Impact-Echo: A technique for determining the mechanical properties of rocks

This paper presents the first application of the Impact-Echo (IE) technique for determining the elastic properties of rocks in the laboratory. The technique was developed at the United States National Institute of Standards and Technology for measuring the thickness and detecting flaws in concrete. It is based on transient stress wave propagation where the stress pulses generated by a mechanical impact at the surface undergo multiple reflections between the top and bottom of a slab. Knowing the thickness of the slab, the P and S wave velocities can be calculated with great accuracy. The dynamic elastic moduli can then be calculated from the measured P and S wave velocities using standard equations. Comparing the results of the Impact-Echo tests with standard static tests and accepted dynamic tests such as ultrasonic and resonance frequency, it is shown that Impact-Echo is the only dynamic testing method that produces the most consistent set of values for elastic properties such as Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio. In addition, Impact-Echo data are obtained in a fraction of time compared to other testing methods because of minimal sample preparation requirements and data processing techniques. The authors provide a detailed description of the Impact-Echo method along with the results of experiments carried out on five different rock types using static and dynamic testing techniques.