A common problem when developing sites in the UK's coalfields is assessing ground stability caused by shallow mining. Most planning authorities insist on a ground investigation, usually with boreholes to a minimum depth of 30m.
Principally on the basis of cost, rotary open hole drilling is used to check for the presence of any workings. With open holing, it is assumed abandoned workings will be identified by one or more drilling characteristics: variations in drilling penetration rates; loss or reduction in flush returns; and changes in groundwater conditions.
However, data quality is greatly affected by the experience of and techniques used by drilling operatives. For example, if too great a torque is used to advance a borehole, it can be difficult to judge changes in penetration rate and disturbed or partially voided ground may appear similar to intact very weak rock.
And, as a case study in the West Midlands shows, results should be interpreted with great care.
The site is underlain by Carboniferous Middle Coal Measures, with only a thin veneer of superficial drift deposits. Coal Authority records indicated deep mining in multiple seams with the probability of unrecorded shallow workings from two shafts close to the site. Before any proposed development could go ahead, it was decided to carry out a survey using rotary open hole drilling.
Results appeared to show that shallow mining had not taken place, as no peculiarities were noted in penetration rate, flush returns or groundwater conditions during drilling.
However, a succession of coal horizons were encountered in all the boreholes, with considerable and random variations in thickness and separation, making correlation between holes very difficult and confusing.
It was decided to sink a number of quality cored boreholes to try to understand the disposition of the various coal horizons.
Detailed logging clearly indicated the site was in fact underlain by two shallow coal seams which had been either partially or totally mined, leaving them intact, reduced to thin coal horizons or absent.
There were no significant voids and the workings appeared to have either collapsed or been backfilled with a clay containing mudstone and coal fragments. This, in part, explained the apparent intact nature of the ground suggested by the open holing.
Although the small number of cored holes did not indicate significant voiding, it was considered this could not be ruled out across the site.
To minimise future ground movements, it was recommended that stabilisation by drilling and grouting was carried out and that any structures should be supported on raft foundations.
This was later confirmed by drilling for a proposed development, which revealed zones of significant voiding together with substantial quantities of groundwater.
Clearly extreme care must be taken in interpreting rotary open hole drilling for detecting shallow mining. Just because no particular anomalies are identified, it does not necessarily mean the ground is stable.
Despite the extra cost, sinking cored holes to establish whether or not shallow mining has taken place is unavoidable.
John Crook and Paul Platt, HB Boring and Company