Bakhtiary Dam & Hydropower Project is located on Bakhtiary River (the main branch of Dez River), about 70 km North-East of the city of Andimeshk in South-West of Iran. The main dam is a double-curvature concrete arch type with maximum height of 325m and crest length of 509m at El. 840 masl. It will be the highest arch dam in the world now at the design stage. The hydropower plant consists of an underground powerhouse complex with a total installed capacity of 1,500MW which is designed for annual energy output of 3,000 GWH.
Bakhtiary dam site and its reservoir are located in the Folded Zagross tectono–sedimentary province. The dam site is completely placed on siliceous limestones of Sarvak formation (Cretaceous period), divided to 7 units from Sv1 to Sv7. Bakhtiary dam and hydropower plant project is unique for its outstanding characteristics. One of them is the extensive engineering geological and rock mechanics investigations carried out during its design studies. Subsurface exploration at Bakhtiary dam site consisted of the drilling of more than 178 boreholes with the total length of about 12,500m and excavation of 11 galleries with the total length of 2,850m for performing engineering geological investigations and in-situ rock mechanics tests. Geotechnical investigations performed in all the boreholes included RQD measurements, lithological and joint survey and analysis of permeability tests' results.
A large number of laboratory rock mechanics tests were performed on rock samples selected from the drill cores of the exploratory boreholes including index tests, ultrasonic wave velocity measurements, unconfined compression tests, triaxial tests and direct shear tests. In order to make a realistic estimate on the geomechanical parameters of the existing rock masses at the dam site, an extensive in-situ rock mechanics test program was performed in the exploratory galleries excavated in the dam foundation and underground powerhouse areas. This test program consisted of deformability tests including Plate Load, Large Flat Jack and Dilatometer tests, in-situ stress measurement including Borehole Slotter and Hydraulic Fracturing tests and Direct Shear test for determining the shear strength parameters of the bedding planes as the most frequent and dominant discontinuity at the dam site. Since the Plate Load Tests provided the most reliable results for estimating the deformability characteristics of the rock masses therefore; a correlation was defined between the overall modulus of deformation of the rock masses and their ratings based on GSI classification system. Results of Hydraulic Fracturing tests at Bakhtiary dam site revealed that the average orientation of the major horizontal principal stress at the dam site (N57° ± 36° ENE) fitted well with the general stress regime by which the Zagross mountain ranges had been created, in the vicinity of the borderline of Arabian and Eurasian tectonic plates. Engineering classification of the existing rock masses at Bakhtiary dam site were performed based on the detail information gathered from engineering geological mapping and discontinuity survey data on the surface and in the exploratory galleries using RMR89, GSI and Q systems. Then the modulus of deformation of the rock masses were estimated based on the correlation between Plate Load Tests results and GSI values of the tested rock masses using Hoek and Diederichs formula (2006) and their ultimate bearing capacity were calculated using the Serrano et. al. method (2000). The estimated geomechanical parameters for the "Group A" rock masses confirm that a 325m high arch dam can be constructed at Bakhtiary dam site.
