Following detailed review of previous investigations, including identification of inadequate historical ground treatment associated with the Swansea Four Feet coal seam, we undertook targeted intrusive works to provide resolution on potential ground hazards and allow an assessment of impacts on the proposed development. This enabled risk zoning to inform future ground treatment options.
We have recently been involved at this site to provide a Mining Risk Assessment for a new Dyfed Powys Police facility in Llanelli. Previous treatment works undertaken in the 1990’s identified historical coal mining, including extensive voids and collapse of supporting roof rock.
A mine shaft is also located to the south of the south-eastern corner of the site (St Patrick’s Pit – Coal Authority ref. 253202-017). The Coal Authority also indicate a possible mine adit, orientated to the west in the south-eastern margins of the development area (Coal Authority ref. 253202-044).
Following a targeted investigation, comprising trial pitting, trial trenching and rotary drilling, we were able to refine the ground model and provide accurate risk zoning, detailing areas requiring mitigation, as well as identifying low risk zones which would not require any remedial works. The investigation also confirmed a low risk linked to a mine adit.
The additional information obtained during this advanced Coal Mining Risk Assessment allowed us to zone the site into four separate areas after accurately locating the outcrop of the Swansea Four Feet coal seam and areas of surface and shallow mine workings.
- Zone A: The area to the north of the Swansea Four Feet coal seam outcrop (north portion of the site). No shallow coal (or workings) identified and therefore no subsidence risk mitigation required.
- Zone B1: A narrow strip of around 10m width immediately south of the seam outcrop. Within this zone the coal is at very shallow depth beneath the site (less than 5m) and has been worked and backfilled. These workings could pose a risk to the proposed development and risk mitigation measures are required across the building footprint.
- Zone B2: The area south of the outcrop and south of Zone B1. Within this zone the coal is at shallow depth (from around 5 to 30m depth) beneath the present site surface. Past workings have reportedly been previously treated by drilling and grouting by others, however potentially high subsidence risk appear to remain and risk mitigation is likely required.
- Zone C: Located outside the southern margins of the site where the coal/workings are at sufficient depth not to pose an unacceptable subsidence risk to the proposed development.
Through detailed review, experience and focussed investigation, we were able to identify remaining risks at the site and refine the ground model to enable accurate design of future subsidence risk mitigation works, as well as establish low risk zones.