Updated: Sep 26
Among the different sorts of geotechnical instruments used for measuring the soil behavior, there is one especially used in civil construction: we are talking about the inclinometers. They can be used for measuring relative horizontal displacements of the ground, concrete structures deformations and settlement of embankment fills.
The instrument consists of a casing (of aluminum in case that it is embedded in concrete, or ABS plastic when inserted into a borehole), an inclinometer probe with a cable attached, and a readout unit for storing the readings.
An Inclinometer probe contains two biaxial servo-accelerometers (MEMS-accelerometers nowadays) and is fitted with two sets of spring-pressured wheels that help the probe slide along the casing.
When measuring, one holds the probe every 0,5m to take a reading so that the sensors detect the change in tilt (from the vertical axis) in two different perpendicular planes (A and B).
It’s important to consider some good practices to install the inclinometer casing so that we can ensure that the readings will be taken properly:
1. Ideally, an inclinometer casing installed in the ground, needs to be inserted by ensuring that the bottom end of the casing is anchored below the layer of expected soil movement (i.e. rock or soil layer with no displacements expected). The reason is that one will measure the casing relative inclination every half meter from bottom to top in order to represent the cumulative deformations thus the bottom end of the casing is considered a “fixed” point.
However, this is something that hardly ever happens as sometimes it’s quite expensive to drill those extra meters of soil to get the rock mass (or harder soil). Therefore, in this case it’s recommended to measure the top end of the casing with conventional topography so that we can monitor the displacement (in absolute) of the casing.
2. When placing the casing, it’s important to carefully fix it by ensuring that one of the grooves axis is perfectly oriented to the expected horizontal movement direction (i.e. towards the excavation pit, the tunnel or landslide natural direction).
3. When installing the casing into the ground (drill), the annular space is grouted with a mixture of bentonite, cement, and water (proportions depend on the soil hardness and are usually defined in the project specifications). This grout is meant to reproduce “similar” ground conditions of the soil around the drill. Therefore, when the soil is apparently suffering deformations, all these stress changes are transmitted to the grout and hence the inclinometer casing deforming accordingly.
In some cases, in-place inclinometer (IPI) are used instead. In this case, one or more probes are positioned at fixed locations in the casing. IPI probes can be combined with automatic data acquisition systems (ADAS) for continuous monitoring in real time. civil engineering, geotechnical, software engineer, geotechnical engineering, borehole
Geotechnical Instrumentation Engineer