Stability and deformation analyses of dams require estimation of rock mass strength. Hoek-Brown failure criterion can be adopted to dam foundations ''where closely jointed rock with several sets of joints are not oriented unfavorably in respect to potential failure surfaces; or failure surface are parallel to the base of the dam where the joints, bedding and other defects are not oriented close to this failure surface,'' as stated by Fell (2015). Douglas (2002) indicates that such criterion can define the equivalent Mohr-Coulomb parameters which are an integral part of any design of dams.
Mechanical properties of rock masses (i.e., strength and stiffness) can be reduced by blast damage and stress relief caused by excavation. These effects would result in relaxation and dilation of the rock masses (Hoek-Brown 2018). This degradation is commonly referred to as damage, blast, or disturbance factor (D). Sönmez and Ulusay (1999) proposed a disturbance factor approach in which rock mass can be continuously defined depending on the type of excavation in the case of five slope failures. Also, Hoek et al (2002) introduced the blast damage factor of Hoek-Brown criterion.
Hoek et al (2002) and Hoek-Brown (2018) provided tables as a practical guidance for estimating initial values of blast factors and presented several tunnel and slope cases to assign reasonable D values for practical projects. Also, Hoek (2012) provided a technical note with some suggestions on how this parameter can be chosen and applied in each project. On the other hand, a tentative guideline for dams is given by Romana (2003) to determine D.
Although some descriptive guidelines are available to determine the D, it is still not easy to determine a suitable D value in practical applications. For instance, Edelbro (2004) evaluated the estimation of the D by 11 technical staff for the same rock mass in the Laisvall mine in Sweden. Interestingly, the values ranging from 0 to 0.7 were selected for the same rock mass by the participants. This study shows how differently the D, consequently rock mass strength and deformation modulus, can be calculated by engineers depending on the use of the D.
Researchers have tried to establish quantification methods for the D. For this purpose, Luo and Fu (2012) proposed relationships between the D and the P-wave velocity of rock masses using the elastic damage theory. Wen et al (2019) measured the acoustic velocity of the rock mass to calculate the equivalent disturbance effect. Also, Yang et al (2020) used the acoustic P-wave velocity to quantify the D by statistically analyzing a large amount of in-situ test data measured on rock slopes. Rose et al (2018) proposed an empirical disturbance rating for open pit slopes. Ahangaran et al (2022) performed numerical analyses of rock slopes with a different types of discontinuities to offer a detailed guideline for blast-induced D.
The D for dams, on the other hand, is generally estimated according to guidelines for tunnels, pit quarries, and slopes. However, the disturbance effects (i.e., blasting, or mechanical excavation) on dam foundations might considerably differ from those applications. Therefore, dam engineers have frequently been struggling to find out what the proper D factor should be used in their applications. In this study, the authors have evaluated the root of this confusion, developed a comprehensive dataset of D from dam applications, and evaluated the effect of D on dam design. In the end, the main emphasis is to provide a judgmental and convenient guideline for the use of D in dam foundations considering several factors with the aid of conducted field observations.