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RMR Rock Properties for Shallow Foundation Design

Journal of Engineering Technology, Fall 2004 by Rose, Andrew T

Abstract

This paper discusses the use of the Rock Mass Rating (RMR) system, also known as the Geomechanics Classification System, to determine rock strength and deformation properties for use in shallow foundation design. Many civil engineering projects are relatively small and involve limited budgets for geotechnical exploration and testing. For these projects, design of shallow foundations on rock is usually not of concern, and extensive rock testing is not conducted. Design calculations, however, are often necessary for project completeness and in some localities for permits and approvals. The RMR system provides an economical approach for estimating rock strength and deformation parameters for shallow foundation design using conventional geotechnical explorations.

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Introduction

Although most civil engineering technology graduates do not specialize in the geotechnical field, familiarity with foundation design principles is important for career development and interaction with other design professionals. In addition to understanding soil strength and deformation behavior when designing foundations in soil, which is typically the focus of most undergraduate geotechnical courses, inclusion of rock classification and behavior and basic design applications is also desirable. At some institutions, extensive coverage of foundation design in rock is accomplished through elective undergraduate engineering geology or rock mechanics courses. In other programs, selected topics related to rock must be incorporated into introductory soil mechanics or foundation design courses. The RMR system provides a way to introduce students to rock classification and foundation design within an undergraduate curriculum in which time for multiple elective topics may be limited.1,2

Design of Foundations on Rock

One of the most common topics covered in foundation design courses and used in geotechnical engineering practice is the design of shallow foundations. Buildings and other structures are often supported by shallow spread footing foundations. When rock is shallow, these foundations can be directly supported on rock. Published tables and building codes list presumptive allowable bearing pressures commonly used for designing foundations on rock.3,4 When rock types and conditions are not consistent with those described in building codes, foundation design models utilizing appropriate rock strength and deformation parameters can be used.4 Design procedures presented by the American Society of Civil Engineers (ASCE) and the American Association of State Highway and Transportation Officials (AASHTO) for foundations on rock are available for building and transportation structures, respectively.5,6 Rock strength and deformation parameters needed to apply these design procedures can be determined by field and laboratory tests when project scope and budget allow for such testing. Smaller projects, however, may not support extensive field and laboratory testing, and the engineer is left to either select strength and deformation parameters from published tables or use a presumptive allowable bearing pressure that may or may not be conservative, depending on the rock conditions at the project site.3,4 Tabulated values for rock strength and deformation properties are available for numerous rock types.6 However, these published values are not based on site-specific studies and often vary widely, making selection of appropriate values difficult. Rather than randomly selecting strength and deformation parameters from a published table, the RMR approach utilizes site-specific data with limited field and laboratory testing to estimate rock strength and deformation parameters that can then be used in the ASCE and AASHTO shallow foundation design models. Although originally developed for tunneling applications in deep rock, the RMR rating has been applied to shallow rock conditions to estimate strength and deformation parameters for dam foundations and laterally loaded drilled shafts socketed in shallow rock.1,2,7,8 Application of the RMR system to vertically loaded spread footing foundations is a natural extension of previous work and is appropriate for many structures for which more sophisticated testing, analysis, and design procedures are not economical.

Rock Classification Systems

Rock classification systems are used by geotechnical engineers and engineering geologists to group rock masses with similar engineering behavior. The Rock Quality Designation (RQD), the Rock Mass Rating (Geomechanics Classification System), and the Q-System (NGI System) all classify rock for engineering purposes based on various input parameters.1,2,9,10 Both the RMR and Q-System use the RQD of the rock as an input parameter. The RMR system requires easily obtained rock characteristics or properties as input, and through correlations, it provides estimated strength and deformation parameters similar to those normally discussed in introductory soil mechanics courses. As a result, the RMR system is easily applied by students and practicing engineers familiar with soil mechanics principles. Additionally, both ASCE and AASHTO recognize use of the RMR classification system for engineering design in rock.5,6

Manufacturing Industry

RMR Rock Properties for Shallow Foundation Design

Journal of Engineering Technology, Fall 2004 by Rose, Andrew T

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