Slab-buckle failures in
coal sequences involve thin slab deformation in strata dipping parallel to the
footwall, and are a consequence of both unloading and the weak nature of the
thinly bedded rock mass. Methods of modelling and predicting such occurrences
include:

a conceptual model using a base friction table,

a mathematical model using the Euler solution for column
and beam bending (after Goodman, 1980), and

computer analysis techniques such as UDEC (Universal
Distinct Element Code after Cundall and Hart, 1993).

Slab-buckle failures
have the potential to seriously disrupt production because of their volume and
rapidity, and to endanger both equipment and personnel working on the pit
floor.

In
July 2004 a large slab-buckle failure occurred overnight after a new batter and
bench had been formed at the Malvern Hills Opencast Mine in inland
Canterbury.
This involved the entire length of the pit (85m along strike) when a 2m thick
intact slab with a total volume of ~3500m3 translated down dip 6.2m on the base
of a thin coal seam to form a pronounced buckle at the toe. The coal measures
consist of finely laminated mudrocks with multiple coal seams of varying
thickness dipping ~45 to the southeast, and the opencast mine had been designed
with footwall batters formed parallel to bedding with a vertical bench
separation of 15m.

Back
analysis of the Malvern Hills failure was
necessary to investigate the controls on footwall instability, and for future
mine design. An engineering geological model was created and samples of the slab
material and failure surface were collected and tested to establish the required
parameters for use in the Euler solution for column and beam bending. Back
analysis using the Euler solution provided unrealistic results that
overestimated the overall length of a stable slope by more than 10 times.

No
further large scale slab-buckle failures have developed at the mine site, in
part because of the slow rate of coal extraction, but precautionary drainage of
the footwall slopes has been undertaken to improve overall batter stability. The
location of the slab-buckle failure on a critically positioned pre-sheared thin
coal seam with full hydrostatic head is considered the most probable cause of
the July 2004 failure, rather than inherent instability of the generic bench and
batter arrangement adopted. The use of a precedent-based engineering geology
approach to future mine design is considered the most appropriate solution in
the circumstances.