The paper discusses an essentially iterative design and construction process for a rock shaft excavation undertaken to provide a pump chamber sited on the fringes of one of South Africa's primary water storage reservoirs, Vaal Dam, to service a major inter-basin water transfer project. Preliminary design for the facility was based on a limited geotechnical drilling investigation which broadly covered the intended footprint of the intended abstraction works. The project then proceeded rapidly to construction, the outcome of which indicated a significantly different response to rock excavation from that envisaged by the designers. Detailed design for rock support necessitated several phases of additional drilling and geotechnical evaluation in parallel with construction, each of which was designed to inform the ongoing excavation and rock support programme. The experience provides further vindication of the well-established view of geotechnical practitioners, that as a consequence of the variability inherent in geological formations, some more than others, the initial investigation provides only a limited appreciation of circumstances that will be encountered and thus a preliminary guide for design, details of which need to be developed, refined, and updated throughout the construction process.

INTRODUCTION

The Vaal River Eastern sub-System Augmentation Project (VRESAP) is a South African highveld interbasin water transfer scheme, originating in the Vaal River catchment and feeding onto the watershed and headwaters of the Olifants River basin. Planning for the project was initiated around 2004/2005 by the Trans-Caledon Tunnel Authority (TCTA) on behalf of the Department of Water Affairs (DWA).

VRESAP was designed by the Project Engineer, Vaal Pipeline Consultants (VPC), as an emergency water supply to supplement the water resources of the Mpumalanga coalfields, which feed numerous surrounding Eskom power stations and the Sasol petrochemical plants around Secunda. The project comprises an intake works and low-lift pump station facility located at the Vaal Dam reservoir, desilting works, a high-lift pump station – known as the Boschkop Abstraction Works, and roughly 120 km of 1.9 m diameter pipeline terminating at the Knoppiesfontein diversion weir near Secunda.

Rock excavation and support work for the intake/low-lift pump station are the subject of this paper.

This content is only available via PDF.
You can access this article if you purchase or spend a download.