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The Soil Conservation Service uses the Rock Material Field Classification Procedure (TR-71) to classify rock and assess rock performance for engineering purposes. The classification process consists of identifying the rock units at the site of investigation and describing the rock in terms of its rock material, rock mass, and hydrogeologic properties. The performance assessment is the selection of performance objectives according to the engineering use of the rock. Objectives include erosion resistance, excavation characteristics, construction quality, water transmission, and rock mass stability. The rock is then classified according to criteria in the appropriate performance assessment tables. The procedure uses established methods and tests developed in some widely accepted rock mass classifications, particularly the Unified Rock Classification System (URCS). The URCS is included in its entirety in Appendix I of TR-71 as a convenient reference, since it is a major part of three of the five performance assessment tables. The prominent attributes of the procedure include: (1) rock can be classified according to its intended use, (2) terms are nontechnical and unambiguous, (3) classification elements are based on observable and measurable field conditions, 4) significant properties can be quickly tested in the field, (5) the need for additional evaluation can be recognized, and (6) a rock mass can be assigned to groups with similar behavior. The main caveats of the procedure include: (1) some of the criteria in the performance assessment tables are empirical, (2) the procedure is new and not widely accepted, and (3) the classification elements are not weighted.
engineering geology, rock material property, rock mass property, rock classification, rock performance assessment, rock excavation, construction
Moore, John S.
Engineering Geologist, USDA Soil Conservation Service, Chester, PA