Residential Development Coal Mining Risks
Introduction:
Our Coal Mining Risk Assessment identified the possible presence of recorded and unrecorded mining features (including probable bell pits) on a proposed residential development site in Swansea.
A Conceptual Ground Model enabled us to demonstrate the hazards and risks to the Coal Authority and refine the site layout with the designers, taking into consideration the identified mining hazards and exclusion zones.
Knowledge of the mining features/hazards at the site enabled us to provide detailed assessment and recommendations for targeted remedial works to enable the construction phase.
Details:
Our Coal Mining Risk Assessment (CMRA) identified evidence that the site was developed as part of the former Waunarlwydd Colliery, with a winding pit, air shaft, two adits and recorded underground mine workings. Additional information obtained from the Coal Authority confirmed the above features to be present based on abandonment plans.
Phased investigations identified the horizon of the Swansea 5ft coal seam to be present at shallow depth across the development area, from surface level at the outcrop to >30m depth. We also proved mining features identified within the Desk Study showing that the winding pit shaft had been loosely filled to its full depth. Drilling in the area of the possible air shaft/adit identified highly variable ground conditions, with evidence of an adit entry identified through trial pitting and trenching.
In other areas of the site, shallow trial pitting and trenching identified the presence of Bell Pits which were previously unrecorded. These explained anomalous ground conditions noted during drilling of individual residential plots.
There is evidence of Bell Pit mining in the nearby Cwmllwyd Wood Nature Reserve; the area has history of coal mining since the 17th Century and the remains of about 35 bell pits can be seen within the reserve boundary.
In Cwmllwyd Woods there are many birch, holly, willow and rowan trees as well as some large apple trees near the coal workings which may be the result of miners throwing away their apple cores.
Locating the winding pit enabled us to calculate the potential risk zone associated with potential future collapse, in line with published guidance (CIRIA C758), using the information obtained during the investigative works (e.g. bedrock depth, shaft diameter and approximate angle of shearing resistance of the identified superficial deposits).
Identified Bell Pits on site:
Based on the information obtained through desk-based and intrusive investigation works, we provided targeted mitigation strategies for each identified mining hazard and outlined a phased approach to ensure the future development could progress with risks to the end-users was reduced to an acceptable level.