Rod Extensometer

Rod Extensometer


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.
Rod Extensometer


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.

Multipoint Monitoring

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.

schematic diagram of rod extensometer in use

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

Please download the Rod Extensometer datasheet and the VW Displacement Transducer datasheet.

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 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
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
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

Range Length Part Number
12.5mm 195mm 92636135
25mm 243mm 92636255
50mm 275mm 92636055
75mm 369mm 92636075
100mm 400mm 92636335
150mm 530mm 92636155
200mm 660mm 92636205
300mm 917mm 92636305
500mm 1433mm 92636505
Sensor Type Vibrating wire. A built-in thermistor provides temperature measurements
Thermistor 3k Ohms at 25 °C
Over-Range Range + 20%
Resolution 0.025% FS
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