Hammer Time: Using the Schmidt Hammer to Improve the Rockfall Activity Index (RAI) Forecasting Accuracy

The Schmidt hammer is a widely-used and inexpensive instrument used to estimate rock strength either in the lab or in the field. Our research team tested the accuracy and repeatability of the Schmidt hammer to estimate rock strength for six Alaskan rock slopes and four Washington/Oregon rock slopes. We determined in situ rock hardness using two different Schmidt hammers (Types L and N); conducted unconfined compressive strength (UCS) testing for select Alaskan rock samples; and summarized the pros and cons of using the Schmidt hammer in the field. Parameters that potentially affect Schmidt hammer results include testing methodology, sample testing conditions, and data reduction. Our results indicate that major structures within a rock unit (such as bedding or foliation), variation in mineralogy, and moisture content will significantly impact Schmidt hammer results. After data collection, several correction methods can be used to process the Schmidt hammer results. Choice of method depends on the intent of the measurements (i.e., strength of the intact rock or the rock mass), and the application of any method can alter the final results. Our UCS results generally correlate to the Schmidt hammer rebound values (e.g., rock types with high rebound values also had high UCS values). Before using the Schmidt hammer, we suggest to: determine the final goal before selecting Schmidt hammer and testing methodology; identify rock type and potential discontinuities that can influence results; identify bias in sample or site selection; and select the most applicable data reduction method for identified goal.