Inclinometers – Measurement of Displacement Perpendicular to the Line
In many parts of Croatia, landslides are a natural occurrence. In the urbanized areas below the slopes, the condition of instability and conditional stability of the slopes is a constant presence. To timely detect the potential danger of such phenomena and prevent them in due time, continuous monitoring is necessary, and one of the most well-known and widely used methods is displacement measurement using inclinometers.
Inclinometers have a wide range of applications in civil and geotechnical engineering and are used for monitoring ground displacement and deformation. An inclinometer is a geotechnical device used to measure the angle of inclination or deformation of any structure. The measurement of horizontal ground displacements in a vertically positioned inclinometer is based on measuring the relative angles of rotation within the embedded PVC pipes.
It is applied as monitoring for ground displacement, structural interactions with the ground, embankments, and similar constructions. This can include the following:
- monitoring the inclination of surfaces and sliding planes
- monitoring underground lateral movements, inclinations within settlements, dams, levees, open pits, etc.
- monitoring the deflection of diaphragms, piles, retaining walls, and various other structures
Measuring Equipment consists of two parts: the inclinometer casing and the inclinometer measurement system.
The inclinometer casings that are installed must be fitted with a reference mark. In vertical boreholes, the reference direction should align with the expected displacements (e.g., towards the construction pit, in the direction of the slope's base in landslide areas, etc.). The reference mark should be permanently indicated in an appropriate manner (e.g., by engraving, although most casings used are already factory-marked with a red line, in which case no additional marking is necessary).
The casing, like other equipment, should be handled carefully. Once the casing is introduced into the borehole, it is prohibited to attempt to reorient (rotate) it.
The space between the casing and the borehole wall needs to be filled to ensure more accurate displacement measurements. A cement-bentonite suspension is used as the fill material between the casing and the borehole wall. In certain cases, granular fill (based on gravel, sand, glass beads, etc.) may also be used. Granular fill can be utilized when measurements are taken in fractured rock, karst, or highly permeable gravelly soils. Before installation, the casing should be sealed at the joints with adhesive tape, and protective caps should be placed at the bottom and top.
The top of the inclinometer is sometimes protected with a cap secured by a padlock and key (in case there is a risk of unauthorized access), and a concrete block must be placed on top to prevent the inclinometer's tip from shifting.
The inclinometer probe contains vertical sensors that measure the angle formed between the probe body and the vertical (vertical inclinometers) or horizontal (horizontal inclinometers).
To establish a reference (zero) measurement, it is necessary to perform two measurements in the normal (0˚) and opposite (180˚) directions, yielding repeatable results. The measured values should fall within acceptable measurement tolerances. For a vertical inclinometer, acceptable measurement values for depths up to 30 meters are ±2 mm.
Data Processing
Data processing is conducted in the office on a personal computer. The principle of measuring displacement in vertical inclinometers through the measuring line is based on the relative displacements between adjacent measuring points, determined by changes in angles α and β.
The vertical inclinometer probe consists of a casing housing two tilt sensors (in two mutually perpendicular planes). These planes are designated as Plane A and Plane B. The measured values yield displacement values, and this process is automated within a standardized computer program, Klion, produced by Sisgeo.
Data processing involves selecting the reference (zero) measurement and reviewing measurements over time.