Slope mass rating
Slope mass rating (SMR) is a rock mass classification scheme developed by Manuel Romana [1][2][3] to describe the strength of an individual rock outcrop or slope.
The system is founded upon the more widely used RMR scheme,[4] which is modified with quantitative guidelines to the rate the influence of adverse joint orientations (e.g. joints dipping steeply out of the slope).
It has been included in the technical regulations of some countries as a classification system by itself or as a quality index for rocky slopes (e.g., India, Serbia, Italy).
[5][6] Rock mass classification schemes are designed to account for a number of factors influencing the strength and deformability of a rock mass (e.g. joint orientations, fracture density, intact strength), and may be used to quantify the competence of an outcrop or particular geologic material.
While it is relatively straightforward to test the mechanical properties of either intact rock or joints individually, describing their interaction is difficult and several empirical rating schemes (such as RMR and SMR) are available for this purpose.
SMR uses the same first five scoring categories as RMR: The final sixth category is a rating adjustment or penalization for adverse joint orientations, which is particularly important for evaluating the competence of a rock slope.
SMR provides quantitative guidelines to evaluate this rating penalization in the form of four sub-categories, three that describe the relative rock slope and joint set geometries and a fourth which accounts for the method of slope excavation.
SMR addresses both planar sliding and toppling failure modes, no additional consideration was made originally for sliding on multiple joint planes.
However, Anbalagan et al.[7] adapted the original classification for wedge failure mode.
Most of the observed inaccuracies are related to the calculation of the ancillary angular relationships between dips and dip directions of the discontinuities and the slope required to determine F1, F2 and F3 factors.
Moreover, the proposed functions for SMR correction factors calculus reduce doubts about what score to assign to values near the border of the discrete classification.
where parameter A is the angle formed between the discontinuity and the slope strikes for planar and toppling failures modes and the angle formed between the intersection of the two discontinuities (the plunge direction) and the dip direction of the slope for wedge failure.
Graphical SMR (GSMR) Alternatively, Tomás et al.[14] also proposed a graphical method based on the stereographic representation of the discontinuities and the slope to obtain correction parameters of the SMR (F1, F2 and F3).
This method allows the SMR correction factors to be easily obtained for a simple slope or for several practical applications as linear infrastructures slopes, open pit mining or trench excavations.
[15][16] A four-dimensional visual analysis of SMR geomechanical classification, performed by Tomás et al.[17] by means of the Worlds within Worlds methodology to explore, analyze and visualize the relationship among the main controlling parameters of this geomechanical classification, revealed that several cases exist where the slope-discontinuity geometrical relationship scarcely affects slope stability (i.e. F1×F2×F3≃0), and as a consequence SMR can be computed by correcting basic RMR only with the F4 factor using next equation with a maximum error lower than nine points:
Considering the previous situations, we can assess that the SMR index is insensitive to the geometrical conditions of the slope for a significant number of plausible discontinuity-slope geometries.
In these situations, we can ignore the calculation of factors F1, F2, and F3, which depend on the geometry of the slope and the discontinuities, considering that F1 × F2 × F3 ≃ 0.
[17] This insight can be very useful for field engineers and geologists, as it helps provide preliminary acceptable field values of SMR when any of the aforementioned circumstances are identified in the studied slope, resulting in significant time savings.
[17] SMR index can be calculated through the open source software SMRTool,[18] which permits to compute SMR from the geomechanical data of the rock mass and the orientation of the slope and the discontinuities.
[19] Some authors have proposed different methodologies to map the failure susceptibility in rock slopes by computing the SMR index using a Geographical Information System (GIS).
