Problematic incidents were experienced with the upper pond of a pumped storage project constructed on a mountain top underlain by sedimentary rocks consisting of sandstone, claystone and siltstone. An asphalt face was used to line the pond slope and floor, created by rock cut and rockfill embankments. During the first trial filling, cracks and bulges in the asphalt face occurred in three slope areas where claystone cut faces existed underneath. Slippage along the contact between claystone and geotextile sheet used for treatment of rock cut surface led to failure of the asphalt face system above. The second incident was a severe rupture of the asphalt lining layer in the pond floor. Piping of fine grained materials used in the leveling layer over the excavated claystone and sandstone face was the triggering cause of the damage. The third problem was the large deformation of the embankment that led to severe cracks on the dam crest. Possible causes include time-dependent compressibility behavior of the Zone 3C rockfill materials consisting of claystone, siltstone and weathered rock, and sliding along the near horizontal claystone bed in the dam base. Creep behavior of the claystone could be also the reason for the pronounced time-dependent lateral movement of the embankment. All these incidents were attributed to the adverse moisture sensitive behavior of the weak rocks of the project.
Asphalt face was adopted as impervious lining for a cut and rock fill reservoir to serve as the upper pond of a 4x250-MW pumped storage project in Thailand. The project marked the first underground cavern construction and the use of asphalt face for reservoir lining in the country. The site, located 200 km northeast of Bangkok, was situated at the southwestern rim of the Khorat Plateau at the escarpment of a long valley, where there existed an irrigation reservoir which was to be used as the lower pond of the scheme. The valley was bordered by flat-topped mountains with gently dipping slopes. The topography, which is the common feature in the region, is attributed to the near horizontal orientation of the sedimentary rock beds. The construction work started in November 1995 and was completed in March 1998. Despite of the weak rock conditions at the site, the construction was completed on schedule without major mishaps. However, moisture sensitive properties of the weak rocks at the project caused complication in all stages of the project development: from design, construction and operation. Much was learnt from the project, particularly that related to the cavern construction and the asphalt facing of the reservoir built in the rocks of Khorat group, which happens to be the predominant rock group in the vast northeast region of Thailand and parts of the neighboring Lao PDR and Cambodia.
Rocks at the project mainly consisted of red-bed siltstone, sandstone and claystone of the Phu Kradung-Pra Wiharn formations of Khorat group. Siltstone was the predominant rock type while claystone was only found at shallow zone in the Upper Pond area.
