1. Summary of issue
On the 4th January 2018, a 1.2m deep, 0.6m wide ‘’sinkhole’’ opened next to the Whitehaven – Workington railway line, near the coastal village of Harrington in Cumbria. Amidst fears of the hole expanding and damaging the rail line, services were delayed in order to complete emergency repairs and infill the void. Initial observations have led rail engineers speculating that a collapsed drain or coastal erosion accelerated by Storm Eleanor are the cause of the collapse. However, the correlation with known mine entries and collapses along the railway line have prompted Terrafirma to research further to see if past coal extraction may in fact be the cause.
2. What is the cause of the issue?
There are numerous recorded mine entries located within 15 metres of the railway line near Harrington, many of which are reported to be adits: near-horizontal tunnels providing entry into subsurface mines. Several of these adits extend beneath the railway line providing access to the underlying recorded shallow workings. It is likely that increased erosion rates caused by Storm Eleanor will have facilitated the migration of subsurface voids, associated with these mine entries and shallow workings, to the surface resulting in the “sinkhole”.
3. Can data relevant to the site be sourced, provided and used as evidence
The Coal Authority holds data pertaining to all recorded coal mining in the region. It is from this data that numerous mine entries (including adits) have been identified, including a previously recorded coal-mining related collapse beneath the railway line approx. one kilometre south of Harrington. This Coal Authority data can be supported by historic mapping, high resolution lidar and British Geological Survey bedrock and superficial geology data.
4. What is the professional interpretation according to Terrafirma
Adverse and rapidly changing weather, such as that experienced during Storm Eleanor, provide prime conditions for ‘’sinkhole’’ formation. A rising water table combined with high-speed winds, rough seas and increased surface run-off all accelerate the movement of water through loose, unconsolidated and porous material which increases the rate of erosion and thus the likelihood of void migration and/or creation. It is possible that an overburdened drain or high sea erosion rates are responsible for the collapse. However, the presence of numerous mine entries along the railway line in correlation with 12 previously recorded coal-mining related collapses within five kilometres of Harrington is evidence of extensive coal mining in the area but also of the propensity for these old workings to collapse. It is believed likely that these prime sinkhole-forming conditions facilitated the migration of subsurface, shallow coal, Fireclay or ironstone mining-related voids which manifested as a “sinkhole” beneath the railway line. Only further on-site investigation of the collapse can determine the exact cause, however, given the extent of coal mining in the region and beneath the railway line, it is considered prudent to exhaust the possibility of a mining-related cause to ensure that future collapses do not plague the infrastructure in the area.