1. Summary of issue
On 28th June 2016, a 10 metre by 10 metre sinkhole consumed gardens just metres behind homes on Towy View Park, Carmarthen, Wales. The sinkhole appeared early in the morning and almost destroyed a storage shed that was left precariously close to the edge.
2. What is the cause of the issue?
Sinkholes develop when voids in the subsurface become large enough for the overlying ground to collapse. The Ordovician bedrock, named the ‘Tetragraptus Beds’, is a sandstone formation, a rock type known to allow the migration of voids through internal water erosion washing away material. Towy View Park is also the location of a former lead mine (Glan Towy Mine) where recorded underground extraction has occurred. Over time, roof material collapsed into the subterranean mine and allowed the migration of voids to the near surface. Eventually the voids lost structural integrity through further growth and erosion by water resulting in the collapse and formation of a large sinkhole.
3. Can data relevant to the site be sourced, provided and used as evidence?
Many sources can be used to supply data for this site: The British Geological Survey (BGS) hold the most relevant data which include Britpits, linear features, borehole records and bedrock geology. Historical mapping and land use data can also be used in conjunction to build upon and support the BGS data. With these data sources and interpretation by a qualified geologist, evidence can be constructed on the cause of this sinkhole as well as other potential geological hazards in the area.
4. What is the professional interpretation according to Terrafirma?
All compiled relevant data sources lead to the conclusion with a high level of confidence that this sinkhole was resultant of historical mining activities. There are 3 recorded lead mine features within 150m of the sinkhole as well as historic sandstone quarries directly on the site. It is likely these quarries acted as access points to the mineral vein that trends SSE-NNW through the housing estate of Towy View Park. The presence of several lead mine shafts to the North of the site imply that subterranean working of the mineral vein has occurred and likely extends beneath the residential site.
The quarries will have been infilled, possibly with waste from the lead mine, covering the clear evidence of mineral extraction in this area and acting as loose, highly erodible material that can compress and allow the rapid migration of voids.
It seems in this case that the developers did not conduct an appropriate site survey or mine search prior to construction and so the underlying lead mines went undetected. Over time the permeable and erodible sandstone and/or quarry infill has allowed the migration of the man-made voids to the near surface where they collapsed (facilitated by rainfall), forming the sinkhole. Had the developers commissioned the appropriate site survey and discovered the recorded mines then remediation of the workings or alterations to the foundations would accommodate the historic mining and restore site stability.