Pile Design and Construction Practice, Fifth edition

448 Miscellaneous piling problems
the Coal Measures are overlying the partly worked seam they may form a bridge over the
cavities such that the collapse of the weak strata forming the roof of the working will
not extend above the base of the massive rock stratum (Figure 9.12a). Provided that the
coal pillars themselves do not decay, the workings may remain in a stable condition for
centuries and it will be quite satisfactory to construct piled foundations overlying them.
Again it is desirable to terminate the piles in a soil layer to avoid any concentration of
loading at the rockhead.
Where massive rock strata are not present and the overburden consists only of weak and
thinly bedded shales, mudstones and sandstone bands overlain by soil, a collapse of the roof
will eventually work its way up to the ground surface to form a chimney-like cavity known
as a ‘crownhole’ (Figure 9.12b).
Piling should be avoided above these unstable, or potentially unstable, areas, but if the
workings lie at a fairly shallow depth, it is possible to install bored and cast-in-place piles
completely through the overburden, terminating them in a stable stratum below the coal
seam as shown in Figure 9.12b. The pile shaft must be isolated from the soils and rocks of
the overburden in the manner illustrated in Figure 9.11. Any collapse of the strata over pillar
and stall workings usually takes place in a vertical direction with little lateral movement, but
nevertheless a generous space (a minimum of 150 mm) should be allowed between the pile
shaft and the walls of the drill hole. As noted above, it is necessary to seal the annulus
around the piles to prevent gas seepage to the surface. Large-diameter piles are preferable
to small sections because of their higher resistance to lateral loading that may be due to local
distortions of the rock strata. The large-diameter drill holes also serve as a means of access
for inspecting any cavities and to enable geologists to judge the stability conditions of
the overburden.
(a) (b)
Crown hole
Unworked
coal pillars
Cavity extending
only to base of
sandstone
Soil overburden
Weak thinly-bedded
shales and mudstones
Collapsed overburden
Abandoned workings
Figure 9.12 Piling in areas of abandoned ‘pillar and stall’ mine workings (a) Where massive rock forms
stable roof over workings (b) Where roof over workings is weak and unstable.
Pillar
Piles sleeved
over whole of
shaft down to
base of
coal seam
Coal
seam
Rock socket