Extensometers are used to measure movements of soil and rock along a single axis. Applications for extensometers include:
- Monitoring settlement in excavations, foundations, and embankments.
- Monitoring subsidence above mines and tunnels.
- Monitoring movements in rock slides, walls, and abutments.
- Monitoring consolidation of soil under embankments and surcharges.
- Monitoring compression of piles and soil under piles.
- Monitoring spread in embankments.
- Monitoring convergence in underground openings, such as tunnels.
The Rod Extensometer consists of anchors set at specified depths, rods inside protective tubing, and a reference head. Measurements are taken at the reference head by micrometer or by an electric sensor.
Advantages: Can be automated, can be read remotely, works in any orientation, can measure multiple points.
Limitations: Limited measurement range (50 to 100 mm).
The Magnet Extensometer consists of a series of magnets that are installed with an access pipe. The magnets are anchored at specified depths. Measurements are taken by lowering a probe through the access pipe to detect the depth of the magnets.
Advantages: Can monitor large settlements; works with inclinometer casing and can supplement inclinometer data, relatively easy to operate, indicates incremental settlements.
Limitations: Cannot be automated, practical limit of 15 or 20 magnets, vertical installation only.
The Sondex system consists of a series of rings attached to a flexible corrugated pipe. Measurements are lowering a probe through an inner access pipe to detect the position of the rings.
Advantages: Can monitor large settlements; works with inclinometer casing and can supplement inclinometer data, indicates incremental settlements, no limitation on number of measured rings.
Limitations: Cannot be automated, vertical installation only.
The Borros Anchor Settlement Point settlement point is used to monitor settlement of soil under an embankment. It consists of an anchor and and two concentric riser pipes that are extended up through the embankment. Measurements are made with a graduated tape and optical survey.
Advantages: Simple to install and inexpensive.
Limitations: Provides only measure of total settlement; requires a man on site; extensions to pipe must be recorded carefully; top of pipe must be surveyed; anchor works best in soft clays, vertical installation only.
The Settlement Hook is used to monitor settlement in telescoping inclinometer casing. Measurements are taken by lowering the hook device through the casing. The hook is catches on the telescoping joints and a depth reading is obtained from a steel tape.
Advantages: Works with inclinometer casing, nothing extra to install. The USBR-type settlement hook is easy to use and delivers reliable readings.
Limitations: Cannot be automated, works only with telescoping casing, requires a careful operator.
The soil strainmeter is used to monitor spreading of embankment material. It is typically installed in trenches.
The tape extensometer is used to monitor convergence in underground openings, such as tunnels.