What is a GTSM Strainmeter?
A GTSM strainmeter measures the change of diameter of a borehole in at least three directions usually at 120 degrees from each other. The instrument is grouted into the borehole using an expansive grout to ensure it is always under compression. Areal strains are characterized by observations which are equal on all gauges. Shear strains are identified by examining the differences between gauge responses. There can be two kinds of shear strain. The GTSM strain meter measures all three kinds of strain in the plane perpendicular to the borehole, and this requires a minimum of three gauges. In practice, GTSM strain meters have a fourth gauge included in case one of the primary gauges fail.
