A site investigation program was carried out for a concrete dam construction in Iran. The investigations are aimed to provide geotechnical parameters in order to analyze the suitability of this location and also to obtain required design parameters. As part of the program several Plate Load Test (PLT) and Flexible Dilatometer Test (FDT) were carried out in specially prepared galleries. Both tests are based on the theory of elasticity which considers the rock mass as an elastic, isotropic and homogeneous medium. Due to these assumptions, the modulus derived from both tests should be similar. In real conditions, however, this is not the case and the modulus determined from PLT is usually greater than values derived from the FDT. A number of uncertainties are associated with each of the above mentioned tests. The aim of this paper is to discuss the results obtained from these tests and compare the results from the two mentioned methods.


The static modulus of deformation is among the parameters that best represent the mechanical behaviour of a rock and of a rock mass, in particular when it comes to underground excavations or to the abutments of a concrete dam. This is why most numerical finite element and boundary element analyses for studies of the stress and displacement distribution around these types of projects are based on this parameter. The deformation modulus is, therefore, a cornerstone of many geomechanical analyses.

Deformability is characterized by a modulus describing the relationship between the applied load and the resulting strain. The fact that jointed rock masses do not behave elastically has prompted the usage of the term modulus of deformation, rather than modulus of elasticity, or Young's modulus. The commission of terminology, symbols and graphic representation of the International Society for Rock Mechanics (ISRM).has given the following definitions (ISRM, 1975.):

  • Modulus of elasticity or Young's modulus (E): The ratio of stress to corresponding strain below the proportionality limit of a material.

(Figure in full paper)
  • Modulus of deformation of a rock mass (Em): The ratio of stress to corresponding strain during loading of a rock mass, including elastic and inelastic behaviour (wd), as shown in Fig. 1.

  • Modulus of elasticity of a rock mass (Eem): The ratio of stress to corresponding strain during loading of a rock mass, including only the elastic behaviour (we), see Fig. 1.


The most common in-situ test for the determination of the deformation modulus of a rock mass is the Plate Loading Test (PLT) and Flexible Dilatometer Test (FDT) such as those shown in Figs. 2, 3 and 4. These tests involve either a set of hydraulic jacks which apply the load to a prepared surface in a gallery or a pressurized inflatable jacket acting against the walls of a borehole. Displacement of loading plates in PLT is measured by means of multi-point extensometers in the rock mass as shown in Fig. 2 and for the FDT by three LVDTs installed inside the probe.

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