Within specific areas of the United Kingdom, the potential for former shallow mineworkings is often identified during a Phase I Desk Study. Workings can be resultant from extraction of various materials including Coal, Ironstone, Fireclay, Limestone and Chalk. Where the potential for shallow mineworkings is identified during a desk study, this normally requires additional more detailed research. This includes a detailed geological assessment based on large-scale geological maps, and a review of data held by the British Geological Survey, Coal Authority [Abandonment Plans] and/or Local Authority. Following on from this, the Phase II physical investigation can be appropriately designed to incorporate the needs of the mining investigation, alongside other routine geotechnical and environmental requirements.
A programme of drilling using rotary techniques is normally required to establish the nature of the superficial and solid geology and determine the presence, extent and potential risk posed by any workings. Dependent on the specifics of the site, other more innovative techniques may also be appropriate, including geophysics and down-hole camera surveys. Once a sufficient understanding of the ground has been established, an assessment of the potential for future surficial movement and collapse can be undertaken and the need for treatment or remedial measures identified based on detailed site proposals.
Applied Geology has extensive experience in the investigation, assessment and treatment of shallow mineworkings, to include the specification of drilling and grouting programmes, site supervision and validation.
Case Study 1
Applied Geology was contacted after a large hole appeared in the back garden of a newly constructed property in South Oxfordshire. The problem was quickly assessed and the hole was stabilised and made safe by the introduction of pea gravel. This allowed time for the reason for the development of the hole to be understood. A research phase was undertaken where existing site investigation and foundation data by others was appraised. From this, it was apparent that the site was in an area that was at risk from both the presence of naturally occurring solution features and the possible presence of relatively shallow workings within the Chalk. A programme of drilling was designed by Applied Geology and confirmed the presence of Chalk workings in a pillar and stall system. It was concluded that the initial hole seen was formed by a combination of collapse of these workings and the presence of a solution feature. A drilling and grouting scheme was then designed and approved by the client and the National House-Building Council and then undertaken with the majority of the residents still in residency. The grouting was controlled by the use of down-the-hole cameras followed by proof drilling.
Case Study 2
Applied Geology was instructed to carry out a ground investigation for new housing at a site in Wednesbury in the Black Country.
Reference to the British Geological Survey Maps for the site suggested that the Thick Coal outcropped on the site. Reference to nearby abandoned shaft records suggested that the Thick Coal is between 3.6m and 4.48m thick in this area. As such a significant thickness of Made Ground was expected where the Coal had been worked from the surface or evidence of Bell Pits where the Coal has been worked by creating hand dug shafts opening into pits that are bell shaped.
The ground investigation to investigate the coal mining comprised the drilling of three rotary open boreholes to 30m bgl and logging the arisings which were returned up the borehole. These boreholes suggested thin seams of Coal near surface with broken ground at around 25m bgl. The broken ground was suggestive of the collapse of abandoned Coal seams at depth below the site. The near surface seams however were not worked and not of a thickness which would suggest the presence of the Thick Coal.
Building Control were still concerned about the presence of the Thick Coal beneath the site and reference was therefore made to abandoned mining plans which indicated that Coal seams underlying the site had not been worked at that time however it also gave the depth of seams that had been worked under the site. Using these plans and the evidence of the Coal Authority Report, Applied Geology was able to interpret the results of the rotary open hole boreholes and confirm that the Thick Coal should either appear at surface or not be present at all under the site. In addition due to the accuracy of the mining plans, we were able to significantly reduce the depth of treatment required by drilling and grouting and thereby reduce the overall cost of the development to the client.