Basic Rock Mass Properties for Design: Information that can be Collected in the Field

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Technical Insights by William C.B .Gates, PhD, PE, D.GE., PG, PEng

Failure to collect the proper rock engineering information during field investigations can result in costly project delays due to erroneous assumptions based on limited data. This information is straightforward to obtain and collect with the right tools.

McMillen Jacobs geotechnical engineers and engineering geologists use several geomechanical rock mass classification systems to inform design decisions for tunnel, rock slope stability, foundation, and rock scour projects. These systems include Rock Mass Rating (RMR), Rock Mass Quality (Q-system), Geological Strength Index (GSI), and Scour Erodibility Index (EI). Properties common to each classification system include:

  • Strength of intact rock
  • Rock quality designation (RQD)
  • Spacing, condition, and orientation of discontinuities
  • Groundwater conditions

Each system requires extensive information on rock engineering properties for design that can be readily collected in the field. We use two field mapping sheets to collect this information: the Discontinuity Survey Data Sheet and Rock Mass Description Data Sheet, both adapted from Rock Slope Engineering by Duncan Wyllie and Chris Mah (2004).

Julia Irizarry geologic mapping at Lewis County Public Utility District’s Cowlitz Falls Hydroelectric Project in Washington State

Discontinuities are beds, joints, faults, shears, and other fractures that interrupt the rock mass. Our Discontinuity Survey Data Sheet provides a logical system to tabulate the characteristics of discontinuities such as dip and dip direction azimuths, persistence, aperture width, and nature and strength of filling, shape, and roughness.

Our Rock Mass Description Data Sheet provides a summary checklist of rock mass characteristics. This includes general information on the site and information attendant to the rock material and rock mass—such as compressive strength tables, RQD computation, slake durability, discontinuity sets, spacing and conditions, and groundwater conditions.

Use of standardized data sheets in the field ensures that all investigators obtain consistent and comparable engineering properties required for the geomechanical rock mass classification systems and eventual design. This reduces project delays, standardizes the input, and improves the quality of final reports.