Inclinometers: Geotechnical Measurement

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What is an inclinometer?

An inclinometer is an instrument that uses the effects of gravity to measure its own tilt, or the angle of the slope at which it is lying, and by extension to measure changes in the substance supporting it—usually the ground or a part of a structure. Inclinometer readings are normally given in degrees of the angle of deviation from a vertical or horizontal plane, as a grade percentage, or as absolute deviation in millimeters or inches.

What are inclinometers used for in geotechnical monitoring?

Because inclinometers can detect their own orientation relative to gravity, they can also identify movement in the material they are installed in by comparing several readings taken at different points in time. In geotechnical monitoring, this information is used to detect many kinds of movement and deformation, both in the ground and in built structures. These movements include lateral movement, subsidence, and heave in the ground, as well as settlement and deflection in structures. Monitoring these movements is important in tracking stability during construction or repair work as well, and also in following long-term stability once work is complete.

Inclinometers are used in dams and retaining walls of all kinds—from the smallest earthfill retaining walls to the largest concrete dams—to monitor stability in the structure and the ground that supports it.

Roadways, bridges, and tunnels also have applications for inclinometers, where they can be used to monitor the integrity of the box girders that support a bridge’s deck or alert to potential subsidence in the ground below an asphalt road surface.

Inclinometers are used in any project that involves excavation, such as open-pit mines or landfill sites. In these cases, inclinometers monitor deformation in the ground surrounding the excavated area in order to detect stability problems before they occur.

How do inclinometers work?

Geotechnical inclinometers use an accelerometer to measure tilt. In an accelerometer, the relative position of two electrodes—one fixed to the instrument’s body and one suspended in a way that allows it to move freely along one axis—produce a capacitance. This capacitance when the instrument is level—either vertically or horizontally, depending on the application—is used as a baseline. As the inclinometer tilts, and as the floating electrode moves along its axis, the change in capacitance is used to calculate the instrument’s orientation.

Most modern inclinometers used in geotechnical measurement use what are known as micro-electrico-mechanical systems (or MEMS). This technology allows for the production of tools on a literally microscopic scale—with components between 1 and 100 micrometres in size. While the principles are the same, MEMS inclinometers are much more sophisticated than the simple two-electrode model described above. This increased sophistication brings with it an increase in the precision and accuracy of the inclinometers’ readings.

Inclinometer systems

The inclinometer itself is only one part of a larger system that turns the electric signals it produces into useable information. Although the specifics of an inclinometer system may vary according to the application, there are some essential components. The system starts with a probe, which contains the inclinometer. The probe is installed in a rigid casing, which both protects the probe and transfers deformations in the ground to its sensors. A cable suspends the inclinometer in its casing and provides power to units that do not run on batteries. Portable inclinometers may also contain a data logger, which processes and stores the readings, in the same instrument. Other systems may use external data loggers housed in separate instruments connected by cable to the probe.

What to look for in an inclinometer

There are a number of factors to consider when choosing the right inclinometer for your particular geotechnical application:

Single versus dual axis. Inclinometers normally measure tilt along a single axis or along two perpendicular axes. Single-axis inclinometers are more common in horizontal installations.

Manual inclinometers: Manual inclinometer probes are designed for manual data collection.

In-place inclinometers (IPI): IPIs are intended to be installed on site for continuous monitoring are called in-place inclinometers

Vertical or horizontal installation: Inclinometers are designed for either vertical or horizontal installation. Horizontal instruments are used to measure settlement and heave, while vertical instruments are used for lateral movement.

Measurement range: Given in degrees, the measurement range tells you how much deviation the instrument is calibrated to detect. A measurement range of ±30° means that the inclinometer will give accurate readings for deviations of up to 30° in either direction from the baseline.

Resolution: Resolution represents the smallest change in measurement that the inclinometer will register.

Accuracy: An inclinometer system’s accuracy is the maximum difference between the displayed measurement and the actual displacement.

RTS Instruments’ inclinometers

MEMS Digital In-Place Inclinometer System. This industry-leading inclinometer system is made for continuous monitoring of lateral displacement.

Digital Inclinometer System. This all-in-one portable system contains everything needed to take manual lateral-displacement readings.

Digital Horizontal Inclinometer System. For monitoring vertical heave and settlement displacements.

Digital Inclinometer Spiral Sensor. This sensor is designed to measure helical deformation in borehole casing.