Open Cut » Environment
This report presents the outcomes of an extension of
ACARP Project C9030 to investigate Capping of Coal Coarse Rejects.
The aim of the research was to provide guidance on design of capping for
coarse rejects based on understanding the effect of capping material and
thickness on the fate of moisture and dissolved salts.
The research was a preliminary study to examine the extent to which capping could affect
the movement of solute in coarse rejects heaps. No specific laboratory testing or fieldwork
was undertaken and instead available physical and chemical data were used to characterise
materials.
Comparison of the collated coarse rejects and spoil data sets suggested the following conclusions:
- Rejects had the physical composition of sandy gravel; spoil was almost any composition
except sand or gravelly sand;
- Rejects showed mainly consistent, low, water holding characteristics; spoil water retentivity
covered the range of behaviours and was related to silt-and-clay fraction;
- Both rejects and spoil showed median pH slightly above 8, however a proportion of rejects was acidic.
For rejects, pH showed broad correlation with EC, but this was not apparent for spoil;
- Both materials had median electrical conductivity less than 500?S/cm, but about 20% of spoil samples
were moderately or highly saline. For rejects, sulphate correlated to salinity but chloride
did not; for spoil both anions were involved.
Data collated for rejects were limited in the number of results and mines. The consensus of industry personnel
consulted was that rejects are generally more saline than indicated.
Finite element modelling was undertaken to simulate solute movement through rejects and capping on a heap
plateau over 25 years. Various configurations of capping material and thickness, and climatic
conditions representing the Bowen Basin and the Hunter Valley, were modelled. /p>
Within the context and limitations of a preliminary study, the modelling suggested that:
- Uncapped coarse rejects in the Bowen Basin would experience long-term average drying conditions. However, individual large rainfall events could produce infiltration that, in the absence of vegetation, would eventually exit the base of the heap.
- In the Hunter Valley, uncapped rejects would experience average wetting conditions. Solute could start seeping from the base sooner and at higher volume than in the Bowen Basin.
- Any thickness of capping with properties similar to those modelled would result in lower long-term salt concentration near the landform surface. For a given material, salt concentration near the surface appeared relatively insensitive to capping depth.
- Capping materials with lower moisture retentivity produced lower salt concentration near the surface. However, as those materials had higher permeability, infiltration during major rainfall events would be greater. A thin sub-layer of finer spoil overlain by coarser spoil might combine the advantages of both materials.
In summary, a thicker cap of coarser spoil would have significantly lower salt concentration in the vegetation zone. Despite this, for all configurations modelled, the long-term salinity was such that salt tolerant vegetation species would be advisable.
The following tentative guidelines for capping are suggested to manage long-term solute movement in coarse rejects heaps are suggested:
- Select capping material with the lowest moisture retentivity consistent with maintenance of vegetation;
- Consider 1m nominal capping depth, however thinner capping could be validated in field trials;
- If the consequences of seepage in the very long term are judged unacceptable, the lower 0.2-0.5m of cap should be material with lower permeability and higher water retentivity;
- Conditions in the Hunter Valley suggest that a finer capping or capping sub-layer may be appropriate more often;
- In all situations, vegetation is essential to minimising infiltration into the body of the heap. Salt tolerant species should be considered.
There is a need to develop defensible procedures for the rehabilitation of existing heaps and for design and construction of future coarse rejects emplacements. To this end, it is recommended that the findings of this project be incorporated into a comprehensive risk assessment for rejects heaps during operation and after closure.