An analysis of drill and grout records from the South Wales coalfields

An examination of the records of over 40 contracts to stabilise abandoned mine workings by drilling and grouting has revealed the following:

• In over 25% of the contracts, grout takes were negligible. Sometimes this was attributable to the mines being completely collapsed but in many cases, the coal seam had not been worked.

• Grout takes varied widely from borehole to borehole. The pattern suggests that high grout takes usually are attributable to open roadways. The workings themselves are typically collapsed or back-stowed, except in ironstone horizons, where very high residual voidage is common.

• Migration of voids to any significant distance above the level of the original workings was hardly ever encountered during drilling.

• Grouting is usually restricted to the ground immediately beneath structures. External spaces, even those intensely used, are rarely treated.

The findings indicate that drilling and grouting is a very ‘hit and miss’ procedure for locating and eliminating the residual voids which are often very small parts of abandoned mines which constitute a real threat to the stability of the ground surface. Subsidence incidents in South Wales are rare, given the size of the coalfield, most relate to mine entrance or outcrop workings and their effects are generally limited to a small area. It is considered that a careful assessment of risk and cost-benefit would reveal that, in many instances, drilling and grouting gives a poor return when the increased security of the site is compared against cost. Protective measures which concentrate on the structures themselves may be cheaper and release resources to tackle the associated open spaces, where people are often as much at risk as within the buildings. These conclusions are drawn for South Wales only although they may be applicable to other coalfields where the geological conditions and mining methods were similar.

• Introduction

• The relationship between mining and subsidence

• Residual voidage

• The subsidence record

• Grouting contracts

• Discussion

• Conclusions

• References