Construction of the 30 m arch-buttress Mndwaka dam, in the Eastern Cape, started in April and is scheduled for completion by the end of 2013, reports consulting engineering firm ARQ Consulting Engineers dams and hydro director Dr Quentin Shaw.
The R70-million project makes up Phase 1C of the R350.7-million Mncwasa Water Supply Scheme and is aimed at providing water supply storage for the Amathole district municipality.
The scheme, which was initiated in 2010, is being built in phases up to 2012/13 and will provide water to more than 6 000 households.
The Mndwaka dam makes use of rubble masonry concrete (RMC) technology, which blends ancient manual construction methods with state-of-the-art design techniques, Shaw says.
“RMC dam construction has been demon- strated to be an efficient and highly cost-effective method for the labour-based construction of small and medium-sized dams,” he notes.
Structures created through this method are durable and substantially less sensitive to exceptional flooding conditions than a common fill embankment. This durability reduces the requirement for river diversion and allows for a dam to be built safely in phases over an indefinite period, or to be built for future raising, he adds.
Further, RMC dam construction usually creates about five times more employment opportunities than are created during the construction of an equivalent embankment dam and the masonry skills developed through the application of this technology have been found to be of value in many other types of construction, Shaw points out.
Meanwhile, construction on the 24 m earthfill embankment Mukurumudzi dam, which will supply water to Australia-based mineral sands developer Base Resources’ Kwale project, in southern Kenya, started in January.
The Kwale project entails the extraction of titanium and zircon minerals from the dunes in the area south of Mombasa.
The dam is configured as a homogeneous embankment of about 210 000 m3 of silty sand originating from the local area. With little suitable clay materials available, the impermeable membrane of the dam will comprise a central bentonite-cement-soil cutoff constructed progressively with the embankment and will include graded blanket and chimney drains for seepage control.
Flood handling will be managed through an uncontrolled ogee spillway over a saddle on the left bank.
To date, the construction process has included excavation across the full dam footprint, partial excavation of the core trench, partial construction of the river diversion culvert and the establishment of the contractor’s facilities and plant.
The project is expected to be completed in April 2013.
ARQ’s RMC technology, combined with its spillway capacity enhancement technology, was also recently used on the Department of Water Affairs’ (DWA’s) Bospoort and Molepo dams to increase the dams’ spillway capacity fourfold, adds Shaw.
ARQ, in a joint venture with consulting engineering and project management firm Goba, is also working on the DWA’s Chuniespoort Dam Safety Rehabilitation Project, in Limpopo, which is currently nearing completion.
Chuniespoort is a 16-m-high homogeneous earthfill embankment dam.
The rehabilitation project included increasing the spillway capacity, raising the earthfill embankment, installing embankment drainage, up- and downstream erosion protection and upgrading the outlet works.
The granitic fill used for the construction of the dam is dispersive and careful treatment, as well as design and construction specifications, were needed to ensure a durable, low-maintenance solution.
“ARQ’s involvement included the conceptual design, the full detailed design, preparation of tender documents, as well as supervision assistance,” says Shaw.
Changuinola 1 Dam
Last year, ARQ also completed the design for the Changuinola 1 dam, in Panama, which is the impoundment for a $430-million engineering, procurement and construction (EPC) contract.
“The 230 MW hydropower scheme comprises the dam, a 4 km headrace tunnel and a surface power station,” says Shaw.
At a completed cost of about $150-million, the dam consists of a 105-m-high roller-compacted concrete (RCC) arch/gravity structure, with a crest length of 590 m and a concrete volume of 895 000 m3.
“ARQ’s involvement in the design phase of the project included the feasibility design, the full detailed dam design, the spillway design, the instrumentation design, the river diversion design, as well as the foundation rock mass analysis and the design for the EPC contractor,” he says.
The dam was initially configured as a gravity structure, but as more rock mass information became available, the company was able to prove the technical feasibility of an arch/gravity structure to the satisfaction of the project owner.
On the basis of the associated time and cost savings, the design was accordingly revised.
“The Changuinola 1 dam is the first RCC dam built outside China and South Africa,” Shaw points out.
Future of Water Engineering
Meanwhile, Shaw states that the most significant challenge facing the South African water engineering sector is retaining high-quality engineers.
Compared with the rest of the world, the country’s water engineering industry’s standards were high, until a recent reduction in large projects and an increase in competition in a shrinking international market led to the best minds no longer entering the field of civil engineering, he says.
“Retaining qualified and experienced engin- eers can only be done by accessing bigger markets, which is why ARQ is working on a number of dams and hydropower projects abroad,” he adds.
The biggest potential for South African consulting engineers to grow is to expand further north into Africa, says Shaw.
“We are ideally placed, both geographically and in terms of our understanding of the correct engineering approach to provide services for the development of water resources and hydropower projects in Africa,” he says.