I have just published a new tool and this post will detail the methods that are being used in this tool. What it does: Performs Hoek-Brown analyses for rock and recommends additional parameters based on the inputs.
The Hoek-Brown material model is the most widely used rock mechanic model due to its simplicity and ease-of-use in continuum based numerical models such as finite element or finite difference models. Hoek-Brown model is published in Hoek & Brown (1980) and after that, it is constantly updated. Latest update was published in 2019.
There are 4 main input parameters for Hoek-Brown material model. Additional inputs are required to estimate rock mass modulus or equivalent Mohr-Coulomb parameters.
Uniaxial compressive strength is the compressive strength of intact rock. In field, the intact samples are selected to be tested in UCS test. If required samples are not found, point load tests can also be performed and the results of these tests can be converted to UCS using various correlations.
GSI is a number between 0-100 that defines the weathering or joint degree of a rock mass. This value is usually determined on the field based on the tunnel or slope faces, boreholes or outcrops. It is possible to correlate GSI to RMR (Rock Mass Rating) value using variety of correlations available in the literature. The most commonly used correlation is GSI=RMR-5. However, it should be noted that RMR’ should be recalculated by neglecting the effect of groundwater and tunnel orientation.
GSI can also be estimated using the graph below. It is usually advised to keep the GSI above 25 for rock mass conditions.
Material constant (mi) is a fitting parameter which can be determined using the curve fitting technique on high quality triaxial tests or can be estimated using the ranges given in the table or figure below.
Disturbance Factor is used to reflect the disturbance due to blast damage or stress relaxation. It is very unlikely to reach very high disturbance factors or very thick disturbed zones with the current technology. Hoek and Brown (2019) emphases that disturbance factor should not be applied to the whole rock mass. It should only be applied to a limited thickness. Literature shows that the usual thicknesses for disturbed zones are around 0.5-1.0 m. However, it is very usual to apply 3 m disturbed zone in drill and blast tunnelling.
A common error is to assume that the disturbance factor D should be applied to the entire rock mass in which the excavation is conducted. This will result in an extremely conservative and inappropriate design.