Bayfield Estate development been identified at the beginning steps could have been taken then to make the ground safe including stabilising it by filling the workings with cement and supporting new buildings on specially strengthened foundations. Alternatively building could have been avoided altogether in the area currently subject to demolition although this would have been an extreme measure.
Coal mining risk assessment or CMRA aims to avoid problems like those currently being experienced at the Bayfield Estate. The resulting increased scrutiny and management of mining risk in recent years following the introduction of CMRA in 2011 has significantly improved matters. CMRA aims to identify and mitigate coal legacy hazards arising from shallow mine workings, mine shafts, mine gas and infilled opencast so they do not lead to unexpected building delays and unforeseen costs.
The positive outcome from the coal mining subsidence problems at West Allotment may be that when the dust has settled and the details become clear it will help to highlight past deficiencies in Coal mining risk assessment and coal drilling investigation.
Bayfield Estate may also increase public awareness of the hazards from coal mining legacy and make builders and developers less reluctant to spend money on exploratory drilling investigation which still remains the only conclusive way of confirming whether a site is underlain by mine workings or not.
In exploratory coal mine investigation 2 Boreholes are always better than 1 and 3 holes are even better. This is because with 1 hole there is the risk that the intersection of coal in the borehole may be wrongly interpreted as an area of solid unworked coal seam whereas in fact the area has been subject to room and pillar working (aka Stoop and Room in Scotland) with the existence of both cavities and coal pillars the latter left in-situ to provide support to the roof. The model below clearly highlights the danger of drilling too few Boreholes, in this case only 1.
In this instance the drilling machine was only capable of drilling 1 borehole in 1 day whereas Geoinvestigates New Microdrill can complete 3 holes and sometimes more in 1 day depending on depth and using water flush drilling.
The same applies not only to the investigation of coal mining legacy but also building development over historical Chalk workings in the South of England and the well publicised problem of natural Gypsum sinkholes and building subsidence in the Ripon area – the latter having recently been described as the “Sinkhole Capital of Britain”. Put quite simply more drilling investigation is needed to develop these areas safely and to avoid more problems like Bayfield Estate in the future.
With the recent introduction into the UK of safe, high speed, compact, site friendly and cost effective microdrilling borehole technology there is now no excuse not to drill in gardens, in small restricted spaces and below houses and buildings where mine workings or mine shaft hazards are suspected. Only by drilling can the presence, nature and the cost of putting these defects right be accurately assessed.
Contact Geoinvestigate for a Microdrill quote. Microdrilling makes sense because it offers safer, faster, cheaper and more environmentally site friendly coal drilling and mine working investigation.