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Field Methods for Characterizing Weak Rock for Engineering

¹ Department of Geological Engineering, Colorado School of Mines, Golden, CO 80401

Many classification and testing methods for shale, slaking rock, weak rock, and weathered rock have been developed. However, few of these methods are suitable for field use or are applicable to a wide range of material types. The purpose of this paper is to present a summary of recent research and analysis evaluating classification and testing methods. This summary will assist the practitioner in properly identifying problem shale and weak rock units, in adequately describing their characteristics in the field, and in selecting and performing field tests to quantify their behavior. For identification of shale and weak rock units, three increasingly site-specific tools are presented. First is a map showing occurrences of weak rock units in the United States. Second is a detailed list of suspect rock types that may appear on regional or local geologic maps. Third is a summary of engineering properties that describe weak rock units, based on a review of technical literature. Some of these properties are compressive strengths between 1 and 20 MPa, slake durability less than 90 percent, clay content greater than 15 percent, poor induration, a significant amount (50 to 75 percent) of matrix between hard blocks, or high moisture content. Methods of field description were chosen based on a review of existing methods to assess their ease of application, breadth of application, and the usefulness of the engineering properties indicated by each system. Two modified methods are presented to indicate proportion and nature of corestones and matrix, strength, influence of discontinuities, and reactivity to water. Finally, field-testing methods that estimate strength, permeability, durability, and reaction to water are identified. These include point load index, Norwegian Geotechnical Institute Rock Mass Classification, jar slake, and hammer rebound classification. Predictive equations to estimate unconfined compressive strength, slake durability, and slake index are given.

Key Words: Weak Rock • Weathered Rock • Shales • Field Testing