441
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Settlement and shear strength of uncemented coal mine overburden materials
placed loose under dry and wet conditions
Tassement et résistance au cisaillement de matériaux de couverture non cimentés extraits de
mines de charbon et mis en dépôt en vrac dans des conditions sèches et humides
Williams D.J., Kho A.K.
The University of Queensland, Brisbane, Australia
ABSTRACT: The overburden materials in the coalfields of South East Queensland, Australia, are dominated by essentially
uncemented rocks, which rapidly break down on excavation to extract coal. On excavation, these uncemented materials bulk up to a
very loose density. On end-dumping by haul truck in spoil piles, these loose materials then undergo three forms of settlement: that due
to their self-weight, “collapse” settlement on wetting-up by rainfall, and settlement due to degradation on exposure to weather,
resulting in a substantial increase in density. Due to their lack of cementation, these overburden materials degrade rapidly on exposure
to the weather, leading to significant settlement, followed by some reversal on re-agglomeration and swell. Collapse and weathering-
induced settlements, both being associated with exposure to water, occur simultaneously on wetting-up. Wetting-up also causes a
substantial reduction in the shear strength of the materials. The paper quantifies the settlement and shear strength of uncemented
overburden materials excavated on open pit mining in the coalfields of South East Queensland.
RÉSUMÉ: Les matériaux de couverture dans les bassins miniers du Sud-Est du Queensland, en Australie, sont principalement
constitués par des roches non cimentées, qui se décomposent rapidement lors de l'excavation pour extraire le charbon. Lors de
l’excavation, ces matériaux non cimentés sont mis en dépôt en vrac dans un état très lâche (très faible densité). Après mise en dépôt
par des camions dans des zones de stockage, ces matériaux subissent trois formes de tassement : celui résultant de leur poids propre,
le tassement d’"effondrement" résultant du mouillage par la pluie et le tassement dû à la dégradation lié à l'exposition au climat. Ces
tassements entraînent une augmentation substantielle de la densité. En raison de leur non cimentation, l’exposition au climat entraîne
une dégradation rapide de ces matériaux de couverture, se traduisant par des tassements significatifs, suivis par des processus de
reformation d’agrégats et de gonflement. Les tassements résultant du phénomène d’effondrement et des dégradations dues au climat,
tous deux liés à l’exposition à l’eau, se développent simultanément lors du mouillage. Le mouillage entraîne aussi une réduction
significative de la résistance au cisaillement des matériaux. La communication présente une quantification du tassement et de la
résistance au cisaillement de ces matériaux de couverture excavés dans les mines à ciel ouvert des bassins miniers du sud-est du
Queensland.
KEYWORDS: coal mine, compression, degradation, dry, overburden materials, settlement, shear strength, wet.
MOTS CLES : mine de charbon, compression, dégradation, sec, matériaux de couverture, tassement, résistance au cisaillement,
humide
1 INTRODUCTION
The net bulking of coal mine overburden materials from their
in
situ
state on open cut mining results in an increased volume
required to accommodate the excavated spoil. Knowing the
bulking and subsequent self-weight, collapse and degradation-
induced settlement of the spoil over time is important for
estimating the storage volume required in mined-out pits and in
out-of-pit piles.
The majority of the self-weight settlement occurs during
placement ((Naderian
et al.
, 1996), with only a residual 20%, or
so, occurring after the end of construction. Collapse settlement
on wetting-up of the placed spoil by rainfall and/or groundwater
infiltration requires just sufficient wetting-up to saturate the
micro-cracks that occur at highly-stressed particle contacts. On
collapse of these particle contacts, the coarse-grained particles
break down, creating multiple, less-stressed particle contacts.
Further wetting-up of the spoil then essentially fills the pores
between the broken particles, without leading to significant
further settlement.
Uncemented coal mine overburden materials bulk-up
substantially on excavation, but also settle significantly due to
their self-weight or the height of spoil, “collapse” on wetting-
up, and degradation on exposure to weather cycles. Uncemented
weathered rock spoil sampled from Jeebropilly Coal Mine in the
Ipswich Coalfields of South East Queensland, Australia is
characterised and its shear strength, compression under loading,
and degradation due to exposure to weather cycles are
described. Laboratory testing was carried out on scalped
specimens at the as-sampled gravimetric moisture content (that
is, “dry”) and in a water bath (that is, “wet”), which highlighted
the high potential for this material to slake and disperse, and to
collapse and breakdown, on wetting-up.
2 SAMPLING OF JEEBROPILLY WEATHERED ROCK
The uncemented weathered rock overburden found in the depth
range from about 5 to 20 m in an open pit at Jeebropilly Coal
Mine was sampled from haul truck dumps. It was shaken
through a 19 mm sieve fitted to the top of a 20 litre bucket. The
sample passing 19 mm (-19 mm) and the +19 mm oversize were
both weighed, and the oversize was photographed for later
estimation of its particle size distribution.
3 SPOIL CHARACTERISATION
The characterisation testing of the Jeebropilly weathered rock
was broadly carried out in accordance with AS 1289. The as-
sampled gravimetric moisture content of the -19 mm scalped
Jeebropilly weathered rock was 14.9%, and the Liquid and
Plastic limits of the -0.0425 mm fraction were 71.0% and
21.0%, respectively, giving a Plasticity Index of 50.0% and
indicating a Unified Soil Classification of CH (Clay of High
