Knowing the Poisson's rate value of the rock mass is one of the an important rock mechanical and rock engineering parameters. This value has to be used for calculating the deformations of the tunnel or the dam, among the others. Unfortunately, measuring this value is very difficult and time consuming for rock masses. The goal of this paper is to present a simple method for estimating the Poisson's rate value for rock masses if it is known for the intact rock. A linear equation was found: decreasing the quality of the rock mass, the Poisson's rate is increasing. The presented calculation is also good, if the Poisson's rate of the intact rock is not determinable. Note, the suggested equation is not valid if any other horizontal stress is existing.
The Poisson's rate value of the intact rock can be relatively easily measured. It is used for several rock mechanics and rock engineering calculations, necessary to know it for the in situ measurements. In Figure 1, typical ranges of values are presented for Poisson's ratio of some rock types (after the collection of Gercek ). Generally, Poisson's ratio of intact rocks can be determined in the laboratory either indirectly by dynamic methods or directly by static tests. The behavior of rock masses are influenced by the mechanical behavior and properties of the discontinuities and those of the intact rock bounded by discontinuities. It has also been well known that structural features induce some degree of anisotropy in rock masses. It was found, that the value of Poisson's ratio for the rock mass was found to be about 20 % higher than the value for the intact rock . Note, this paper did not classify the rock mass according to one of the widely used rock mass classification system. Unfortunately, measuring the Poisson's rate value of the rock mass is very difficult, so usually the intact rock's is used for the calculations.
In rock engineering applications involving underground openings, Poisson's ratio of the rock mass is utilized for estimating in situ stresses and in expressions involving induced stresses. The internal friction angle (φ) for rock masses can be determinable using Geological Strength Index - GSI (Hoek et al, ). Originally, the GSI was developed for the Hoek-Brown failure criteria, but knowing the Hoek-Brown material constants, the Mohr-Coulomb parameters (namely the internal friction angle, φ and the cohesion, c) can be also calculated.
The strength of a jointed rock mass depends on the properties of the intact rock pieces and also upon the freedom of these pieces to slide and rotate under different stress conditions. This freedom is controlled by the geometrical shape of the intact rock pieces as well as the condition of the surfaces separating the pieces.