The authors have developed a testing machine for use in boreholes to evaluate various physical characterist.ics (e.g. strength, deformability) through a simple procedure. in which the inner wall of a borehole is struck and responses are measured. At the same time. propagation velocity is obtained by two built-in receivers. Laboratory and in situ tests have confirmed the usefulness of this machine. It is applicable for investigations of dams and their foundations. The paper describes the structure of the instrument and presents some test results.
To date, many in situ testing methods and core specimen tests in the laboratory have been carried out for evaluation of rock properties and for classification of rock foundations. These tests include in situ rock shear tests, plate bearing tests and uni-axial strength tests. These tests require large instruments and facilities, and are expensive when large amount of data are desired. Boreholes used in field testing make it Possible to obtain continuous profiles of nearly undisturbed rock. Schmidt Hammer testing is a simple testing method used when a large surface is available. such as an adit tunnel. Adits may be excavated before a construction project is begun. The Schmidt Hammer is widely used for indirect estimation of rock properties. The new testing machine developed by the authors can be used as conveniently as the Schmidt Hammer. It has performed successfully in boreholes with diameter of only 66 mm.
Figure 1 shows the basic principles of The system. An external view of the system is shown in Photo 1. The system can be divided into two parts; borehole probe. and data recording and processing unit. The probe contains an electromagnetic hammer. a drive circuit, an impacting hammer with a small accelerometer, two accelerometers for propagating elastic waves along the borehole wall, and a mechanism for fixing the probe against the borehole wall. The above-ground unit has various functions:
to control 1 the impact of the hammer in the probe,
to amplify and to print or display the response of the impacting hammer and the elastic waves,
to record the digitized data on 3.5 inch floppy disks, and
to transmit the digital data to an external CPU.
2 Experiment Using the system's hammer drive (see Figure 1), fundamental experiments were carried out for several materials having different characteristics. The objective of the experiments was to examine the difference of the response patterns obtained with different materials. All specimens had flat surfaces. The electromagnetic drive was chosen after the comparison among electromagnetic, spring and pneumatic drives. The advantages of the electromagnetic drive is its small size and the fact that strength of impact can be regulated. An accelerometer provides data of higher quality on the behavior of the hammer than that from a small load cell. The characteristics of this piezo-electric type accelerometer are as follows: In order to obtain a comparison of impact behavior on various materials, sixteen materials were used, as shown in Table 1. Figure 3 shows that response accelerations are different in shape for different materials.