THE PROJECT
A flow of approximately 50 L/s was observed, when the bellmouths were closed, indicating a leak had developed within the HLDC. To protect mining operations in Pits C and D of the Curragh Coal Mine, Blackwater Creek low flows and floodwaters needed to be diverted and a protection levee constructed.
PSM was engaged to propose solutions that would enable permanent sealing (grouting). These could not be achieved without halting or reducing water flow from Blackwater Creek.
OPPORTUNITIES
A catchment area greater than 700 km² and a seasonal (cyclonic) rainfall coupled with limited hydrological records were key challenges that required careful evaluation in line with the client’s risk profile.
Engineering and technical challenges included:
- Levee construction materials limited to predominantly clay and sandy soils
- Potential for sandy soils in alluvial channels underlying the levee increasing the risk of potential piping through the foundation
- Predominantly stiff to hard clays of the dragline road subgrade raising the prospect of poor subgrade conditions and the potential of failure in wet conditions
- Dominant role of vegetative roughness on the Backwater Creek floodplain and the very small main (low flow) channel precluded using HEC RAS Backwater program, a de facto world standard.
APPROACH AND INNOVATION
PSM’s tailored approach included:
- Assessment of geological and geotechnical conditions along the alignment with limited site investigation coupled with resource drilling data
- Revision of previous 1985 hydrological studies with additional data eliminating the need for a new study
- Completion of hydraulic designs using PSM’s in-house model using a variable vegetative roughness resistance algorithm and automatic partitioning of flow at a cross section. HEC RAS Backwater requires the user’s manual input
- Material sampling and assessment of potential construction materials and levee/road foundations to assess in-situ conditions
- Provision of geotechnical design parameters for the levee and pavement
- Mechanistic analysis (using FLEA/CIRCLY) of pavement and subgrade to assess pavement thickness. Development of dragline road and LV pavement design
OUTCOMES
Low flows and floodwaters were successfully diverted with dragline access on a road along the pit-side of the levee together with LV access along the bund crest.
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