Rock Mass Rating Classification
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Rock mass rating (RMR) classification is one of the key outputs of geological & geotechnical investigations, which are useful for design purposes in infrastructure projects. It is upon this classification that several significant decisions are made during different stages of construction. Our Geologist, Mr. Sreeraj M writes.
In most aspects of deep underground excavations such as tunnels, the main focus of geological and geotechnical investigations is on understanding rock mass properties, which include mechanical properties of intact rock, and the orientation and geometry of various discontinuities present in the rock mass. The geotechnical investigations carried out for underground excavations give the properties of intact rock. The combination of intact rock properties, discontinuity properties and ground water properties gives rock mass property, which is useful for design purposes. Q, RMR, and GSI are widely used geo-mechanical classifications of rocks.
Bieniawski (1976) published the details of rock mass classification called the Rock Mass Rating (RMR) system. Over the years, RMR system has got refined as more case records have been examined and Bieniawski has made changes in the ratings assigned to different parameters. The following discussion is based upon the 1989 version of the classification (Bieniawski, 1989). Both this version and the 1976 version deal with estimating the strength of rock masses. This engineering classification system, utilises the following six rock mass parameters:
All these are measurable in the field and can be obtained from a combination of geotechnical investigation and geological field investigations. The rating of each of these parameters are added to give a value of RMR.
Uniaxial Compressive Strength (UCS)
Uniaxial Compressive Strength (UCS) stands for the maximum axial compressive stress that a rock specimen can bear under zero confining stress. Uniaxial compressive strength of a rock can be evaluated using a laboratory compression test on a prepared rock core sample. But for rock classification purposes it is satisfactory to determine compressive strength approximately using the point load test on intact pieces of drill core. UCS of a material is verified by applying compressive load until failure occurs due to fracture in core sample. When stress exceeds the bearing limit it cracks the core sample. These cracks are produced along the weaker zones. When cracks are produced, we can note the gauge reading which shows the maximum compressive strength of the rock.
The rating for the rock strength depends on the unconfined compressive strength of the rock. The maximum rating is 15 for rocks having a UCS more than 250 megapascal (MPa) and for rocks having an unconfined compressive strength below 1MPa the rating is 0 (See the figure below).
Rock Quality Designation (RQD)
Rock Quality Designation (RQD) is a modified core recovery percentage in which the lengths of all rock core piece over 100 mm in length are summed and divided by the length of the core run. Pieces of core that are not hard and sound should not be included in the RQD evaluation even if they are at least 100 mm in length. The general equation is expressed as:
RQD index (%) = 100 × Σ (Length of core pieces ≥ 0.10 m)/ (Total length of core run).
The maximum rating is 20, for an RQD of 90% to 100% and the lower rating is 3, for an RQD below 25%. The stronger the rock mass, the less broken the cores and therefore RQD is relevant to the classification of the rock mass. RQD calculation procedure of a core sample and the RQD rating table for RMR calculation is given below.
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Spacing of Discontinuities
It is the perpendicular distance between adjacent discontinuities. Description of spacing and RMR rating for spacing of discontinuities are given in the tables below.
Condition of Discontinuities
Condition of the discontinuities depends on the roughness of the discontinuity surface, persistence, fillings, aperture, and weathering conditions of the discontinuities. RMR rating according to different conditions of discontinuities are given in the classification parameters and their rating table. Groundwater and discontinuity orientation ratings are also given in the same table.
By adding the rating obtained from the six parameters mentioned above we will get a RMR rating, ranging from less than 20, for very poor rock, to 100 for very good rock. Every rating is associated with a class of rock – class I for a very good rock, and class V for a very poor rock. The overall rating is obtained by adding up the six parameters mentioned above.
The RMR obtained for the rock mass is useful for the excavation and support of underground excavations. A sample RMR classification guide for the excavation and support of rock tunnel (Bieniawski,1989) is given below, (Shape: horseshoe; Width: 10 m; Vertical stress: below 25 MPa; Excavation by drill & blast)
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