ENGINEERING GEOLOGICAL EVALUATION OF RESERVOIR LEAKAGE PROBLEM

The 260 mill m3 capacity Samanalawewa reservoir, formed by a 100m high embankment dam is the key element of the 120 MW Samanalawewa Hydroelectric Project, constructed on the Walawe, one of the main rivers in Sri Lanka. The reservoir developed a water leakage in its right bank right along with the impounding due to the adverse geological conditions existed. Geologically the project is lying in an area within the Highland series of the Sri Lankan Precambrian complex comprising crystalline metamorphic rocks. Further the area has been subjected to extensive folding, faulting and hydrothermal reactions converting the project area in to a geologically complex nature. The site has been studied by number of agencies in several occasions involving detailed geological, hydrogeological geophysical etc. investigations. However amidst the detailed investigations conducted the signs of a possible leakage through the right bank were surfaced during dam construction stage itself with the poor geological conditions encountered in the right abutment excavation. Immediate additional investigations coupled with improvements to the cut off works had to be implemented. In the initial trial impounding at the end of construction with partially filled reservoir the leakage was appeared and the impounding suspended for remedial measures. A 100m deep 1800m long grout curtain was constructed as the remedial measure and the impounding recommenced. The leakage continued to increase amidst the massive grout curtain placed behind and finally resulted in a severe earth slip. Again the impounding had to be suspended and the detailed investigations and remedial measures were called for. As the second attempt of sealing the leakage, laying of a clay blanket over the suspected ingress zones was planned. However even after the construction of the blanket the reservoir leakage could not be arrested though it helped to reduce the right bank ground water pressures. At present the reservoir is operated with a continuous leakage of 1800 litres per second amidst the various investigations and remedial measures implemented. As discussed in detail the failure in remedying the leakage problem is due to the inability in establishing exact leakage mechanism owing to the complex geological conditions prevailing at site.