Rainfall defies Botswana’s new dam12 January 1999
Alan George explains how a lack of rain and inadequate pre-tendering surveys have caused problems at the Letsibogo dam in Botswana
Eighteen months after impoundment started, Botswana’s Letsibogo dam, a key element in the North-South Carrier Project, had a reservoir only 8m deep, compared with the design capacity of 28m. The shortfall was the result of two rainy seasons in which precipitation was well below average and which could delay the commissioning of the carrier scheme as a whole.
Botswana suffers a highly variable rainfall and the North-South Carrier Project is designed to assure water supplies for the capital Gaberone and a series of other towns and villages. The dam, sited on the Shashe river (a tributary of the Limpopo which flows for only about three months per year) is an embankment structure 32m high with a crest length of 1.27km. The dam has a storage capacity of 25M m3.
Underlining the extreme variability of the rainfall, the dam has a 300m wide spillway designed to cope with a once-per-century river flow of 1500m3/sec. This compares with an average anticipated annual flow of 55M m3 per year.
The main contractor for the Letsibogo dam was a joint venture between Belgium’s BESIX and Brazil’s Companhia Norberto Odebrecht (CNO). BESIX is Belgium’s biggest construction company and it has long and varied experience in Africa. CNO, which specialises in dam construction, is Brazil’s biggest international construction firm. The consultant was Australia’s Snowy Mountains Engineering Company International (SMEC).
The failure of the rains is especially poignant as the CNO/BESIX venture had won the contract on the basis of a proposal which offered a significantly earlier competition date than its competitors.
‘We were awarded the contract even though we were not the lowest bidder,’ explained Jean-Claude Palgen, BESIX’s International Operations Manager. ‘On price we were in second place to Spie Batignolle’s of France but we offered an alternative programme to allow impoundment to start a year earlier than specified in the tender documents.’ He went on to point out that the company met its target. If water cannot flow into the pipeline it is because of the weather and not because of any construction delays.
This is not to suggest that the job was problem-free. Letsibogo has a clay core with rockfill and sand filters on both sides. In all, it comprises 1.3M m3 of materials. According to the original specifications, almost all the materials for the embankment were to come from the spillway excavation. Only the clay was to have been procured off site. The contract was signed in October 1995 and work started without delay.
‘In January 1996 we started excavating for the spillways but we soon discovered that the materials being excavated from the spillway were totally different from what had been specified in the tender,’ Palgen said. ‘First, the geology was completely different. Second, the quality of the materials was quite different. Basically, the spillway location was specified as first class rock. Instead we found boulders and extremely disturbed soils.’ CNO/BESIX had to take drastic steps. The poor quality of the material created an immense drop in the productivity of all the equipment on site. The solution was to work a double shift which continued for the duration of the project. In addition, a quarry was created near the dam site. ‘Despite this major problem we maintained our schedule and attained our original completion target,’ Palgen stressed.
The geological problems stemming from inadequate surveys in the pre-tendering phase prompted the CNO/BESIX venture to lodge a claim which brought the overall contract value to US$28M, compared with an original figure of US$20M.
Botswana sees the North-South Carrier Project as a key to its long term development, assuring good quality water to a significant part of its population. In the longer term there are plans to double the carrier’s capacity in a project which would involve the construction of a new dam on another river in the north of the country.
|The North-South Carrier Project|
|Via a 28.3m high intake tower, water from the Letsibogo reservoir will be fed to a pipeline with an average diameter of 1.2m running 360km to the reservoir behind the Gaberone dam near the city. Some of the water in the pipeline will go to large villages in the region between Letsibogo and Gaberone. The scheme includes three treatment plants along the pipeline at key abstraction points, and a series of pumping stations. Another pumping station, sited below the dam wall, is unrelated to the trunk pipeline. Its task will be to pump water into a pipeline leading to the mining town of Selebi Phikwe, Botswana's fourth biggest town 20km away. The pipeline contractor is a consortium of South Africa's LTA, Athens-based Consolidated Contractors International and Balfour Beatty of the UK. Most of the line is of GRP, with higher pressure section of steel. The treatment plants are being built by South Africa's Group Five, with Degremont of France. The North-South Carrier Project has attracted a range of international funding. The dam was part-funded by the European Investment Bank. The African Development Bank contributed towards the costs of the pipeline while Japan's Overseas Economic Co-operation Fund helped to finance the water treatment plants.|