In the present work we discuss the results of a number of Schmidt hammer tests (total number of impacts N > 2,400) that were performed in situ on rock outcrops of different lithology (marl, calcareous marl, limestone, sandstone, quartz sandstone and rhyolite) that occur in Italy. Firstly, a specific field procedure to choose the reference value of the rebound index adopted to calculate UCS of intact rock is suggested. A relationship between L and N hammer rebound index values (RL and RN, respectively) is subsequently assessed. Considering the experimental datasets provided by a Schmidt hammer construction company and other research available in literature, a new exponential equation for the correlation between RL and UCS of intact rock has been derived. Considering the here-proposed RL−RN relationship, a new exponential correlation between RN and UCS has also been defined. The newly proposed procedure and relationships were successfully utilised to determine the intact rock strength of different rocks. The calculated UCS values are very similar when using both types of Schmidt hammer (L and N) and are generally in line with previous determinations from experimental data available in literature.
In common practice of rock engineering, the Schmidt hammer is widely used to obtain an indirect estimation of the intact rock strength, specifically of its uniaxial compressive strength (UCS) [1–4]. UCS can be easily calculated by means of empirical equations that correlate the Schmidt hammer rebound index (R) with the intact rock strength. Many empirical equations are available in literature, for both L- and N-type hammers. These empirical equations were established on the basis of hammer tests that were performed on various types of rocks [3, 5–10]. The indirect estimation of the intact rock strength on using the Schmidt hammer was found to be strongly influenced by some basic issues concerning the specific test conditions [7–12]. Among these issues, the differences between field and laboratory testing, the specific test procedures and the assessment of the reference rebound value to calculate UCS have widely been discussed in literature and represent key features when using the Schmidt hammer in rock mechanics practice. Notably, most research investigates the intact rock properties on the basis of laboratory tests carried out on rock specimens. However, laboratory testing on rock specimens requires specific apparatus and specimen preparation [2, 4, 13] that may result in complex, expensive and time-consuming procedures. On the contrary, research published on in situ Schmidt hammer tests is fairly limited. Nevertheless, the Schmidt hammer is more commonly used by engineers and geologists to obtain a quick estimation of the intact rock strength directly on the field, where conditions are rather different when compared with those designed in the laboratory.