The Coal Authority has called for geotechnical engineers to ensure coal mining legacy issues are considered in light of its findings from an investigation into subsidence event in north-east England.
The authority said it was sharing the details of the ground movement issues at a housing development in North Tyneside to highlight the risks of not carrying out a thorough investigation.
The National House Building Council contacted the Coal Authority in July 2016 after the housing development experienced extensive ground movement. The subsidence affected 35 properties and resulted in a 150m by 70m elliptical subsidence zone.
Ground investigations undertaken by the authority revealed coal mine workings extending to more than 30m below the site which dated from 120 years ago but were not recorded on historical mining plans.
Detailed surveys showed that extraction rates were over 70% resulting in narrow residual supporting coal pillars and wide extraction rooms in the High Main coal seam, leading to compression on the remaining coal pillars and roof instability. This, together with a fractured sandstone layer above the coal seam, resulted in an underground collapse and subsequent movement at the surface.
Stabilisation of the historical mine working is now underway using drill and grout techniques and is expected to be completed later this month.
Coal Authority chief operating officer Simon Reed said: “We have released our initial recommendations to ensure there is awareness of the risks posed by historical coal mining legacy, in light of our findings from this recent subsidence event.
“In this instance, given the depth and age of the workings, and knowledge about risks of subsidence at the time, we cannot criticise the developer’s approach taken at the time with respect to these mining circumstances, but we must now build on this new knowledge to address these risks better moving forward.
“This was an area of unrecorded mine workings and caution must be adopted in assuming that the absence of a record means the absence of mining. Although our historic plans did not, in this case, reveal the coal workings they are a vital part of any site assessment prior to development therefore, in future we may ask for more information, or for more works to take place, to reduce the likelihood of a similar subsidence event happening again.”
The Coal Authority is expected to publish an in-depth technical guidance nots on the incident later this year once monitoring work is completed.