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Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal

Creator:

Medwedeff, William, G (University of Michigan Earth & Environmental Science), Clark, Marin, K (University of Michigan Earth & Environmental Science), Zekkos, Dimitrios (University of California, Berkeley), West, A., Joshua (University of Southern California), and Chamlagain, Deepak (Tribhuvan University, Kathmandu 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.

Keyword:

rock strength, critical zone, shallow seismic, and chemical weathering

Discipline:

Science

Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal

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Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal

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