Improvement of the Laboratory Calibration Experiment Method for a Borehole Extensometer Measurement Using a Straddle-sliding-packer System

A borehole extensometer using a conventional straddle-sliding-packer system has been developed as a method for evaluating the shear compliance of a fault. In this study, we re-evaluated the laboratory calibration experiment method for determining the compliance of the packer system which is required when applying the borehole extensometer method. The packer compliance measured by the previous method using a single packer was compared with that derived from a method using double packers, where the latter method more reproduces the actual in-situ test conditions. There is a difference between the results using a single packer and double packers, and the results showed that the previous method using a single packer may over- or under-estimate the packer compliance, and the difference between those from the methods using a single packer and double packers is greater with smaller packer pressure. We also found that the packer compliance depends on the packer pressure and increases with decreasing packer pressure. Therefore, it is desired to conduct laboratory calibration experiments using double packers at packer pressure close to the in-situ test conditions. The knowledge obtained by this study improves the reliability of the borehole extensometer measurement using a straddle-sliding-packer system.