Abstract:
Anthropomorphic fill failures are common on slopes within mining areas in New
Zealand and globally. They are shown to be catastrophic and occur with minimal warning,
often resulting in significant damage to property and infrastructure. This study focuses on two
anthropogenic fill failures at the Southern Wall at the K1 pit at the Maramarua Coal Mine.
Referred to as the 2019 February initial failure and 2019 December reactivation. A preliminary
investigation into the initial failure revealed a formation of a sizeable headscarp measuring
~250m wide and ~6m high at the top of the Southern Wall, with visible signs of surface
instability present towards the bottom. Eight months later, the reactivation showed a sign of a
significant flow slide occurring, resulting in an increase in surface deformations around the
Southern Wall. This study into the Southern Wall failures utilises a combination of
investigative techniques to characterise the mineralogical and geological properties of the fill
material, construct geomorphological and geological models of the failures and identify
significant mechanisms of both failures. A review of existing data compared the results and
failure mechanisms with anthropomorphic fill failures reported in New Zealand and globally.
Laboratory testing revealed the fill material to comprise a complex heterogeneity,
indicating a matrix of moderate to active clays and silt. The fill comprises moderate moisture
contents and plastics limits and high liquid limits, and linear shrinkage. SPT borehole data and
LEM indicated the possibility of a low permeability layer within the Wall, resulting in high
porewater pressures and subsequent groundwater levels. LEM modelling established low FoS
of both failures (FoS <1.3), indicating the presence of both shallow and deep-seated instability
at the Southern Wall. The possibility of surface-sliding liquefaction along a weak
basement/shear zone between the fill and basement material was suggested as a critical trigger
for the initial failure. The removal of a wedge of WCM/coal at the toe acting as a stability berm
resulted in a variation of ‘toe buckling’ at the base acting as the primary trigger of the
reactivation. The potential of rising groundwater levels and seismic activity are also attributed
as potential future failure triggers with methods of monitoring and slope stabilisation
suggested.