Pile Design and Construction Practice, Fifth edition

The magnitude of the strain can range from as much as plus or minus 0.8% of the overburden
thickness above shallow workings to 0.2% over deep seams.
The horizontal ground movements make it virtually impossible to use piled foundations
in areas where longwall mining is proposed, or is currently being practised. The horizontal
shearing forces accompanying the strains are so high that it is quite uneconomical to attempt
to resist them by heavily strengthening the pile shafts. However, in areas where subsidence
following longwall mining has virtually ceased it is possible to use piled foundations if it is
recognized that some residual movement will take place as the collapsed strata slowly reach
final equilibrium. In these cases it is desirable to terminate piles in a soil layer overlying
rockhead, as shown on the left-hand side of Figure 9.10. The soil acts as a cushion, preventing
any concentration of load on the broken rock strata. Long-term movements may be substantial
near the boundary of the worked-out seam. If the workings are shallow, piles may be taken
down through the collapsed overburden to intact rock layers below the coal seam as shown
on the right-hand side of Figure 9.10. Bored and cast-in-place piles are used for this pur-
pose, but it is essential to isolate the shaft of the pile from the overburden above the coal
seam in order to avoid heavy compressive loading caused by downdrag from the collapsing
strata. This isolation is achieved by placing the concrete within a shell formed from stiffened
light-gauge steel sheeting, the sheeting terminating at the base of the coal seam. Below this
level the concrete can be cast against the surface of the stable strata to form a ‘rock socket’,
as shown in Figure 9.11. The space between the shell and the wall of the drill hole through
the overburden can be filled with bentonite slurry, soft bitumen or loosely placed rock
fragments. A minimum clearance of 150 mm should be provided to accommodate minor

446 Miscellaneous piling problems

Figure 9.10Piling through collapsed ground over longwall mine workings.

Piling

Subsidence complete

Long-continuing

small subsidence

No

subsidence

Very soft clay

Stiff boulder clay

Unworked

seam

Stowage

Collapsed

over burden