to base intact modulus values on test results done on good samples of intact rock from locations with competent rock, using either laboratory measurements of intact modulus or on an assumed ratio between intact strength and modulus for a particular rock type. This ignores the possibility that the material in zones with poor rock will often be highly weathered, and it ignores the possibility that even without weathering a zone of poor rock may represent rock which simply has a lower intact strength, and that is why it became disturbed while zones of stronger rock on the same project did not. The tunnel support guidelines based on RMR were provided originally in the form of a table giving support recommendations for a tunnel span/diameter of 10 meters. In view of the improving technology for rock bolting, shotcrete and steel ribs, it was left to tunnel designers to modify these guidelines for other tunnel sizes, which served its purpose well. Today, after 40 years of use, it has become apparent that it would be convenient for practical tunnel designers to have charts for the selection of rock support as a function of both tunnel size and rock mass quality. This is depicted in the chart below (see Lawson 2013). See also Core recovery parameters SMR classification References ASTM (1988). "Standard Guide for using the Rock Mass Rating (RMR) System (Geomechanics Classification) in Engineering Practices." American Society for Testing and Materials, Book of Standards D5878-08, v.04.09, Philadelphia, PA. Further reading Pantelidis L. (2009) "Rock slope stability assessment through rock mass classification systems" Int. J.Rock Mech. Min.Sci., 46(2):315–325. Rock mass classification