7 JUNE 2021 WorldWide Drilling Resource ® Geotechnical Advice May Prevent a Collapse Compiled by Carol C. Schimpf, Editorial Assistant WorldWide Drilling Resource ® In the UK, the continuing dangers of historic coal mineshafts are revealed each year with an average of 15 mineshaft collapses annually. Photographs of collapsed roads, buildings, and large chasms, which suddenly appear, raise concerns for residents close to the collapses. A recent collapse in Chesterfield highlighted the problem of unseen, unstable abandoned mineshafts. The hole collapsed in undergrowth to the left of the second hole at Chesterfield Golf Club. It measured around 12 feet by 12 feet at the surface, and its depth extended beyond sight, even with a light. Luckily, the Chesterfield Golf Club col- lapse was in an area where golfers would not be playing, but it was noted some people use the area for dog walking. The area was fenced off with warning signs installed. For new developments in high-risk areas, the Coal Authority, headquartered in Mansfield, now requires a Coal Mining Risk Assessment (CMRA). The coalfield is divided into two areas: Development High Risk Area and Development Low Risk Area. The High Risk Area, comprising 15% of the coalfield, is where coal mining risks are present at shallow depth, which are likely to affect new development. The Low Risk Area, about 85% of the coalfield, is where past coal mining activity has taken place at sufficient depth, posing low risk to new development. The CMRA, submitted to the local planning authority, iden- tifies site- specific coal mining risks and sets out the mitigation strategy to show the site can be made safe and stable for the proposed development. To complete these assessments, some geotechnical expertise is needed. For residential, commercial, and industrial properties, the CMRA includes an assessment of the Coal Authority Mining Report, a visit to the Coal Authority offices to assess mining plans, a review of modern and historical pub- lished geological maps, and review of techni- cal reports/memoirs. The point of contact for coal exploration, water well, and site investiga- tion borehole information is the British Geological Survey. Geotechnical investigations, processes in which the physical properties of a site are assessed for the purpose of determin- ing which uses of the site will be safe, are most often performed to complete the risk assess- ment. APhase 1 Desk Study Report details the likely presence of any contamination within the sub- surface and the potential risks this may pres- ent to future site users, developments, and the immediate environment. This always precedes any ground investigation, but is also accompa- nied by a walkover survey conducted by an experienced practitioner. The walkover survey will assess areas identified within the desk study as potential development issues, Photo by Daniel Maddison. C&G Geotechnical cont’d on page 8.