Slope Inclinometers, Borehole Inclinometers, Slope Indicators

Slope Inclinometers - Displacements Monitoring

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. Slope inclinometers are the most important source of geotechnical data during construction (Erikson et. al., 1992). Slope inclinometers are widely used to measure lateral soil and wall movements in slurry wall and deep excavations (Fig. 3.4). They fall into two categories, 1) probe inclinometers and 2) fixed- in-place inclinometers (Laplante, 1998). Only probe inclinometers have been used in all the studied projects. In either type, a casing with four orthogonal grooves is installed in a borehole in the ground or within a retaining wall. The grooves are designed to fit the wheels of the inclinometer probe. The angle of the probe from the vertical axis is measured in both directions with the use of a sensitive gravity pendulum, tiltmeters, or a servo accelerometer. The deflections are calculated automatically from this angular measurement and from the distance between the wheels (which is known). Deflections are always compared to an initial reading since the casing may not be installed in the vertical position.

Typically measurements are reported at two foot intervals and a process requires up to 15 minutes (Fig. 3.7). The reduced data maybe erroneous and certainly will be less accurate if the measurements are taken at steps greater than the typical two-foot distance between the probe wheels (W3). For better results it is advisable that the inclinometer casing extends to a firm stratum (or to a great enough depth) so that the inclinometer base does not move. For example, walls in floating excavations tend to translate at their bases (B3, B7) and these movements will not be picked up if the inclinometers do not extend beneath the base of the wall (B6). Movements during slurry wall construction can not be measured when all the inclinometers are installed within the diaphragm wall. Thus in order to get a more realistic view of soil movements during slurry wall construction it is suggested that a few inclinometers be installed within soil. In ideal conditions, soil inclinometers should be installed before any construction takes place.

The limitation of inclinometers is that they measure only horizontal ground deformations. Deformation data is necessary to appease abutters worried about potential damage to their property (Laplante, 1998). Slope inclinometers are essential for monitoring deep seated movements especially for land slide stabilization and other slope stability related issues.