Andy Pepperell head of Jeffares & Green water division recently visited Botswana to assess the progress being made on the construction of the new Dikgathlong Dam. With an independent, on-site view, we take a look at this interesting project.
Construction of the Dikgatlhong Dam, the largest dam in north-east Botswana, commenced in March 2008 and is currently progressing well. The dam is on the lower Shashe River just downstream of the Shashe/Tati confluence approximately 55 km north-east of Selebi Phikwe, and 5 km upstream of the border between Botswana and Zimbabwe. It is about 60 km downstream of the existing Shashe Dam, which is situated closer to Francistown on the Shashe River.
The dam is being constructed for Botswana’s Department of Water Affairs and is due to be completed at the end of 2011 when it will be capable of impounding some 400 million cubic metres of water. This will easily surpass the capacity of the next largest dam in Botswana, the Gaborone Dam, which can impound a maximum of 141 million cubic metres at full capacity. The project was precipitated by rapid water demand growth in the 1980’s which prompted the Botswana Government to undertake studies to meet the projected growth.
This contract, No 1, the construction of the Dikgatlhong Dam and associated works and access road, is being constructed by Sinohydro Corporation of China, under the direction of supervising engineers Bergstan, Gauff, Jeffares and Green Joint Venture, (BGJGJV). Messrs EHES are the environmental consultants on the project and Messrs G4 Consulting Engineers are associated with BGJGJV on electrical matters.
Contract No 1 is one of four contracts making up the project. The housing contract comprising the construction of some 15 units was recently completed by a separate contractor.
A third contract comprises the construction of the transfer system from the dam to the existing north-south carriers. This will include the construction of a 73 km-long, 1.4 m-diameter steel or ductile iron transfer pipeline, a major pump station at the dam and a feeder tank between the dam and existing break pressure tank.
A smaller fourth contract entails the provision of a permanent power supply to the project. It is anticipated that permanent power will be available to the site by the end of 2009.
The Dikgatlhong Dam Project is a massive project by any standards with an overall estimated value of some P2 billion. The new dam and associated infrastructure will ultimately provide an additional 3 000 ℓ per second of raw water delivery to the existing national north-south carrier pipeline that transports water from the Letsibogo dam near Selebi Phikwe to Botswana’s capital, Gaborone.
The Dikgatlhong dam wall is to be random fill earth embankment with a clay core, founded on bedrock. The bedrock is overlain by approximately 4 m of sandy material, which will be removed for the construction of the clay core and reused in the embankment construction. Owing to the fissured and fractured nature of the upper bedrock layers, extensive curtain grouting is being undertaken below the clay core to prevent seepage from the dam.
The dam wall will be approximately 4.5 km long, and the maximum height is 41 m above river bed level. The main spillway will be a 200 m-long mass concrete structure, situated on the left flank of the dam wall, with a wide concrete channel with energy breakers to lead the overflow back to the river bed. An auxiliary spillway approximately 1 km in length will be excavated in a rocky ridge on the left bank of the impounded area. Total earthworks inclusive of the clay core will be of the order of 5 million cubic metres. Four borrow pits have been identified and tested in order to be able to source sufficient quality and quantity of clay for the core of the dam.
The earth embankment will be protected on the upstream side against wave action by a 1 500 mm-thick layer of broken rock rip-rap. Rock is to come from a quarry site on the northern bank of the river that was one of two identified, drilled and tested during the project investigations. Coarse aggregate for concrete production will also be sourced from this quarry. On the downstream side the dam wall will be protected by a 500 mm-thick layer of less coarse rockfill.
Other infrastructure forming part of the project include a 7 m-diameter, 45 m-high circular concrete outlet tower, with multiple level openings, feeding a 3 m-diameter steel outlet conduit With concrete surround, passing under the embankment before bifurcating to a pump station and a river outlet. The outlet tower will be connected to the dam wall by means of a structural steel walkway/bridge with a clear span of 61 m.
The project also includes the upgrading of the existing 35 km-long low-grade gravel road between Selebi Phikwe and the village of Robelela – the closest settlement to the dam site. A number of particular challenges are faced on a project of this nature. The construction of a major structure across a large river that is prone to flooding every year obviously poses a major problem that has to be planned for and overcome. In this case, provision for the passing of annual floods during the construction period will have to be made over three rainfall seasons before the embankment is brought to final height.
The success or otherwise of a structure constructed principally of natural materials sourced from within the dam area will be dependent largely upon the quality of those materials and the effective and controlled treatment of them to ensure optimum performance and adherence to specification. The quality of the clay core as well as the sand making up the filters in the dam are particularly important in ensuring an effective water retaining structure. The quality of much of the clay available for construction of the core is considered marginal, therefore necessitating strict selection, treatment methods and quality management. In addition, naturally occurring sands in the river bed required for the embankment filters are generally on the course side, therefore necessitating the same very strict selection and acceptance criteria.
Materials and the processing plant required for the construction of the embankment and associated works are generally located on the left bank of the river, while critical works such as the intake tower and outlet conduit are on the right bank. The programming and executing of certain works during times of the river flooding are major issues that have to be taken into account.
Working with contractors from abroad who have limited knowledge of the English language poses particular problems. Communication between engineer and contractor becomes problematic in that one often cannot be sure that the other side has fully understood what has been said. The use on non-technically qualified interpreters can also be difficult because of the fact that the interpreter might not fully understand and interpret technical issues. International contractors who have not worked in the country before often take time to become familiar with the country’s laws, rules and regulations, sometimes to the detriment of the project.
The construction of the Dikgatlhong Dam is in its early stages and will take time to complete. Progress during the first seven months of a 47-month contract period has been acceptable but the success of the project will be dependent upon the continued diligent performance of all parties concerned as well as the respective cooperation of those parties. We look forward to 31 October 2011 when substantial completion of the dam is expected to be achieved.