BULLETIN of the International Association of ENGINEERING GEOLOGY No 47 PARIS 1 993 / de I'Association Internationale de G#OLOGIE DE L'INGENIEUR / / ENGINEERING GEOLOGICAL EVALUATION OF THE OF-SOLAKLI HYDROPOWER DEVELOPMENT PROJECT (N.E. TURKEY) F.VALUATION GI2OTECHNIQUE DU PRO JET DE CENTRALE HYDROI2LECTRIQUE D'OF-SOLAKLI (N.E. TURQUIE) V. DOYURAN*, V. TOPRAK*, T. TOPAL*, A. KO(~Y|O|T*, F.B. ROJAY*, and E. BOZKURT* Abstract The Of-So[akli Hydropower Development Project IHDP/ is designed to generate 380 MW of electricity through the diversion of six streams into a headrace tunnel and maintaining a head difference of t 000 m above the underground power station, The tunnel system is located within felsic and mafic magmatic rocks, and wflcanogcnic tlysch. In this paper an engineering geological evaluation of the HDP is introduced. "l'he field investigations included geological strip mapping, core drilling, and intense discontinuity surveying. A portion of the headrace tunnel was re-located due to adverse foundation conditions. The rock mass qualities of various lithological units were determined and the necessary support systems are described. Rdsumd Le projet de construction d'une centrale hydraulique ",5. Of-Solakli a pour objectif de produire 380 MW d'dlectricitd par la diversion de six tours d'eau darts un tunnel principal en conservant une difference de charge hydraulique de 1 000 m au-dessus de la station souterraine. L.e svstbme de tunnels se situe dans des roches magmatiques felsitiques et mafiques et dans un flysch d'origine volcanique. Cet article pr6sente une dvaluation gdotechnique du projet. Los investigations sur le terrain inluent la prdparation d'une carte g~ologique, des carottages, et un levd detailld des discontinuites. Une partie du tunnel principal a did relocalisde en raison de mauvaises conditions de fondation. Los caracldristiques qualitatives des diff6.renles unitds lithologiques sont ddterminees et les syst~mes de support necessaires sont ddcrits. 1. Introduction The Of-Solakh hydropower development project com- prises four diversion tunnels, a headrace tunnel, a steel- lined pressure shaft, an underground power station, and a tailrace tunnel (Fig. 1). The total length of the tunnel system is 47.6 km. Through diversion tunnels the dis- charges of the Balkodu, Kavlatan, Biiytik, Haldizan, Cunis, and Maki streams will be collected and diverted into the headrace tunnel. The pressure shaft is [ 414 m long. and inclined at 45 ~ to create a head difference of 1000 m. The power station is designed to generate 380 MW of electricity annually. The project area is located in the south of the Of- county, 48 km east of Trabzon, and power station being 15 km inland from the Black Sea coast (Fig. 1). The area is characterized by a rugged terrain where north- ward flowing streams are deeply incised within steeply sloping topographic troughs. Narrow canyons, steep stream gradients, valley walls which are highly suscep- tible to slope failures, and low discharge rates of in- dividual streams are the major factors which preclude construction of surface reservoirs. Thus, the project originally designed by NORENDEL (Norway)-MAPA (Turkey) joint venture does not only eliminate all these shortcomings but also helps to improve the hydroelec- tric potential of the basin. In this paper the geological aspects of the tunnel align- ments will be described and the mass characteristics of the foundation rocks will be discussed. The geological strip map covers a I-2 km broad corridor, along which various lithological units are distinguished and struc- tural features are identified. Engineering geological evaluation of the lithological units is based on their mass characteristics involving the assessment of degree of weathering and discontinuity pattern. * Middle East Technical University, Geological Engineering Department, 06531 Ankara, Turkey. Tel.: (4) 210 1000 / 2682, 2686. Fax: ~4) 210 1263.