So, if you are a fancy engineer and dare to make some calculations, you can find much lower values. You can say it depends on many factors as much as you like, everybody has a rough idea already: 100,000 kN/m3. However, let’s think about a weathered rock with E=500 MPa and a foundation to be built on top of this rock with 20 x 50 m dimensions. It depends on loading type, temperature, bitcoin prices and others.We all know some typical statements about it: Exploration Geophysics, 37(1), 108-113.īy now, everything should have been settled about modulus of subgrade reaction. Application of linear-array microtremor surveys for rock mass classification in urban tunnel design. Bulletin of the seismological Society of America, 95(6), 2081-2092. Empirical relations between elastic wavespeeds and density in the Earth’s crust. “Analysis of laterally loaded drilled shafts in rock.” PhD Thesis, Univ. Hoek-Brown failure criterion-2002 edition. Hoek, E., Carranza-Torres, C., & Corkum, B.Evaluation of rock mass deformability using empirical methods–A review. International journal of rock mechanics and mining sciences, 43(2), 203-215. Empirical estimation of rock mass modulus. Putting geological focus back into rock engineering design. Journal of Rock Mechanics and Geotechnical Engineering, 11(3), 445-463. The Hoek–Brown failure criterion and GSI–2018 edition. General equation for maximum and minimum effective principal stress at failure is calculated using the following equation: In case of absence of any laboratory data, following ranges can be used.Ĭarter and Marinos (2020) General Equations Modulus ratio is the ratio of elastic modulus of intact rock to compressive strength of intact rock. This will result in an extremely conservative and inappropriate design. However, it is very usual to apply 3 m disturbed zone in drill and blast tunnelling.Ī common error is to assume that the disturbance factor D should be applied to the entire rock mass in which the excavation is conducted. Literature shows that the usual thicknesses for disturbed zones are around 0.5-1.0 m. It should only be applied to a limited thickness. Hoek and Brown (2019) emphases that disturbance factor should not be applied to the whole rock mass. It is very unlikely to reach very high disturbance factors or very thick disturbed zones with the current technology. 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.Ĭarter & Marinos (2020) Disturbance Factorĭisturbance Factor is used to reflect the disturbance due to blast damage or stress relaxation. It is usually advised to keep the GSI above 25 for rock mass conditions. GSI can also be estimated using the graph below. However, it should be noted that RMR’ should be recalculated by neglecting the effect of groundwater and tunnel orientation. The most commonly used correlation is GSI=RMR-5. It is possible to correlate GSI to RMR (Rock Mass Rating) value using variety of correlations available in the literature. This value is usually determined on the field based on the tunnel or slope faces, boreholes or outcrops. GSI is a number between 0-100 that defines the weathering or joint degree of a rock mass. 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. In field, the intact samples are selected to be tested in UCS test. Uniaxial compressive strength is the compressive strength of intact rock. Additional inputs are required to estimate rock mass modulus or equivalent Mohr-Coulomb parameters. There are 4 main input parameters for Hoek-Brown material model. Hoek-Brown model is published in Hoek & Brown (1980) and after that, it is constantly updated. 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. What it does: Performs Hoek-Brown analyses for rock and recommends additional parameters based on the inputs. I have just published a new tool and this post will detail the methods that are being used in this tool.