The common standards (ISRM and ASTM) indicate that the cone penetration depth
should be considered in determining Is(50) particularly for weak rocks. However, accu-
ratemeasurements of cone penetration depth by a laser distance sensor attached to a point
load frame revealed that deeper penetration occurs in fresher=stronger rocks and when
the penetration depth is considered, the scatter in the data is minimized leading to a
significantly better regression coefficient and lower conversion factor for Is(50)f in pre-
dicting UCS. The UCS vs. Is(50)i correlation curves from the literature confirm that the
scatter in the data lie mainly in the higher UCS range. The UCS value below which the
depth of penetration has little or practically no influence on the correlations largely
depends on the rock type and itsmicrostructure. Therefore, the standards should acknowl-
edge that the cone penetration could be significant particularly for fresher=stronger rocks.
In brittle rocks, penetration depth should decreasewith decreasing tensile strength but
the rate of change in depth may be smaller in fine grained rocks. Thus, specimens with
significant differences in grain size should be evaluated separately, despite their composi-
tional similarities, to enhance the efficacy of Is(50) as a predictive tool for UCS and as an
indicator of weathering grades or for objective confirmation of their visual identification.
