Rock mass quality and structural geology observations in northwest Prince William Sound, Alaska from the summer of 2021

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 (Dai and others, 2020; Barnhart and others, 2021; Barnhart and others, 2022) that could impact 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 (~500 M 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 assessed rock mass quality and collected structural geology data in a large part of northwest Prince William Sound (including Barry Arm) in June and July, 2021. 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 the summer of 2021, most of our fieldwork was boat-based and was therefore conducted at sites along the coastline. A small number of sites in and near Barry Arm were accessed by helicopter, and sites near the town of Whittier were accessed by driving and hiking. 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 73 sites in the field. Most sites were in metamorphosed Cretaceous flysch, but a few were in Tertiary granitic rocks (Nelson and others, 1985; Winkler, 1992; Wilson and others, 2015). Of the 73 sites, we collected rock mass quality data and structural data at 54 sites, and only strike and dip of bedding in flysch at 19 sites. At each of the 54 sites, 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, Romana, 1995, Moore and others, 2009) and Geologic Strength Index (GSI, for example, Marinos and Hoek, 2000, 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 N hammer (see RatingsReadMe.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 N Schmidt hammer were converted to UCS by first converting Type N rebound values to Type L rebound values, then using these Type L values in the equation shown in Table 3 and Figure 3 of Morales and others (2004). For granitic rocks, UCS values were calculated using Type N rebound values in equation 2 of Katz and others (2000). Additionally, we collected strikes and dips 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. A file of a blank field data collection sheet (FieldDataCollectionSheet) is included in this data release. Numerical ratings for each of these factors are assigned based on the correlation of field measurements and observations with descriptive rankings. The rankings used for Q, RMR, SMR, and GSI classification schemes are shown in Table 1, Table 2, Table 3, and Figures 1 and 2. Additional details regarding descriptive rankings and numerical ratings not shown in the tables and figures are provided in the RatingsReadMe.pdf.

All field measurements, numerical ranking values, and calculated Q, RMR, SMR, GSI, and RQD values are included in the RMQMeasurements_Ratings_Values2021 file (.csv and .xlsx). Site names beginning with “JAC”, followed by numbers, are locations where both rock mass quality and structural data were collected. Site names beginning with “JACSD”, “srl”, and “fault” are locations where only the strike and dip of bedding was measured. Question marks in the data files indicate a lack of certainty in field observations. Abbreviations of rating parameters (for example, R4e, Jw, etc.) for the RMR, SMR, and Q classification systems used in column headings are defined in more detail in Tables 1 and 2. All structural measurements are provided in the StructuralData2021 file (.csv and .xlsx). The planar and toppling calculations used for determining SMR values are included in the SMRCalculationsWorksheet2021 file (.csv and .xlsx). Final Q, RMR, SMR, GSI, and RQD values for each site are presented in a separate file (FinalRockStength_QualityValues2021, .csv and .xlsx). All rock mass quality values are positively correlated with rock quality. That is, as Q, RMR, SMR, GSI, and RQD values increase, rock quality increases.

Additional information in this release includes photos, field sketches, and geographic data. Photos from each site are included in a separate folder (2021PhotosbySiteName), organized by the individual site names and the names of the photographers. Field sketches for eight sites are in a SketchesinFieldNotesbySiteName zipped folder. A Google Earth 2021SiteLocations.kml file showing site locations, site names, and geographic coordinates is also included. Samples of rock were collected at some of the 2021 sites in the summer of 2022. These sample names are noted in a column in the RMQMeasurements_Rating_Values2021 file. Physcial samples are held by Lauren N. Schaefer with the U.S. Geological Survey, Geologic Hazards Science Center in Golden, Colorado.  

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.

Barnhart, K.R., Collins, A. L., Avdievitch, N. N., Jones, R.P., George, D.L., Coe, J.A., and Staley, D.M., 2022, Simulated inundation extent and depth in Harriman Fjord and Barry Arm, western Prince William Sound, Alaska resulting from the hypothetical rapid motion of landslides into Barry Arm Fjord, Prince William Sound, Alaska: U.S. Geological Survey data release, https://doi.org/10.5066/P9QGWH9Z

Bieniawski, Z.T., 1989, Engineering rock mass classifications a complete manual for engineers and geologist in mining, civil, and petroleum engineering: John Wiley & Sons, New York, 251 p.

Coe, J.A., Harp, E.L., Tarr, A.C., and Michael, J.A., 2005, Rock-fall hazard assessment of Little Mill campground, American Fork Canyon, Uinta National Forest, Utah: U.S. Geological Survey Open File Report 2005-1229, 48 p., two 1:3000-scale plates. http://pubs.usgs.gov/of/2005/1229/    

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Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest: U.S. Geological Survey Miscellaneous Field Studies Map MF–1645–B, scale 1:250,000. https://doi.org/10.3133/mf1645B.

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Schaefer, L.N., Kim, J., Staley, D.M., Lu, Z., and Barnhart, K.R., 2024, Satellite interferometry landslide detection and preliminary tsunamigenic plausibility assessment in Prince William Sound, southcentral Alaska: U.S. Geological Survey Open-File Report 2023–1099, 22 p., https://doi.org/10.3133/ofr20231099.

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