Several authors have reported specific applications of the Schmidt hammer for rock mass characterization, resulting in its acceptance as a convenient tool to index rock quality and to estimate engineering rock properties. The Bureau of Mines advanced the use of this instrument by testing 10 U.S. coals to determine the utility of the Schmidt hammer in designing underground coal mine pillars. Specifically, the tests investigated the correlation of hammer rebound index to uniaxial compressive strength of laboratory-prepared coal samples. Unconfined strength is of particular interest in coal mining since it is used in the majority of published equations on coal pillar design. A description of the Schmidt hammer, a summary of related research performed by several noted investigators, and results of the Bureau's investigation are presented in this paper.
2 THE SCHMIDT HAMMER
Developed in 1948 by Swiss engineer Ernest Schmidt, the Schmidt hammer is a portable, cost-effective instrument capable of estimating intact rock strength with distinct advantages over traditional laboratory testing (Schmidt, 1951). Laboratory tests are time consuming, expensive, and nearly always subject to bias due to platen effects, integ- rity loss during coring and preparation, and sample alteration from environmental conditions. Conversely, a large number of nondestructive Schmidt hammer tests can be performed quickly and efficiently in either the laboratory or the field. Significant ranges of scatter are typically produced when many hammer tests are performed. Statistically, this provides an excellent description of rock mass homogeneity and allows determination of realistic degrees of confidence to incorporate in design performance evaluations. Straightforward principles apply when operating the Schmidt hammer, shown in figure 1. A constant amount of stored spring energy is imparted through a hammer mass to the plunger, causing the mass to rebound a distance proportional to the total energy absorbed by the impact surface. The rebound distance is shown by the indicator and is defined as the "rebound index." The degree of rebound varies, depending upon the rock elastic properties. Studies also show that a variety of additional factors may affect laboratory and field-determined index values, including the following:
Varying degrees of surface irregularity.
Impact surface moisture content.
Inhomogeneities in the rock fabric.
Presence of cleavage slips, bedding planes, porous cavities, and other local anomalies.
Orientation and size of test surface.
Duration and degree of test surface weathering.
Rock mass confinement; in place versus unconfined laboratory setting.
After reviewing literature describing the consistency and reliability of Schmidt hammer data, Poole and Farmer (Poole, 1980) concluded that rebound values have a tendency to increase and show considerable variation during the first three to four individual impacts at a point. They added that the most consistent results are obtained by selecting the peak values from at least five discrete impacts at a point. In contrast, Kazi and Al-mansour, in an empirical study comparing the Los Angeles abrasion and Schmidt hammer tests, decided that at least 35 rebound index values should be taken at each point (Kazi, 1980). The 10 lowest values are discarded, and the average rebound index is then calculated from the remaining 25 readings.