Slope inclinometers are geotechnical instruments used to measure horizontal displacements along various points on a borehole. For this reason, sometimes they are also called borehole inclinometers or simply inclinometers. These are ideally suited to long-term, precise monitoring of the position of a borehole over its entire length. By making a series of readings over time, it is also possible to monitor the rate of movement. The components of the inclinometer are a plastic casing with four longitudinal grooves cut in the inside wall, and a probe that is lowered down the casing on an electrical cable with graduated depth markings (Figures 5 &6). The probe contains two mutually aligned accelerometers, to measure the tilt of the probe in mutually perpendicular directions. The probe is also equipped with a pair of wheels that run in the grooves in the casing and maintain the rotational stability of the probe. It is required to extend the borehole below the depth of movement so that readings made from the end of the hole are referenced to a stable base. The depth at which shear movement is detected by the slope inclinometer is the depth of the failure surface. The portion of the casing that has not sheared, represents the area above and below the failure surface, if there is one failure plane impacting the casing.
Figure 5: Various components of inclinometers
Figure 6: Inclinometer for measuring borehole deflection: (a) arrangement of grooved casing and inclinometer probe, (b) principle of calculating deflection from tilt measurement (Dunnicliff, 1993)