Rock mass quality and structural geology observations in Prince William Sound, Alaska (2024)

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 (USGS) 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 began rock mass quality assessments and collection of structural geology data in 2021. Data from 2021–2023 are contained in USGS data releases by Coe and others (2024), Belair and others (2025a), and Belair and others (2025b), respectively. This data release contains data from June-August 2024 that were collected in southwestern and eastern Prince William Sound. 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 2024, we accessed sites by boat and helicopter. 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 65 sites in the field. Site naming begins with a two-digit year, followed by a two letter location abbreviation, initials of a field investigator, and a three-digit site number (for example, 24PWSL001 was the first site collected by Sean Lahusen in 2024 in Prince William Sound). 24PWSLOBS1 was a landslide investigation, not a full rock mass quality data collection. Most sites were in metamorphosed Cretaceous flysch and intrusive or extrusive igneous rocks, whereas a few were in metamorphic 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 without adjustment for discontinuity orientations (RMR, for example, Bieniawski, 1989)
Slope Mass Rating (SMR, for example, Moore and others, 2009; Romana, 1995)
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 L Hammer (see details below). 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 (often a fracture set), 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 Tables_Figures.pdf. A blank field data collection sheet is also included on the last two pages of the Tables_Figures.pdf.
 
Details regarding field measurements and parameter scoring
The following section outlines details regarding field measurements and parameter scoring guidelines for the Q, RMR, and SMR rock mass quality classification schemes. Original parameter scoring guidelines can be found in Tables 1 and 2, while Table 3 outlines specific details of scoring used when calculating the rock mass quality values.

Discontinuity measurements: All measurements were taken using a magnetic declination of 16°E.
RMR and SMR, Uniaxial compressive strength (UCS) of intact rock: In the field we took 10 Schmidt Hammer measurements parallel to bedding and 10 measurements perpendicular to bedding, and recorded the mean, range, and standard deviation. When no bedding was present, we only took 10 Schmidt Hammer measurements. We used a Type L Proceq Rock Schmidt Rebound Test Hammer for all measurements.

​​For intrusive igneous rocks, the mean rebound value of the measurements (SHL,mean) was used to calculate UCS using the following equation from Aydin and Basu (2005):

UCS = 1.4459 exp(0.0706SHL,mean)


​For flysch, SHL,meanwas used to calculate UCS using the following equation from Morales and others (2004):

UCS = exp(1.332 + 0.053SHL,mean)


For extrusive igneous rocks, SHL,meanwas used to calculate UCS using the following equation from Karaman and Kesimal (2015):

UCS = 4.2423SHL,mean - 81.92


We include all Schmidt hammer measurements in our spreadsheet. So if a robust UCS equation is developed specifically for rock units in Prince William Sound (Cretaceous flysch, for example), a revised UCS value could be calculated.


RMR and SMR, Rock Quality Designation (RQD): This measurement was made along a tape measure oriented approximately perpendicular to bedding (if present) and covering a distance of 2 m. Assuming that a core of rock was taken along the tape measure, RQD is the percentage (in decimal form) of that core that would have intact lengths greater than 10 cm.
RMR and SMR, Joint Persistence: We used the estimated length of discontinuities to estimate this value. In the field, the longest length measurement that we used was “greater than 5 m.” It was impractical to measure or estimate lengths greater than 5 m in a reasonable amount of time. For joint persistence scoring, the maximum distance used was 5 m.
RMR and SMR, Joint Aperture: For RMR/SMR, the scoring ratings are finer resolution than the measurements taken in the field. Our field measurements ranged from "all tight" to "Gaping open, many joints open >20 cm," whereas RMR/SMR uses measurements on the millimeter scale. For this reason, our aperture ratings (R4b, Table 2) ranged from 0 to 4, as illustrated in Table 3.
Q, RMR, and SMR, Joint Roughness: The RMR/SMR joint roughness scoring ratings do not consider whether the fractures are planar or undulating. To account for this, we ordered observations from more likely to slide along fractures to less likely to slide (as in the Q rating system) and assign values from 0 to 5 accordingly (see R4c in Table 3). For example, “smooth planar” is more likely to slide than “rough or irregular undulating” fractures, so the ratings of 1 and 5 were assigned respectively.
RMR and SMR, Joint Infilling: This rating does not have a 1:1 relationship to field observations. We ordered observations from less altered/weathered to more altered/weathered and assigned values from 0 to 6 (see R4d and R4e in Table 3).
Q, RMR, SMR, Groundwater: Field conditions in Alaska made this observation particularly difficult. Many field days were wet and rainy, and sites were often at or near the tide line. For RMR/SMR the groundwater options that we used were either “damp” or “wet.” For Q (Table 1) there is a large difference between levels (i.e., “A. Dry excavations or minor inflow” and “B. Medium inflow or pressure occasional outwash of joint fillings”). No sites experienced any water conditions close to medium inflow, so for Q, all observations were considered “dry excavations or minor inflow.”
SMR, Rating adjustment for joint orientation: For this adjustment we: (1) determined the average gradient of the slope and average direction (i.e., aspect) of the slope for each site using GIS; (2) only considered bedding (when present) and prominent, prolific, pervasive, and adverse discontinuities as shown in the SMRCalculationsWorksheet file; (3) calculated the SMR joint adjustment (SMRadj = ((F1*F2*F3) + F4) for both toppling and planar failures; (4) selected the minimum SMRadj from all values at each site. Note: Romana (1995) and Moore and others (2009) have F1 = |joint dip direction – slope dip direction| for planar failures, and F1 = |joint dip direction – slope dip direction - 180°| for toppling failures. This causes an issue for some north facing dip directions where the mathematical difference between angles is more than the physical difference between angles (for example, |350° – 10°| = 340°, but the physical difference in angles is only 20°). In our SMR calculations, we used the physical difference between angles.

This data release includes: (1) a spreadsheet with the final Q, RMR, SMR, GSI, Uniaxial Compressive Strength (UCS), and RQD values for each site (FinalRockStrength_QualityValues2024.csv), (2) a spreadsheet of all field measurements, numerical ranking values, and calculated Q, RMR, SMR, GSI, and RQD values (RMQMeasurements_Ratings_Values2024.csv), (3) a spreadsheet of all structural measurements (StructuralData2024.csv), (4) a spreadsheet of the planar and toppling calculations used for determining SMR values (SMRCalculationsWorksheet2024.csv), (5) photos from each site (PhotosBySite2024.zip), (6) a folder containing 2024 site locations (SiteLocations2024.zip), and (7) a file with tables, figures, and the data collection sheet (Tables_Figures.pdf), and (8) a Metadata file (Rock_mass_quality_and_stuctural_geology_observations_in_PWS_AK_2024.xml). All horizontal positions given in this data release are in the  WGS 1984 geographic coordinate system.

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