Pile Design and Construction Practice, Fifth edition

lateral movements as the rock strata adjust themselves to their equilibrium position. A
bentonite or bitumen infill to seal the annulus completely would be used if there is a risk
of emission of natural gas from the coal seam.
During the nineteenth century and the early years of the present century, coal and other
minerals were extracted using mining techniques variously known as ‘pillar and stall’, ‘board
and pillar’, and ‘stoop and room’. A main heading or road was driven from the shaft to
follow the coal seam to the planned boundary of the workings. Transverse galleries were
then driven from the main roadway to form a rectangular, triangular, or lozenge-shaped
pattern of galleries separated by pillars of unworked coal. These pillars served to support the
overlying rock strata until the general area had been mined. The pillars were then either left
intact or were wholly or partially removed as the coal extraction operations retreated towards
the shaft. Where the pillars were wholly removed the pattern of subsidence followed that of
longwall mining (Figure 9.9). Chalk was mined in south-east England for flints and
agricultural purposes from pre-historic times until comparatively recently. The mining was
usually in the form of a rather haphazard pillar and stall method. Where pillars were left in
place they remained, and still remain, in an unpredictable state of stability which has
resulted in complex problems concerning building over abandoned mineworkings, problems
which are still encountered in the built-up areas of Britain to the present day(9.8).
The instability of coal pillars may be due to the slow decay of the coal, to changes in the
groundwater regime in flooded workings, to increased loading on the ground surface, to an
increase in the load transferred to pillars due to the collapse of neighbouring areas, or to
longwall mining in deeper coal seams. If massive rock strata such as the thick sandstones of

Miscellaneous piling problems 447

Figure 9.11Isolating shafts of bored piles from surrounding collapsed ground.

Space loosely filled

with rock spalls or

bentonite slurry

Concrete

cast in-situ

Rock

socket

Base of worked-out seam

Stowage

Collapsed

overburden

Light-gauge

steel lining

tubes