Dikgatlhong dam construction reaches critical stage : Construction Review

on January-2012

Dikgatlhong dam construction reaches critical stage

DIKGATLHONG DAM CONSTRUCTION REACHES CRITICAL STAGE

The contract for the construction of Botswana’s biggest dam enters its 39th month on programme, for scheduled completion in February 2012.

The current dry season’s activities are focused on the critical closure of a diversion channel purposefully left in the earth embankment to allow the previous season’s river flows to pass through whilst the dam was extended across the Shashe River.  The 200m wide ogee crested spillway is also in an advanced stage of construction and final closure of the diversion and spillway section by October this year will see impoundment of the next wet season’s rains.

The earth embankment dam is located in North-eastern Botswana about 3km below the confluence of the Tati and Shashe Rivers.  At full supply level storage volume will be close to 400 million cubic metres with a backwater reach of about 20 kilometres up river.  This will be Botswana’s largest water supply dam.

Transfer Pumping Station and Pipeline

Work is also due to commence shortly to construct a large raw water pumping station and a 73 km long 1200 mm diameter welded steel pipeline, which will transfer water from the dam to connect with the existing national water carrier pipeline.  From there, the raw water will be conveyed southwards to the capital of Gaborone, as well as serving settlements, new coalfields, and a proposed power station en route, via a second, still to be constructed, North-South carrier pipeline.

Project Team and South African Content

The project implementation by the Botswana Department of Water Affairs, at a cost of Rand 1.4 billion, is being funded by the Botswana Government.  Construction is by Sinohydro Corporation from China and the technical design and contract administration and site supervision aspects are being undertaken by a joint venture partnership between Bergstan Africa, a Botswana based company and Jeffares & Green consulting engineers from South Africa.

South African suppliers have provided steel plate for the outlet conduit, reinforcing steel, as well as the main outlet cone/sleeve and butterfly ranging from 300mm to 2.6 metres diameter.  The cement was supplied by PPC Cement in Botswana.  Sub-contractor Geomechanics also undertook some core drilling and lugeon testing in the early stages of the project before Sinohydro mobilized their own drilling teams.

Challenges Overcome

The 4.5km long by 41m high earthfill embankment required nearly 4 million cubic metres of materials for its construction, including crushed rock aggregates, gravel fill, sand and gravel filters and clay core.  These have all been sourced on site following extensive geotechnical investigations and reconnaissance.  The concrete for the construction of the spillway and intake tower is also manufactured on the site using the quarry sources identified above.  The foundation geology was highly variable and required a multi-stage grout curtain totaling 58 kilometres in length and requiring the pressure grouting of 6 700 tonnes of cement to seal it against leakage.

The construction of a project of this magnitude in a remote part of Botswana presented a number of challenges to the construction and supervisory teams, including the language barrier and sometimes differing perceptions of construction practices, but these have been professionally dealt with and overcome.

In addition, the works also suffered extensive damage caused by an unseasonal flood during the 2009 dry season.  Another challenge was the poor and variable foundation geology and a paucity of good quality clay core materials.  However, ongoing perseverance and dedication by all appears to be drawing the curtain on a project that is headed towards a successive conclusion, on time and on budget.

Client

Botswana Department of Water Affairs

Main Contractor

Sinohydro Corporation (China)

Technical Design (Joint Venture)

Contract Administration

Site Supervision

Bergstan Africa (Botswana)

Jeffares & Green (South Africa)

 

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