A Rock Engineering case study - Boschkop Abstraction Facility,
Vaaldam, South Africa
Alastair T M Morgan Pr.Eng.
Golder Associates Africa, Cape Town, Western Cape Province, South Africa
The VRESAP project is an inter-basin water transfer scheme situated on the South African Highveld, designed to
provide raw water, primarily for the rapidly developing power generation region and oil-from-coal petrochemical
facilities, which are corner-posts of the South African economy.
The project involved construction of a new water intake within the Vaaldam reservoir – the primary source of
water storage for the Gauteng economic hub of South Africa – pumping and water treatment facilities, and a
roughly 125km long, 2m diameter pipeline between Vaaldam and storage near Secunda.
A key element is the low-lift pump chamber constructed within a vertical rock shaft excavated from a steep
hillside fringing the reservoir, to a depth of roughly 20m below reservoir level, (temporarily protected behind a
contiguous-piled cofferdam).
Ongoing geotechnical engineering studies, interpretation, analysis and design accompanied the entire shaft
excavation programme, after initial (above water) excavations uncovered a challenging and unstable (highly nonuniform) variation in rock mass structure; once more providing ample vindication of the need for- and validity of
the “design-as-you-construct” concept characteristic of much geotechnical construction.
Accompanying studies included supplementary drilling, in-situ testing, rock mechanics data gathering, joint
surveys, rock mass classifications, stability analyses and designs, construction monitoring including piling and
support works, interpretations and recommendations to control blasting damage.
Key issues arising from the investigations included:
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Significant structural variations in rock mass at depth;
Notable variance in Rock Quality Designation (RQD) from vertically and inclined boreholes;
Significant variation in discontinuities within a confined (75m diameter footprint) rock mass, and
A need for necessarily conservative support design to accompany shaft-sinking, to account for a rock
mass in which extremes of weathering were experienced to full excavation depth.
The paper describes unique aspects of the geotechnical studies, findings and outcomes, not least of which the
need to provide for safe construction in challenging environmental and construction conditions, and a pro-active
response to highly-varied geotechnical and geological circumstances.
It is, in the author’s opinion, further demonstration of the essential need for a practical engineering approach
based on thorough and ongoing observation and interpretation of rock quality and weathering patterns, rock
mass exposures, straight-forward joint surveys and analysis techniques, to accompany and appropriately inform
the ongoing construction process, and modify designs based on pre-tender geotechnical investigations.