Mine Shaft Treatment, Cross Hands
Introduction:
We developed a clear remedial strategy in accordance with current guidance and led the shaft treatment in phases. Detailed discussions were held with the Coal Authority and the project team and the strategy was updated relative to emerging conditions. Our input enabled the mine shaft to be stabilised, mitigating the potential for future subsidence beneath and adjacent to a public road.
Details:
We have been providing geotechnical investigation and design support, including mine shaft treatment, for the Phase 2 portion of a new link road intended to ease congestion and to provide opportunities for development and economic growth in Cross Hands in the west of the South Wales Coalfield.
The proposed 2km route traverses agricultural land underlain by Quaternary Deposits and Lower/Middle Coal Measures bedrock which has experienced significant localised deformation. Significant historic mining and quarrying has occurred across the route and targeted investigation and remediation has been undertaken in areas of shallow mine workings.
A detailed desk-based review identified the presence of an air shaft within close vicinity of the proposed road footprint, ventilating old workings within the thick Big coal seam, extensively exploited in the area (e.g. California and Emlyn Collieries).
Dimensions were unavailable, however it was indicated to be capped with concrete to an unknown specification. Our exploration was targeted on recorded co-ordinates for the mine entry, but did not identify it within the departure distance or in potential locations outside of this based on surface features and local anecdotal evidence. However, we considered it very likely that it did exist so a watching brief was provided to the contractor to be observed during proposed site clearance.
The shaft was subsequently discovered approximately 12m from the recorded position (1.7m wide and voided to ~14m depth). The cap comprised metal sheeting and sections of metal railway line; the upper portion was dry stone walled; we have found this to be a common historical construction method and falls below current design standards.
 | Following removal of the shaft cap, inspection using a remote camera confirmed that the shaft was lined with stone to around 3m depth and progressed into sandstone. Possible coal (the Big coal seam) was noted near the base in the shaft wall. No groundwater was observed in the shaft. |
Mine shaft treatment commenced with controlled infilling of the shaft using a long reach excavator positioned outside of the potential collapse zone. This was undertaken to limit the potential temporary works stability risks of working above/in the vicinity of the shaft by providing support to the liner and removing the void space.
A self-compacting, free-flowing, coarse grained gritstone aggregate was selected, which would not be susceptible to acid mine groundwater/drainage (in accordance with CIRIA C758 Abandoned mine workings manual) whilst enabling stablisation by subsequently drilling the shaft and grouting.
To limit future loss into connected workings we then consolidated the aggregate with grout in stages.
We are providing design advice for surface mitigation. This will include soil reinforcement at the surface incorporated into the nearby road and pavement construction and will enable future management of subsidence risks in the general area.