Abstract
We investigated the mechanical behavior of the JMA borehole type volume strainmeter by focusing on its interaction with its surrounding medium.
Through a finite element stress analysis of a three dimensional axisymmetrical model containing the cylindrical vessel of a volumetric strainmeter, the state of stress around the vessel was found to vary markedly with the elastic modulus of the surrounding medium under lateral compression. If Young's modulus of the medium becomes less than several persent of that of the vessel, appreciable stress concentrations arise horizontally in the cementing material (mortar) near the upper and lower ends of the vessel, and a tensile stress up to several times the applied one arises vertically in the whole of the mortar. There is a possibility that cracking by tensile fracture occurs in the mortar.
Next, using sensitivities for horizontal compression and atmospheric pressure coefficients in the volumetric strainmeter which we calculated for various elastic moduli of the surrounding media by the FEM analysis, we estimated the elastic moduli of the media by comparing them with the observed ones. The estimated ratios of Young's modulus of the media to that of the vessel range from several percent to as high as several tens percent. If we classify the quality of the surrounding media of the stations into three classes, namely, A, B, and C, according to Young's modulus, the stations in the Tokai district fall into class A or B, and those in the southern Kanto district mostly into class B or C. This classification is largely consistent with the quality of the cored samples in each of the stations. On the variations of the observed volumetric strain records from which long-term trends are subtracted, in the stations of classes B and C we can observe marked effects of precipitation and irregular pulse- or step-like variations, whereas in the stations of class A we cannot observe any marked variations except in the Yugawara and Higashiizu stations.
