Work Description

Title: Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal Open Access Deposited

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Attribute Value
Methodology
  • The data here are Geological Strength Index (GSI) observations of rock outcrop mechanical conditions, raw active seismic data collected in the field, and processed seismic data consisting of dispersion curves and shear-wave velocity profiles. Seismic data were collected with a Geometrics Geode Seismograph and 4.5 Hz vertical component geophones laid out in a linear array. A 10 lb sledgehammer was struck against a 5 cm thick plastic plate to generate the impulsive energy source. We used either 16 or 24 channel seismic arrays (30 and 58 surveys, respectively), and between 0.30 and 3.05 meter (1-10 ft) spacing between geophones depending on the space available at each site. We maximized array length to the space available (typically ~ 40 m), allowing for a depth of investigation of approximately 20 m or deeper depending on the velocity profile. At 3 sites (Vs 6, 9, and 77) we conducted multiple surveys with different receiver spacing to enhance both depth of investigation and resolution near the surface. We used a near offset, i.e., the distance between sledgehammer source and the first geophone, of 15–20% of the total array length to avoid near-field effects (Park et al., 1999a; Yoon and Rix, 2009). All surveys stacked 8 sledgehammer shots to improve the signal-to-noise ratio. We analyzed dispersion curves and performed inversions for Vs using the open-source Matlab-based package SWIP (Surface Wave Inversion and Profiling; Pasquet and Bodet, 2017). We assessed outcrop fractures and surface weathering conditions in the field using the Geological Strength Index (GSI) of Hoek & Brown (2019). We then used the Hoek & Brown (2002) criterion to estimate shear strength as a function of depth at each site, assuming a constant density and that the minimum principal effective stress is the lateral earth pressure. More details on this methodology are presented in the JGR Earth Surface journal article associated with this dataset.
Description
  • These datasets support the findings of Medwedeff et al. (2021) in JGR: Earth Surface. In this article, we present seismic and geotechnical characterizations of the shallow subsurface across a 200 km by 50 km swath of the central Himalayan Range, in Nepal. By pairing widely-distributed 1D shear wave velocity surveys and engineering outcrop descriptions per the Geological Strength Index classification system, we evaluate landscape-scale patterns in near-surface mechanical characteristics and their relation to environmental factors known to affect rock strength. We find that near-surface strength is more dependent on the degree of weathering, rather than the mineral and textural differences between the metamorphic lithologies found in the central Himalaya. Furthermore, weathering varies systematically with topography. Bedrock ridge top sites are highly weathered and have S-wave seismic velocities and shear strength characteristics that are more typical of engineering soils, whereas sites near the bedrock channel bottom tend to be less weathered and characterized by high S-wave velocities and shear strength estimates typical of hard rock. Weathering of bedrock on hillslopes is significantly more variable, resulting in S-wave velocities that range between the ridge and channel endmembers. We hypothesize variability in the hillslope environment may be partly explained by the stochastic nature of mass wasting, which clears away weathered material where landslide scars are recent. These results underscore the mechanical heterogeneity in the shallow subsurface and highlight the need to account for bedrock weathering when estimating strength parameters for regional landslide hazard analysis.
Creator
Depositor
  • wmedwed@umich.edu
Contact information
Discipline
Funding agency
  • National Science Foundation (NSF)
Keyword
Resource type
Last modified
  • 11/18/2022
Published
  • 05/03/2021
Language
DOI
  • https://doi.org/10.7302/s6sw-k853
License
To Cite this Work:
Medwedeff, W. (. O. M. E. &. E. S., Clark, M. (. O. M. E. &. E. S., Zekkos, D. (. O. C., West, A. (. O. S. C., Chamlagain, D. (. U. N. (2021). Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal [Data set], University of Michigan - Deep Blue Data. https://doi.org/10.7302/s6sw-k853

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Files (Count: 5; Size: 574 MB)

Thumbnailthumbnail-column Title Original Upload Last Modified File Size Access Actions
ReadMe.txt 2021-04-30 2021-04-30 5.17 KB Open Access
Medwedeff_et_al_data_repository_...a.zip 2021-04-30 2021-05-01 530 MB Open Access
Medwedeff_et_al_data_repository_...k.pdf 2021-04-30 2021-05-01 42.6 MB Open Access
Medwedeff_et_al_data_repository_....xlsx 2021-04-30 2021-05-01 70.8 KB Open Access
Medwedeff_et_al_data_repository_...s.zip 2021-04-30 2021-05-01 1.35 MB Open Access

Contents of DeepBlue repository associated with Medwedeff et al. (submitted to JGR Earth Surface April 29 2021):

The data here are Geological Strength Index (GSI) observations of rock outcrop mechanical conditions, raw active seismic data collected in the field, and processed
seismic data consisting of dispersion curves and shear-wave velocity profiles. Seismic data were collected with a Geometrics Geode Seismograph and 4.5 Hz vertical
component geophones laid out in a linear array. A 10 lb sledgehammer was struck against a 5 cm thick plastic plate to generate the impulsive energy source. We used
either 16 or 24 channel seismic arrays (30 and 58 surveys, respectively), and between 0.30 and 3.05 meter (1-10 ft) spacing between geophones depending on the space
available at each site. We maximized array length to the space available (typically ~ 40 m), allowing for a depth of investigation of approximately 20 m or deeper
depending on the velocity profile. At 3 sites (Vs 6, 9, and 77) we conducted multiple surveys with different receiver spacing to enhance both depth of investigation
and resolution near the surface. We used a near offset, i.e., the distance between sledgehammer source and the first geophone, of 15–20% of the total array length to
avoid near-field effects (Park et al., 1999; Yoon and Rix, 2009). All surveys stacked 8 sledgehammer shots to improve the signal-to-noise ratio. We analyzed dispersion
curves and performed inversions for Vs using the open-source Matlab-based package SWIP (Surface Wave Inversion and Profiling; Pasquet and Bodet, 2017).
We assessed outcrop fractures and surface weathering conditions in the field using the Geological Strength Index (GSI) of Hoek & Brown (2019). We then used the Hoek
& Brown (2002) criterion to estimate shear strength as a function of depth at each site, assuming a constant density and that the minimum principal effective stress is
the lateral earth pressure. More details on this methodology are presented in the JGR Earth Surface journal article associated with this dataset.

LIST OF FILES IN DATA REPOSITORY:
_____________________________________
File: 'Medwedeff_et_al_data_repository_Raw_Seismic_Data.zip'
Description: Contains raw seismic records collected in the field with a Geometrics Geode Seismograph and 4.5 Hz vertical component geophones.
Sorted into folders corresponding to each site location. Files are binary .dat format - open with seismic sofware such as Geometrics SeisImager.

_____________________________________
File: 'Medwedeff_et_al_data_repository_Vs_inversion_csv_files.zip'
Description: Contains final seismic profiles (shear wave velocity as a function of depth) for all surveys associated with Medwedeff et al. (2021). The files are formatted as follows:

- each column contains data for a single model inversion (best 300 saved in each file)
- Rows containing “9999” are delimiter rows
- The top row through the first row containing '9999' contain shear wave velocity estimates in m/s
- The row below the first set of '9999' through the second set of '9999' contain corresponding depth estimates in m
- The row below the third set of '9999' is the maximum depth of investigation where velocities are resolved
- The row below the fourth set of '9999' is the RMS error of the inversion of dispersion picks
- the last row is the relative misfit of the model run compared to all trial models.

*Note no inversion is included for Vs22 due to an extremely noisy dispersion curve. Vs22 was not used in the Medwedeff et al. (2021) manuscript.

_____________________________________
File: 'Medwedeff_et_al_data_repository_Vs_and_GSI_summary.xlsx'
Description: This file contains three spreadsheets - (1) a cover page (2) summary of Vs field sites and average S-wave characteristics (3) summary of outcrop GSI observations used in Medwedeff et al. (2021)
*Note there are seperate ID numbers for GSI observations and Vs field sites. To see examples where these data were collected at the same site, see the Vs analyses slide deck below.

_____________________________________
File: 'Medwedeff_et_al_data_repository_Vs_analyses_slide_deck.pdf'
Description: slide deck containing raw seismic records, dispersion curves, inversion results, and field photos associated with each Vs site in Medwedeff et al. (2021)

_____________________________________
For questions please contact:
William Medwedeff
wmedwed@umich.edu

_____________________________________
References:
Hoek, E. & Brown, E.T. (2019). The Hoek-Brown failure criterion and GSI – 2018 edition Journal of Rock Mechanics and Geotechnical Engineering, 11(3), 445-463.

Hoek, E., Carranza-Torres, C., Corkum, B. (2002). Hoek-Brown failure critierion – 2002 edition. Proceedings of the fifth North American rock mechanics symposium, Toronto, Canada, vol. 1, 267–73.

Pasquet, S. & Bodet, L. (2017). SWIP: An integrated workflow for surface-wave dispersion inversion and profiling. Geophysics, 82(6), WB47-WB61.

Park, C.B., Miller, R.D., Xia, J. (1999a). Multichannel analysis of surface waves. Geophysics, 64(3), 800-808.

Yoon, S. & Rix, G.J. (2009). Near-field effects on array-based surface wave methods with active sources. Journal of Geotechnical and Geoenvironmental engineering, 135(3), 399-406.

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