Comparison of Rock Strength Testing Methods for Understanding Fluvial Bedrock Erodibility

Abstract:

Bedrock erodibility exerts a fundamental control on fluvial channel incision and ultimately landscape evolution, yet the extent to which specific bedrock properties influence erodibility in fluvial settings is poorly understood. This is in part due to the difficulty in systematically quantifying relationships between bedrock properties and erodibility in the field, where effective methods are not well established. Erodibility is hard to define and measure, and tools for evaluating rock properties are often poorly suited for complex field environments (e.g., the Schmidt hammer, which cannot be used on fissile rock and is sensitive to water content, temperature, unseen fractures, and user technique). Here we focus on quantifying rock strength, which sets the resistance of bedrock to erosion via abrasion, and present a comparison of three methods: point load testing of hand samples, Brazilian split testing of core samples, and in-situ testing using a Schmidt rebound hammer. We employed all three methods at two lithologic knickpoints within bedrock channels in the Valley and Ridge of SW Virginia, in order to compare rock strength between knickpoint and non-knickpoint bedrock (representing end-member erodibility cases).

We find point load testing of hand samples collected in the field to be the best of the three tested methods for making quantitative comparisons of rock strength between groups. Point load and Brazilian split test data show significant differences in rock strength between knickpoint and non-knickpoint bedrock not fully captured by the Schmidt hammer (which does not sufficiently capture the lowest values). Point load tests are more cost-effective and less labor-intensive than collecting cores for Brazilian split testing, and successfully quantify rock strength on thinly-bedded, fissile rock on which the Schmidt hammer is unusable. The point load test is therefore both a more practical alternative to the gold standard of Brazilian split testing for quantifying fluvial bedrock strength without sacrificing data quality, and a better suited tool to test a wide range of bedrock types than the Schmidt hammer. These results will form the basis of a more thorough field-based investigation of the influence of bedrock properties on erodibility in fluvial settings.