With a DMP limit to the tonnage the client could remove and a critical target depth they wanted to excavate to, we designed a safe trench and provided earthworks supervision to this diamond operation in the Kimberley.
The project was located in the Ellendale Diamond Field, in the Kimberley Region of Western Australia. The proposed works involved the staged excavation of an exploration trench (unsupported) to approximately 20m below the existing ground surface using conventional earthworks equipment, to a maximum of 50,000 tonnes.
- Design of the excavation was based on the likely engineering parameters and the behaviours of each of the in-situ soil units likely to be intersected.
- Geotechnical slope stability analyses was undertaken on an inferred design slope profile using the anticipated parameters and Slope W ® software. The Morgenstern Price method of analysis was used. A minimum Factor of Safety of 1.25 was adopted for the design of temporary excavations for this project.
- The trench was excavated progressively in horizontal layers with on-site supervision to allow confirmation of materials types and geotechnical parameters during the excavation phase, enabling design changes to address material variability and the mitigation of risk.
- Excavated material was stockpiled for use as trench backfill on completion.
GGC provided specialist geotechnical engineering and engineering geology design services to the project and earthworks supervision to support and adjust the design during the construction phase.
Key Challenges and Solutions
The available ground information was very limited and consisted of exploration drilling records and surface mapping. The client required the excavation to be progressed to a depth of approximately 20m to intersect the anticipated diamond bearing gravel horizon. The excavation was to produce enough material for screening and later analysis for the feasibility of the project. Due to the project license being for exploration only, the Department of Mines and Petroleum had placed specific conditions on the project.
Using the data provided by the initial slope stability analysis a preliminary pit design was adopted which enabled the safe excavation to the required depth including benching and a shallow slope for the machinery access and egress.
The excavation was monitored by an engineering geologist from GGC to provide real time advice on the material characteristics and on site design changes to maximise the cut slope angles and minimise the risk of slope failure.