DOI: 10.7302/s6sw-k853
title: ¬¬¬¬Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal

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. 
description: 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
description: 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
description: 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.