A comparative study of Schmidt hammer testing procedures with reference to rock cutting machine performance prediction

摘要

The Schmidt hammer is a portable instrument mainly used for testing rock hardness, often related to the mechanical strength of rock. However, there is a wide variation in the employed test procedures with this instrument. Therefore, it would be useful to rationalize the large number of Schmidt hammer test procedures in current use by developing a series of standard test procedures which would prove to be valid for particular applications. The present study is a step in this direction, where extensive raw data obtained from a previous research during the construction of Istanbul's sewerage system tunnels have been evaluated. N-type Schmidt hammer rebound values collected in situ have been correlated to the net cutting rates of a medium-size roadheader for 36 different rock zones. A new Schmidt hammer test procedure proposed in this work has been performed together with two other recognized test procedures in the literature. All the performed test procedures are based on continuous impacts at a point which is claimed to be more consistent and repeatable than single impacts. To check the validity of the performed test procedures for prediction of roadheader performance, each test procedure has statistically been tested. Statistical test results suggest that incorporation of all recorded rebound values at a point, rather than selecting only the peak values, gives a better representation of overall rock hardness and hence a better performance prediction, provided the outliers are statistically discarded. Such an approach may seem quite reasonable for rock excavation purposes, where incorporation of only the relatively higher portion of the spectrum of rebound values would naturally be inappropriate. Finally, it is thought that the proposed Schmidt hammer test procedure in this work may be used as a preliminary guide for roadheader performance prediction purposes, valid for the encountered rock types. However, to be able to check its general applicability, it is emphasized that further in situ observations should be made for other rock types and roadheaders of different classes.