01Open hole or core.
Open-hole drilling, sometimes called rotary probing, recovers no core. The driller watches the rate of drilling, the flush returns and how the rods behave to identify changes in strata. It is the usual way to investigate old coal workings and voids, and it can install monitoring wells quickly.
Rotary coring uses a rotating core barrel and bit to recover intact cylinders of rock. The core is laid out in core boxes, photographed, logged and sampled for testing.
A flushing medium, such as air, water, mist or mud, is chosen to suit the ground and lifts the cuttings out of the hole.
02Measuring the rock.
Core recovery tells your engineer how sound the rock is:
- TCR, total core recovery: the length of core recovered as a percentage of the length drilled.
- SCR, solid core recovery: only full-diameter, solid pieces.
- RQD, rock quality designation: only sound pieces longer than 100mm. As a guide, an RQD above 75% indicates good-quality rock and below 50% poor rock.
Rock is described to BS EN ISO 14689, and samples can go for laboratory testing.
03Finding old workings.
Much of Wigan and the surrounding area sits on the former Lancashire Coalfield, and early shallow mining often has poor or missing records. Where a site is in the Mining Remediation Authority's Development High Risk Area, rotary drilling is the usual way to check for shallow workings and mine entries before building.
Signs such as flush loss or the rods dropping suddenly suggest voids or broken ground, typical of old workings. Our page on coal mining investigation explains the process.
Any drilling that enters or disturbs coal seams, coal mine workings or mine entries needs a permit from the Mining Remediation Authority (formerly the Coal Authority). Standard permits can take 20 working days or more, so we build this into the programme.
04Questions we get asked.
Why open hole rather than coring?
Open hole is faster and cheaper, and it is the usual way to prove whether there are voids or broken ground at seam level. Coring is used when you need to see and test the rock itself.
How deep do you drill for mine workings?
Deep enough to prove the seams of concern and enough sound rock above them, as set out by the coal mining risk assessment or your engineer. There is no single fixed depth.
Do you need a permit to drill on my site?
Only if the boreholes are expected to enter or disturb coal seams, workings or mine entries. We will tell you if your site needs one and allow for it in the programme.
