Open Cut » Environment
The objective of this project was to provide a basis for estimating long term erosion around excavated pit walls in order to assess its effect on final void stability and footprint. The research followed two main lines of investigation:
- Collation of information about existing pit walls to look for key drivers to erosion condition and changes in erosion with walls age; and
- Landform evolution modelling of further erosion for two walls using the computer program SIBERIA, to support extrapolation of the initial findings to long term performance of final voids.
Analyses of the dataset collated for existing pit walls found that wall age, stability, distance to catchment boundary and slope of the top wall batter had significant influence on erosion behind the wall crest; however the regression was either not strong enough or not linear enough to permit reliable predictions. One complicating aspect was a group of older highwalls where the extent of erosion was limited and did not appear to increase over time. This was in contrast to the remaining data which showed a clear trend of erosion extent increasing with wall age and suggests that aspects of either the geology or operational practices in years past may have limited erosion.
The computer modelling found that a highwall crest in moderately erodible weathered Triassic overburden retreated an average of 10m over 200 years, while a wall crest in highly erodible Tertiary overburden retreated on average 30m over the same period. The rate of erosion did not reduce over time.
From the results of these studies it was inferred that, unless mitigative treatments are applied, long term erosion at pit walls could impact adjoining land in a significant proportion of cases.