Unrecorded Shallow Coal Mining, Pembrokeshire
Introduction:
Coal Authority information stated "no probable unrecorded shallow workings" were present. However, our desk based and intrusive Coal Mining Risk Assessment confirmed the presence of unrecorded shallow coal mining features including probable bell pits on a leisure development site in Pembrokeshire. Our preliminary Conceptual Ground Model has enabled design discussion/alteration in advance of planning. More detailed investigation will be used to refine our understanding of the risks and the remedial measures required.
Details:
We are engaged to undertake a Coal Mining Risk Assessment to address Coal Authority objections and enable a leisure development in West Wales. Multiple lines of evidence have been used, including aerial photography, LiDAR, historical mapping, published geological information and Coal Authority data indicated the potential presence of unrecorded bell pits/mine entries, unrecorded underground mine workings and po. These indicated possible surface mine workings associated with outcropping coal seams of the Timber Vein group. A potential mine gas risk was also noted. Our work is in accordance with the abandoned mine workings manual (C758D).
Initial site walkover was undertaken to assess the presence of the mine entries/bell pits identified on aerial photography. Localised topographic depressions were noted in the field, close to the areas of interest and these were surveyed and targeted for investigation. Georeferencing of the aerial photography, localised trenching was undertaken, targeted towards areas of suspected bell pits to inspect for any Made Ground/colliery spoil or anomalous changes in ground model that could confirm their presence.
Colliery spoil was consistently encountered either mixed within the topsoil or as a layer of variable thickness beneath, supporting that mining had likely occurred on the site/nearby. A 1.5m diameter probable bell pit/mine entry was identified in an observed localised topographical depression. The mine entry was infilled with carbonaceous colliery spoil and locally thick Made Ground (colliery spoil) was identified close by.
Further trenches identified variable Made Ground soils with outcropping coal and unrecorded shallow coal mining. It was considered possible that this represented shallow surface mining near the outcrops of the Timber Vein Group; this may also explain the localised topographical depression identified. Differentiating between suspected bell pits and possible shallow surface workings was not easily achieved as it was likely that the near surface soils have been disturbed in the past and masked by agricultral activity, especially close to bell pit/mine entrances.
Our initial investigation confirmed the likely presence of unrecorded historic mining features that pose settlement, subsidence and ground gas risks. Further investigation will help understand the extent and magnitude of these risks, refine the Conceptual Ground Model and define mitigation measures.
Relevant Services Provided:
- Coal Mining Risk Assessment.
- Review of Historical, Geological and Coal Authority Information.
- Site Walkover and Visual and Geomorphological Assessment.
- Supervision and design of trial pit and trench investigation to assess the presence of surface mining.
- Assessment of subsidence risks and outline possible remedial requirements.
- Initial assessment of mine gas risks.
- Initial advice to progress development design.
- Provision of a scope to assess/delineate hazard zones and refine remedial requirements.