A new instrument of 3-component volume borehole strainmeter is developed in China and the primarily function is for borehole strain measurement. The sensor is designed on the base of type TJ-2 volume borehole strainmeter, and it has three independent components which make it being able to observe the strain changes in three different directions. In this paper, the principle, structure, measurement circuit and measuring system of the sensor are presented, and the field observation is also introduced. The filed results show that the measurement resolution of 3-component volume borehole strainmeter is up to 10−9–10−10.
Dr. Shoji (Shoji, 1981) has expressed the rigorous and comprehensive theoretical explaination on the 3- component volume borehole strainmeter. The volume or pressure change of three chambers can be given after little transformation to the analytical formula.
When the stress and strain state of rock are delivered to the probe crust, it is converted to the volume change and finally the stress change based on the incompressibility, and the sensor records the changes. Inside the probe, the elastic cylinder is adopted and the inner circle is a solid stick. The circle is devided into 3 parts with equal volume. Three chambers are distributed in 120 degree, and each one is an independent and closed space as shown in figure 2. However, if the stainless steel plate is too thick„ the volume of the three chambers will be reduced and the sensitivity will be debased. And if the tainless steel plate is too thin, the precision and ndependency will be debased.