As part of the Khwezela LifeEx Project, Anglo American Coal South Africa (AACSA) relocated a dragline, colloquially known as the Pit Bull, at its Kromdraai open cast mine to its Navigation Pit reserves near eMalahleni to enable access to previously mined underground resources.
The Navigation Pit project is strategically important to AACSA as one of the main projects in this overall programme to extend the life of the Khwezela complex; it will bridge the production gap between the Kromdraai operation that ceased in 2019 and the future resource potential, namely the Clydesdale Pit and the Landau 1 and 2 operations identified in the Khwezela resource development plan, which forecasts production up to 2035.
Zutari conducted the pre-feasibility and later the feasibility study for the dragline walkway associated with the relocation. The alignment of the temporary walkway was about 18.5 km long, situated 20 km west of eMalahleni. The walkway traversed green- and brownfield areas, characterised by arable and cultivated land, open veld, existing coal mines, and areas previously subjected to coal mining, as well as manmade and natural wetlands.
Infrastructure traversing along the alignment included National Route 4 (N4), surfaced and unsurfaced provincial roads, a Transnet Freight Rail (TFR) line, and numerous overhead Eskom distribution and transmission electricity lines. Various gas, bulk water, and other buried utilities also traversed along the walk. The project entailed the construction of a temporary dragline walkway and electrical supply infrastructure to energise the dragline. Upon completion of the dragline relocation, the walkway had to be removed, and areas affected by the construction activities rehabilitated to their original state. Zutari’s study included all civil and electrical designs associated with the dragline walk. The geotechnical study undertaken to support the study included understanding and mapping ground risks, geotechnical investigations, modelling, and the design of the dragline walkway.
The objective of the geotechnical investigation was to characterise the subgrade conditions below the structural layer works of the dragline walkway to define appropriate treatment to support the movement of the dragline.
Geotechnical investigations were undertaken in various phases. Test pitting investigations were undertaken during the pre-feasibility study to gain an overall appreciation of the geotechnical conditions along the walkway alignment and to identify ground risks that could impact the walkway alignment. The presence of soft/compressible soils, as well as the presence of a deeply weathered diabase intrusion, necessitated further investigation to gain a better appreciation of the geotechnical conditions at depth. The investigation was undertaken by means of rotary core drilling in conjunction with thin-walled (Shelby tube) sampling and in situ standard penetration testing (SPT). Laboratory testing of representative samples was undertaken during the various phases of fieldwork to establish the geotechnical characteristics of soil and rock horizons encountered along the walkway alignment.
Infrastructure traversing along the alignment included National Route 4 (N4), surfaced and unsurfaced provincial roads, a Transnet Freight Rail (TFR) line, and numerous overhead Eskom distribution and transmission electricity lines.
Innovative modelling and simulation
The ability to modify existing designs timeously to account for the current site conditions, which differed from those assumed for the original design, added significant value to the project.
The Plaxis modelling used during this project considered transient loading representative of the temporary nature of forces exerted by the dragline. For this purpose the duration of each step of the dragline was considered. Similarly the 3D modelling enabled Zutari to simulate the animated movement of the dragline, culminating in a model representative of the complex movement of the dragline.
Owing to the temporary nature of the loads and the higher settlement tolerances, there was more freedom in executing the design. This allowed the design team to deviate from conventional approaches typically adopted for similar analyses.
The sophisticated soil-structure interaction models developed during the project provided a robust tool for evaluating the variable soil conditions encountered along the walkway. These models allowed the designers to simulate the complex movement sequence of the dragline, which resulted in a model representative of the interaction between the in situ soils, the ground-improvement interventions necessary to support the dragline, and the dragline load itself.
The ability to modify existing designs timeously to account for the current site conditions, which differed from those assumed for the original design, added significant value to the project. By adopting a risk-based design approach that assigns probabilities of failures to various construction optimisation options, the project team could make informed decisions about the acceleration of the construction schedule.
*The Aurecon Africa business has been officially renamed Zutari as at 21 July 2020. Zutari acquired Aurecon Middle East on 20 November 2020.