The rod extensometer monitors changes in the distance between one or more downhole anchors and a reference head at the borehole collar. Typical applications include:
- Monitoring settlement in foundations.
- Monitoring subsidence above tunnels and mines.
- Monitoring heave in excavations. Monitoring the stability of tunnels and other underground openings.
- Monitoring deformation in abutments and walls.
Components of a rod extensometer include anchors, rods with protective tubing, and a reference head. The anchors, with rods attached, are installed downhole. The rods span the distance between the anchors and the reference head, which is installed at the borehole collar. Measurements are obtained at the reference head with a sensor or a depth micrometer, either of which measures the distance between the top (near) end of the rod and a reference surface.
A change in this distance, found by comparing the current measurement to the initial measurement, indicates that movement has occurred. Movement may be referenced to a downhole anchor that is installed in stable ground or to the reference head, which can be surveyed. The resulting displacement data can be used to determine the zone, rate, and acceleration of movements, or to calculate strain.
The rod extensometer can monitor up to six points along the borehole. The number of monitored points is limited by the size of the borehole, the type of anchor used, the diameter of the rods, and the amount of tubing required for grouting and activating anchors. A two-point extensometer typically requires a 60 mm (2.4 in) borehole; a four-point extensometer requires a 76 mm (3 in) borehole; and a six-point extensometer requires a 96 mm (3.8 in) borehole. Borehole size for packer anchors depends on number of anchors.
Two types of anchors are available: the groutable anchor is suitable for rock and the hydraulic anchor is suitable for soils.
Rods are made of fiberglass or stainless steel. Extensometers using 3/16-inch fiberglass rods can be pre-assembled at the factory, coiled, and shipped to the site, ready for installation. In general, fiberglass rods are easier to install in confined areas, such as tunnels. Fiberglass rods also resist corrosion and are preferred over stainless in alkaline environments. Extensometers using stainless rods must be assembled on site. Stainless rods are stiffer than fiberglass rods of the same diameter and are preferred when compression, rather than extension, is likely, or when longer lengths of rods are required. Protective tubing or protective pipe is installed with the rods to prevent bonding between the rods and grout.
Extensometer measurements can be obtained with a depth micrometer or a sensor. The depth micrometer is suitable when there is easy access to the reference head. However, when access to the reference head is difficult or where real-time or continuous monitoring is required, a sensor is the better option.
For more information
For further reading, download the Rod Extensometer manual. These documents can be viewed and printed with Adobe Acrobat Reader.
Rod Extensometer Specifications
|Groutable Anchor||51815852||Made from rebar, 19 x 365 mm (0.75 x 14.5")|
|Hydraulic Anchor||51703952||Double-acting hydraulic anchor drives 150 (6") mm prongs into soil. 32 x 620 mm (1.25 x 24.5"). Requires hydraulic tubing & pump|
|Fiberglass Rod||51815855||Fiberglass rod has a diameter of 5 mm (3/16") and is supplied in continuous lengths|
|Protective Tubing||51815860||Protective polyethylene tubing is supplied in continuous lengths|
|Rod Completion Kit||51836240||Rod completion kit includes components for top and bottom of rod. Order 1 kit per anchor|
|STAINLESS STEEL RODS|
|Stainless Steel Rod||51704310||Stainless steel rod has a diameter of 6.4 mm (0.25") and is supplied in 10’ lengths, each threaded and tapped for assembly|
|Protective Pipe||51704321||Protective pipe is supplied in 10’ lengths and includes couplings. Requires PVC solvent cement, which can be obtained locally|
|Rod Completion Kit||51836210||Rod completion kit includes components for top and bottom of rod. Order 1 kit per anchor|
|MECHANICAL REFERENCE HEAD|
|Single-Point Head||51836110||Single-point head works with 1 rod and anchor|
|Multi-Point Head||51836120||Multi-point head works with up to 6 rods and anchors. Readings are obtained with depth micrometer|
|Digital Depth Micrometer||51809620||Digital depth micrometer displays readings in inches and millimeters. 150 mm (6") range, 0.01 mm (0.001") resolution|
|ELECTRIC REFERENCE HEAD|
|Single-Point Head||51836130||Single-point head works with 1 rod and anchor|
|Multi-Point Head||51836140||Multi-point head works with up to 6 rods and anchors|
|VW Sensor, 60 mm range||52636305|
|VW Sensor, 100 mm range||52636325|
|Potentiometer, 60 mm range||51836152|
|Potentiometer, 100 mm range||51836154|
|Signal Cable, 4-Wire||50613824||For one VW sensor|
|Signal Cable, 6-Wire||53102900||For one potentiometer|
|Signal Cable, Multicore||Contact Factory|
|Universal Terminal Box||57711600||For use with VW Data Recorder. Not required with data logger. Splashproof fiberglass box is 290 wide x 345 high x 135 mm deep (11.5 x 13.5 x 5.25" )|
|Pipe Adapter||51835170||Optional adapter for anchoring reference head to 76mm (3") diameter steel pipe installed at collar of borehole|
|Flange||51836175||Optional adapter for anchoring reference head to concrete pad at borehole collar. 190 mm (7.5") plastic flange with 152 mm (6") bolt circle|
|Grout Tubing||50721008||Used to deliver grout from the grout pump to the borehole. 12.7mm OD (0.5") polyethylene tubing rated for 30 bar (425 psi)|
|Hydraulic Tubing||51702701||Used to activate hydraulic anchors. 6.35mm (0.25") nylon tubing filled with oil|
|Hydraulic Anchor Tools||51704600||Hydraulic pump with gauge, T-connection, oil volume indicator, 1 gallon of oil, and adapter for filling hydraulic tubing|
|Spare Nut & Ferrule||51703950||Replacement hardware for connecting hydraulic tubing to anchor|
VW Displacement Sensor Specifications
|VW DISPLACEMENT SENSORS|
|Sensor Type||Vibrating wire. A built-in thermistor provides temperature measurements|
|Thermistor||3k Ohms at 25 °C|
|Over-Range||Range + 20%|
|Calibration Accuracy||±0.1% FS|
|Non linearity||<0.5% FS|
|Frequency Range||1650 – 2700 Hz|
|Nominal zero value||1850 Hz|
|Waterproof Rating||Waterproof crackmeter is rated to 16 bar (232 psi)|
|Diameter||12.5mm to 300mm range are 13mm in diameter. 500mm range is 16mm in diameter|