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Dam repairs partly successful

Repairs to the Samanalaweva hydro project in Sri Lanka (IWP&DC, August 1997, p7) appear to have been only partly successful.

The 120MW hydro project was completed in 1992 but has never operated at its designed full supply level (FSL) because of high seepage from the right abutment of the 100m high rockfill dam. Although extensive grouting was carried out during construction the impounding of the reservoir was stopped in October when the reservoir level was at el440m, 20m lower than the FSL. The reservoir was drawn down to el430m, but an outflow of more than 2m3/s was still recorded.

The reservoir continued operation at el430m between 1992 and 1996 because of concerns related to the safety of that dam and of the 33m high Uda Walawe dam downstream. After considerable study in November 1997 the Ceylon Electricity Board (CEB) raised the level to FSL. At FSL leakage was 27m3/s, and the filling helped identify a number of previously-unknown leakage paths. To repair the dam CEB decided to put in place an impervious blanket 700-1700m upstream of the dam, without interrupting power generation activities. CEB began placing the blanket in March 1998, and over the following five months an area extending to 1000m upstream of the dam has been blanketed to 2m thickness —requiring placement of some 250,000m3 of material. The abutments above the bed were also treated.

The blanket appears to have lowered the downstream groundwater by around 14m and reduced leakage by around 0.5m3/s (at el442m). It is thought that a further 250,000m3 of material is required to reduce leakage to acceptable levels.

CEB’s consultants for the original design and the repair are Nippon Koei of Japan (supervising engineer) and Gibbs of UK. Contractor for the repairs is a Japanese joint venture of Kumagai, Hazama and Kajima.