SYMPOSIA PAPER Published: 01 January 1988

The Unified Rock Classification System


The Unified Rock Classification System (URCS) makes possible the initial assessment of geotechnical rock conditions in the field by verifiable and reproducible means. The URCS is flexible in regard to the scale and scope of the project. In addition, the URCS provides for further refinement by as much laboratory testing as is required to meet the cost and risk considerations of the project. The URCS defines by simple or refined procedures four fundamental properties: Weathering, Strength, Discontinuity, and Density of rock materials or rock masses for analysis and documentation. To be effective, a rock classification for civil engineering or engineering geology purposes must meet the criteria of Objectivity, Reliability, Validity, Sensitivity, Comparability, and Utility. The URCS was designed to meet these criteria. The system has been used for over 27 years in a wide variety of geotechnical investigations for the design and construction of civil engineering projects. Results have demonstrated that the URCS does meet these criteria.

Author Information

Williamson, Douglas, A.
U.S.D.A. Forest Service, Eugene, OR
Kuhn, C., Rodney
U.S.D.A. Forest Service, Cook, WA
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Developed by Committee: D18
Pages: 7–16
DOI: 10.1520/STP48460S
ISBN-EB: 978-0-8031-6775-9
ISBN-13: 978-0-8031-6663-9