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Sensor TYPES

Two types of accelerometers are now being utilized in inclinometer probes: the servo-accelerometer or the recently introduced MEMS accelerometer.

• Servo-accelerometer. The servo-accelerometer probes, commercially available since 1971, have the highest resolution of the available inclinometers on the market. The force-balanced sensing elements detect the change in tilt (from absolute vertical) of the probe that
houses the sensors. The probe contains two biaxial servo-accelerometers and is fitted with two sets of spring-pressured wheels to guide the probe along the longitudinal grooves of the guide casing. A maximum system precision of 0.045 in. per 100 ft (1.1 mm per 30 m) or 1:22,000 is possible with this instrument, but is ordinarily closer to 1:5,000 without corrections for systematic errors. The resolution is nearly linear and constant at inclinations between ± 15° from vertical.
• MEMS Accelerometer. MEMS technology has recently been introduced for inclinometer probes and in-place inclinometers. The probe is the same as for the servoaccelerometer type except that MEMS accelerometers are used instead of servo-accelerometers.

The primary advantages of the MEMS architecture are low power consumption, durability, wireless transmission and low cost. These attributes have resulted in extensive use of MEMS in the automobile industry. The use of MEMS sensors in inclinometer applications
is relatively recent, since 2005. There are limitations to this technology, including temperature sensitivity and related effects, signal noise, and reduced resolution from vertical (±15°). A system accuracy of ±0.05 to 0.25 in. per 100 ft (2.5 to 6 mm per 30 m) or 1:25,000 has been advertised for these probes, but the inclinometer system capability, precision and reliability have not been independently evaluated and demonstrated.

Vertical inclinometers are instruments for measuring relative horizontal displacements affecting the shape of a guide casing embedded in the ground or structure. Inclinometer probes usually measure displacement in two perpendicular planes; therefore, displacement magnitudes and
directions (vectors) can be calculated. The bottom end of the guide casing serves as a stable reference (datum) and must be embedded beyond the displacement zone. Relative displacement over time is determined by repeating measurements at the same depths and comparing data sets.
The guide casing is installed vertically for most applications in order to measure horizontal ground movements. Less common are horizontal installations that use a specialized probe to monitor settlement. In rare cases, guide casings can be installed in inclined boreholes where
access is limited and prevents vertical installations in the preferred locations, or where the feature to be monitored is inclined. However, casing inclination is typically restricted to 30 degrees from vertical due to the potential for errors.

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The inclinometer is used in a variety of applications. Early usage, developed for monitoring landslide movement and slope stability, has evolved to monitoring the impact of excavations on nearby facilities, deformations of structures, and settlement of embankment fills and roadway subgrades.

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Instrumentation of geotechnical structures in the field has been beneficial in evaluating the movements and failures in the structures under real field conditions, as well as assessing the performance of new materials and methods used in the design and construction of geostructures.
Instrumentation that monitors ground deformation response has been used mainly for slope stability studies of natural slopes and performance assessments of earth and pavement structures and the use of new materials such as lightweight embankment materials and construction methods such as mechanically stabilized earth wall systems and deep soil mixing.