The Malvern Hills coal project is now in an advanced stage of development
with the stripping of overburden from above former underground workings in which
a 3 m seam of subbituminous C coal with very low ash and sulphur was previously
mined. The main seam dips at 45 to the south-east, along with other adjacent
potentially recoverable thinner seams, and the strike extent of past coal mining
in the Malvern Hills approaches 10 km with most extraction by underground
methods. Overburden and interburden is predominantly weak mudstone, with only
very minor sandstone, whilst a 1 - 2 m thick veneer of Last Glaciation loess
covers the area. The coal product from the present mine site will supply the
local industrial market, with production planned to peak at about 20 000 tpa.
Settling pond design has been based on several different approaches, including
an estimate of likely inflows from the contributing catchment of about 5 ha
under predicted storms having a ten year recurrence interval. A pond of about
1000 m2 in area has been constructed with
its invert in mudstone bedrock, and soil slopes have been lined with
lime-stabilised clayey loess silt incorporating a gravel armour layer. The pond
outflow takes place via a 2 m high 1.8 m diameter concrete dewatering tower,
with a skimmer arrangement to lower the pond by approximately 500 mm between
storm events. The overflow water discharges to the main stream above a natural
wetland, which will act as a secondary polishing pond, and this is to be
enhanced in conjunction with the neighbouring forestry company.
Discharge consents specify turbidity and pH as the controlling criteria, and
both will be monitored at the settling pond discharge point as well as at the
entry to the main stream. Investigations to date indicate that the pH of water
exiting from previous underground workings is in the range 6.8 to 7.2, and that
acidic drainage will not be an issue with this mine site. No dispersive clays
have been identified in assessing the overburden and batter materials, although
erosion during storm events is expected and all drains will ultimately be routed
through the main settling pond to ensure adequate control of runoff-generated
sediment. Reestablishment of vegetation on cut and fill batters remains a
priority, and ongoing monitoring will ensure satisfactory long-term performance
of the pond system.