A test system using "Rock Test Hammer" is often used to evaluate rock mass properties, simply. The Strike Response Value is measured by using this machine, and the modulus of elasticity is expected. So, in order to apply this principle to the borehole investigation, we develop a new machine "Borehole Hammer"(Figure 1). With regard to this machine, the laboratry testing using rock specimen was carried out and we have a successful results. However, in-situ testing has hardly been made. So we made the in-situ testings using "Borehole Hammer" and borehole expansion tests in order to grasp the applicability of "Borehole Hammer". As the results, the relationship between strike response value obtained by "Borehole Hammer" and elasticity tends to linear and the applicability of "Borehole Hammer" to the in-situ rock masses is clarified at this test yard.
In the case that we examine the foundation a civil engineering construction, it is necessary to grasp the engineering property. These values are obtained by many kinds of in-situ rock tests such as plate bearing test and in-situ shearing test, but these tests are so expensive that we cannot make enough tests to estimate the rock mass properties of the objective region for design. Because of these situations, we have desired the establishment of easier and cheaper in-situ rock test system. A test system using "Rock Test Hammer" is often used to evaluate rock mass properties, simply. Strike response value is measured by using this machine, and the modulus of elasticity is expected. So, in order to apply this principle to the borehole investigation, we develop a new machine "Borehole Hammer"(Figure. 1). With regard to the this machine, the laboratry testing using rock specimen was carried out and we have a successful results. ¹) However, in-situ testing has hardly been made. This paper introduces the outline of the Borehole Hammer and describes the methods and the results of the in-situ testing by the use of this hammer.
The Borehole hammer is broadly divided the insertion part into the borehole (sonde part) and the data collection part. The external appearance of this hammer is shown in Figure 1.
The sonde part consists of the hammer part which strikes the borehole wall, the reception part which receives elastic waves that hae been generated by the strike, and the fixing unit which presses the sonde against the borehole wall before the striking test. The Strike Response Value is measured by the hammer part. In order to obtain the more accute Strike Response Value, it is desirable that strike energy is constant. As the result, a hammer has been conceived which has a construction of driving it by elctro-magnetic forces as shown in Figure 2. Using this mechanism, it has become possible to strike the borehole wall at constant striking forces regardless of whether or not there is water in the borehole. The reception parts are installed at two locations consisting of velocity sensor. At the rear of the sensor, a spring is fitted to improve its contant to the borehole wall. For the fixation unit, a system of the two rigid arms which can be opened and be closed by motor driving has been adopted. The fixing devices have been installed, one at top of the sonde, and the other at its bottom, this sonde can be used for the borehole with a diameter of about 56 to 76 mm.
The data recording device consists of measuring unit CRT display, printer and recorder. These component units are encased in a single accommodating body.