The rock mass deformation modulus can be measured by different methods including field and laboratory testing and considering the relation between the applied load and resulting deformation. The dilatometer is one of the most versatile instruments used for this purpose. Defining an absolute value for the deformation modulus based on this test may be difficult or inaccurate and the aim should rather be the definition of a magnitude for the modulus. However, interpreting the results of dilatometer tests is a matter of engineering judgment that requires an understanding of the rock mass behavioural pattern during the test which is discussed in this paper. Examples of dilatometer test results from Roudbar-Lorestan dam site in Iran are also presented.
The deformability is one of the most important properties that represent the mechanical behavior of rock masses and is used in various rock engineering projects including underground and surface structures. It is characterized by a modulus which describes the relation between the applied load and the resulting strain. The commission of terminology, symbols and graphic representation of the International Society of Rock Mechanics has given the following definitions :
Modulus of deformation of a rock mass is the ratio of stress to corresponding strain during loading of a rock mass, which includes both elastic and inelastic behavior.
Modulus of elasticity of a rock mass is the ratio of stress to corresponding strain during loading of a rock mass which only includes the elastic behavior.
Part of it may also occur from changes in test boundary conditions, from poor test design or incorrect analysis. For instance, Farmer and Kemeny  have found that the deformation modulus of intact rock samples is in the order of 5 to 20 times higher than in-situ values and Pinto de Cunha and Muralha  showed the effect of the volume involved in the test of the deformation modulus measured. Hence large scale techniques offer advantages by testing at a more reasonable scale. However, few projects feature a sufficient number of different tests to allow a meaningful comparison of in situ test data and very different in-situ results may be obtained depending on the test method. Even in an extensive in-situ test program in fairly uniform and good quality rock mass conditions, deformability data may feature a deviation of 25% or as much as 10 GPa for an average in-situ modulus of 40 GPa . Therefore the use of more than one indirect procedure has also been proposed by many other authors, so that the results obtained can be compared and their reliability checked. In this regards the Borehole Expansion Tests, mostly Flexible dilatometers, were found to be one of the most suitable in-situ tests for the determination of the rock mass deformation modulus .
The dilatometer test is a conventional method among in-situ tests and is based on the theory of elasticity which considers the rock mass as an elastic, isotropic and homogeneous medium.