Settlement cells & extensometers are used for a wide range of applications including the monitoring of local settlement for the control of road embankments, dams, and other earth works, the measuring of settlement or heave of building foundations, oil tanks, piers, and other structures, and the control of mining induced subsidence. They provide an easy to-read, high-resolution means of measuring settlement.
Settlement Cells
Applications
Settlement cells are used to monitor settlement and heave in soils. Typical applications include:
- Monitoring settlement or heave in embankments and embankment foundations.
- Monitoring subsidence due to tunneling and mining.
- Monitoring consolidation under storage tanks.
- Monitoring settlement due to dewatering or preloading.
- Monitoring settlement in fills.
Operation
A settlement cell consists of a liquid reservoir, liquid-filled tubing, and the settlement cell, which contains a pressure transducer. One end of the tubing is connected to the settlement cell, which is embedded in fill or installed in a borehole. The other end of the tubing is connected to the reservoir, which is located away from the construction area.
The transducer measures the pressure created by the column of liquid in the tubing. As the transducer settles with the surrounding ground, the height of the column is increased and the settlement cell measures higher pressure. Settlement is calculated by converting the change in pressure to millimeters or inches of liquid head.
Advantages
- The reservoir and readout station can be located away from the construction area. The cell and tubing are buried and do not interfere with construction activities.
- Vibrating wire cells can be automated. Tubing lengths of up to 300 m (1000 feet) are permissible with VW cells.
- Pneumatic cells are less expensive and are a good choice in lightning-prone regions.
Limitations
- Temperature changes in reservoirs and above-ground tubing can affect the repeatability of readings.
- Non-vented VW cells require corrections for barometric pressure, so barometric pressure must be recorded.
- Vented VW cells automatically compensate for changes in barometric pressure, but desiccant chamber must be maintained.
- Pneumatic cells cannot be automated and tubing length should be less than 100m (300 ft). However, barometric compensation is not required, since this is done when the readout is zeroed.
For more information please refer to datasheet.
VW Settlement Cell
Extensometers
Applications
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.
Borehole Extensometers
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, can measure multiple points.
- Limitations: Limited measurement range (50 to 100 mm).
For more information please refer to datasheet.
Rod Extensometer
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: Cannot be automated, practical limit of 15 or 20 magnets, vertical installation only.
For more information please refer to datasheet.
Magnet Extensometer
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: Cannot be automated, vertical installation only.
For more information please refer to datasheet.
Sondex System
Borros Anchor Settlement Point
- 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.
For more information please refer to datasheet.
Borros Anchor
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: Cannot be automated, works only with telescoping casing, requires a careful operator.
For more information please refer to datasheet.
Settlement Hooks
Special-Purpose Borehole Extensometers
Settlement Extensometer
This borehole extensometer is a special purpose, sub-surface extensometer with a range of 25 inches or more and an 4-20mA output. It consists of an anchor set in stable ground, rods inside protective pipe, and a waterproof reference head. Special order only.
For more information please refer to datasheet.
Settlement Extensometer
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. The Increx system must be ordered from Holland and requires a long lead time.
For more information please refer to datasheet.
Increx System
