Rock mass quality and structural geology observations in Prince William Sound, Alaska (2023)

Multiple subaerial landslides adjacent to Prince William Sound, Alaska (for example, Dai and others, 2020; Higman and others, 2023; Schaefer and others, 2024) pose a threat to the public because of their potential to generate ocean waves (Barnhart and others, 2021, 2022; Dai and others, 2020) that could affect towns and marine activities. One bedrock landslide on the west side of Barry Arm fjord drew international attention in 2020 because of its large size (~500M m3) and tsunamigenic potential (Dai and others, 2020). As part of the U.S. Geological Survey response to the detection of the potentially tsunamigenic landslide at Barry Arm, as well as a broader effort to evaluate bedrock landslide and tsunamigenic potential throughout Prince William Sound (for example, Schaefer and others, 2024), we continued rock mass quality assessments and collection of structural geology data in southwest and eastern Prince William Sound in August and September, 2023 (see associated data from 2021-2022 in Coe and others, 2024 and Belair and others, 2025). The quality (strength) of a rock mass depends on the properties of intact rock and the characteristics of discontinuities (for example, bedding, fractures, cleavage) that cut the rock. Rock mass quality can be estimated in the field using a variety of classification schemes.

In 2023, we accessed sites by boat. At each field site, we made our measurements at rock outcrops, which were typically found at the base of cliffs, along ridge lines, in flat areas in coastal zones, and in areas recently scoured and plucked by glaciers. In two dimensions, outcrops ranged in size from about 30 m2 to 100 m2. We visited a total of 79 sites in the field. Most sites were in metamorphosed Cretaceous flysch, but a few were in intrusive and extrusive igneous rocks (Nelson and others, 1985; Wilson and others, 2015; Winkler, 1992). We collected data that we later used to classify rock mass quality according to four commonly used classification schemes:


Rock Mass Quality (Q, for example, Barton and others, 1974; Coe and others, 2005)
Rock Mass Rating (RMR, for example, Bieniawski, 1989)
Slope Mass Rating (SMR, for example, Moore and others, 2009; Romana, 1995)
Geologic Strength Index (GSI, for example, P. Marinos and Hoek, 2000; V. Marinos and others, 2005)

We also determined Rock Quality Designation (RQD, for example, Deere and Deere, 1989; Palmström, 1982) and estimated intact rock strength using a Proceq Rock Schmidt Type L Hammer (see RatingsReadMe2023.pdf for details). Schmidt Hammer rebound values were converted to Uniaxial Compressive Strength (UCS) using equations developed for the same rock types that we observed in the field, but at different locations. For flysch, rebound values from the Type L Schmidt Hammer were converted to UCS by the equation shown in Table 3 and Figure 3 of Morales and others (2004). For intrusive igneous rocks, rebound values were converted to UCS by the equation shown in Figure 3 of Aydin and Basu (2005). For extrusive igneous rocks, rebound values were converted to UCS by the Equation 4 in Karaman and Kesimal (2015). Additionally, we collected strike and dip measurements of any observed bedding, fractures, and cleavage.

All four rock mass quality classification schemes use data from characteristics of discontinuities present in the rock. Discontinuity data that we collected in the field included: total number of discontinuities, roughness of the surface of the discontinuities, number of sets of discontinuities, type of filling or alteration on the surface of discontinuities, aperture or “openness” of discontinuities, and the amount of water present. Numerical ratings for each of these factors are assigned based on the correlation of field measurements and observations with descriptive rankings. The rankings and any additional details used for Q, RMR, SMR, and GSI classification schemes are shown in Tables 1-3 and Figures 1-2 in the RatingsReadMe2023.pdf. A file of a blank field data collection sheet is also included in the RatingsReadMe2023.pdf. Samples of rock were collected at some sites. These sample names are noted in a column in the RMQMeasurements_Rating_Values2023.csv. Physical samples are held in the Geologic Hazards Science Center in Golden, Colorado. 
 
This data release includes: (1) a spreadsheet of all field measurements, numerical ranking values, and calculated Q, RMR, SMR, GSI, and RQD values (RMQMeasurements_Ratings_Values2023.csv), (2) a spreadsheet of all structural measurements (StructuralData2023.csv), (3) a spreadsheet of the planar and toppling calculations used for determining SMR values (SMRCalculationsWorksheet2023.csv), (4) a spreadsheet with the final Q, RMR, SMR, GSI, Uniaxial Compressive Strength (UCS), and RQD values for each site (FinalRockStrength_QualityValues2023.csv), (5) photos from each site and any relevant sketches (PhotosBySite2023.zip), (6) Google Earth file of 2023 site locations (SiteLocations2023.kml), (7) a summary file with extra information about data collection, processing, and each classification system (RatingsReadMe2023.pdf), (8) and a readme (README.txt).

Disclaimer: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References:

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Barnhart, K.R., Jones, R.P., George, D.L., Coe, J.A., and Staley, D.M., 2021, Preliminary Assessment of the Wave Generating Potential from Landslides at Barry Arm, Prince William Sound, Alaska: U.S. Geological Survey Open-File Report 2021–1071, 28 p., https://doi.org/10.3133/ofr20211071

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