Borehole Extensometers
Introduction  |  Borros Anchor |  Increx |  Magnet Extensometer |  Settlement Hook |  Sondex |  Rod Extensometer

Applications

Borehole extensometers are used to measure movements of soil and rock along the the axis of a borehole. A wide range of extensometers is available, each designed for a particular application. In general, typical applications include:

• Monitoring settlement or heave in excavations, foundations, dams, and embankments.
• Monitoring subsidence above mines, tunnels, and other underground openings.
• Monitoring convergence in tunnel walls and other underground openings.
• Monitoring movements in rock slides, walls, and abutments.
• Monitoring consolidation of soil under embankments and surcharges.
• Monitoring compression of pile and soil under pile.

Types of Extensometers

Borros Anchor Settlement Point
The 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.

Increx Mobile Extensometer
The Increx mobile extensometer is used in rock or stiff soils for high-resolution measurements of deformation along the axis of the borehole. It consists of a number of brass rings that are positioned at one-meter intervals along inclinometer casing, and a probe and readout that are used to measure the distance between rings.

Advantages: Provides high-resolution measurements at one meter intervals, can be operated in any orientation, supplements inclinometer measurements.

Limitations: Requires a skilled man on site; cannot be monitored remotely.

Magnet Extensometer
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: Requires a skilled man on site; not easily automated, difficult to install more than 15 or 20 magnets, vertical installation only.

Settlement Hook
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: Works only with telescoping casing. The inexpensive settlement hook requires a skilled operator.

Sondex
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: Requires a skilled man on site; cannot be automated, vertical installation only.

Rod Extensometer
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.

Limitations: Limited measurement range (50 to 100 mm).

Tape Extensometer
This extensometer is used on the walls of structures and underground openings to monitor convergence.