In sensitive projects such as large dams, estimation and evaluation of rock mass deformability parameters is an important component of the field studies. Over the years, various methods have been used for estimation of rock mass deformability, including in-situ rock mechanics tests. Bakhtiary dam and Hydro Power Plant in Iran which is under study at the present would be the highest double curvature concrete dam in the world with the height of 315 meter. Obviously, evaluation of the reaction of the dam foundation and potential ground deformation under the heavy loading is even more sensitive for this project. Various in-situ tests for defining rock mass deformability parameters have been used for this project. This includes Dilatometer test (DL), Extra Large Flat Jack Test (LFJ), and Plate Load Test (PLT). In this paper, these methods are compared and PLT tests are discussed in more details. Results of 41 performed PLT tests are evaluated using ASTM and ISRM formulas as well as Unal method. It seems like the result of Unal analysis is more consistent and reliable.


Bakhtiary dam and hydro power plant project is located in Lorestan Province, southwest of Iran. Dam will be constructed on Bakhtiary River, in Zagros Mountains almost 50 km on the upstream of Dez dam and hydropower plant, constructed in 1963. The mean discharge value of Bakhtiary River is 145 m3/Sec. Dam height would be 315 m from the foundation. The crest elevation is 840 m.a.s.l. and crest length is 435 m. Its catchment area is about 6495 km2 [1]. After construction, it would be the world’s highest double curvature concrete dam. Bakhtiary project is now under study.

Bakhtiary dam site is located in folded Zagros structural zone, which is composed of numerous high anticlines [2]. The main structure is located near core of an anticline and will be built on the limestone layers which were folded severely in some sections. Figure 1 shows folded layers of the right abutment of the dam.

(Figure in full paper)

Deformability is recognized as one of the most important parameters governing the behavior of rock masses [5]. Understanding this phenomenon is vital for large dams, especially Bakhtiary dam which is exceptionally high and some parts of the structure are located on fractured and folded rock mass. Quantitative values of rock mass deformation properties can be obtained through in-situ tests, empirical relations between deformation modulus and rock mass classification, and estimating from laboratory tests result [5]. For smaller size projects in-situ tests normally are ignored due to cost and time constrains. However for larger dams where obtaining reliable deformation behavior of rock mass is necessary, in-situ tests are a major component of site investigation studies.

As a result, extensive in-situ rock mechanics tests have been conducted to obtain reliable data on rock mass properties. While the tests mainly focused on measuring rock mass deformability, some tests were carried out for measurement of in-situ stresses and shear strength of discontinuities.

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