Testing the seismic response of rock pillars using in situ strength measurements

Devin McPhillips, Joaquin Garcia Suarez, Emiliano Gonzalez, Katherine M. Scharer, & Domniki Asimaki

Submitted August 16, 2021, SCEC Contribution #11559, 2021 SCEC Annual Meeting Poster #047

Rock pillars are a type of fragile geologic feature (FGF) that may constrain the intensity of past earthquake shaking. Unlike some other FGFs, such as precariously balanced rocks, rock pillars are materially connected to bedrock at their bases. In order to use rock pillars as ground motion constraints, it is necessary to understand their seismic response as well as material properties like stiffness and strength. Here, we develop and test a hypothesis regarding the progressive weakening of rock pillars in response to shaking during multiple successive earthquake cycles. We also demonstrate the application of rebound hammer as a tool for measuring strength in situ. On the basis of Timoshenko beam theory and finite element modeling, we expect that slender rock pillars are the most vulnerable to earthquake shaking and predict that they accumulate damage near their bases during successive earthquake cycles. Such localization occurs because side-to-side rocking is the primary mode of response by these features. In contrast to slender pillars, stout pillars and rock mounds respond by primarily shearing. Therefore, damage is not localized in stout pillars to the same extent as slender pillars. At the Trona Pinnacles, located within 30 km of the Ridgecrest Earthquake Sequence Mainshock (M7.1), we instrumented several rock pillars and far-field bedrock to compare their responses to aftershocks. We show that slender pillars respond as predicted, by rocking. We also used the rebound hammer to measure near-surface hardness in situ, which is an established proxy for strength in both rock and concrete. Rebound hammer measurements reveal localized weakness near the base of slender pillars, which supports the prediction of damage localization resulting from rocking in response to earthquake shaking. We also show that applying empirical scaling relations to tens of rebound numbers can reproduce laboratory-derived tensile strength values for tufa rock. Ongoing work aims to gather additional rebound hammer data, especially at stout pillars, where we predict little or no localized weakness. This work supports the application of rock pillars as past ground motion constraints and the utility of the rebound hammer for measuring both in situ strength and damage localization.

Key Words
Fragile geologic features, intensity, geology, geomorphology, ground motion

McPhillips, D., Garcia Suarez, J., Gonzalez, E., Scharer, K. M., & Asimaki, D. (2021, 08). Testing the seismic response of rock pillars using in situ strength measurements. Poster Presentation at 2021 SCEC Annual Meeting.

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Ground Motions