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

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 Anchor51815852Made from rebar, 19 x 365 mm (0.75 x 14.5")
Hydraulic Anchor51703952Double-acting hydraulic anchor drives 150 (6") mm prongs into soil. 32 x 620 mm (1.25 x 24.5"). Requires hydraulic tubing & pump
Fiberglass Rod51815855Fiberglass rod has a diameter of 5 mm (3/16") and is supplied in continuous lengths
Protective Tubing51815860Protective polyethylene tubing is supplied in continuous lengths
Rod Completion Kit51836240Rod completion kit includes components for top and bottom of rod. Order 1 kit per anchor
Stainless Steel Rod51704310Stainless steel rod has a diameter of 6.4 mm (0.25") and is supplied in 10’ lengths, each threaded and tapped for assembly
Protective Pipe51704321Protective pipe is supplied in 10’ lengths and includes couplings. Requires PVC solvent cement, which can be obtained locally
Rod Completion Kit51836210Rod completion kit includes components for top and bottom of rod. Order 1 kit per anchor
Single-Point Head51836110Single-point head works with 1 rod and anchor
Multi-Point Head51836120Multi-point head works with up to 6 rods and anchors. Readings are obtained with depth micrometer
Digital Depth Micrometer51809620Digital depth micrometer displays readings in inches and millimeters. 150 mm (6") range, 0.01 mm (0.001") resolution
Single-Point Head51836130Single-point head works with 1 rod and anchor
Multi-Point Head51836140Multi-point head works with up to 6 rods and anchors
VW Sensor, 60 mm range52636305
VW Sensor, 100 mm range52636325
Potentiometer, 60 mm range51836152
Potentiometer, 100 mm range51836154
Signal Cable, 4-Wire50613824For one VW sensor
Signal Cable, 6-Wire53102900For one potentiometer
Signal Cable, MulticoreContact Factory
Universal Terminal Box57711600For 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 Adapter51835170Optional adapter for anchoring reference head to 76mm (3") diameter steel pipe installed at collar of borehole
Flange51836175Optional adapter for anchoring reference head to concrete pad at borehole collar. 190 mm (7.5") plastic flange with 152 mm (6") bolt circle
Grout Tubing50721008Used to deliver grout from the grout pump to the borehole. 12.7mm OD (0.5") polyethylene tubing rated for 30 bar (425 psi)
Hydraulic Tubing51702701Used to activate hydraulic anchors. 6.35mm (0.25") nylon tubing filled with oil
Hydraulic Anchor Tools51704600Hydraulic pump with gauge, T-connection, oil volume indicator, 1 gallon of oil, and adapter for filling hydraulic tubing
Spare Nut & Ferrule51703950Replacement hardware for connecting hydraulic tubing to anchor

VW Displacement Sensor Specifications

RangeLengthPart Number
Sensor TypeVibrating wire. A built-in thermistor provides temperature measurements
Thermistor3k Ohms at 25 °C
Over-RangeRange + 20%
Resolution0.025% FS
Calibration Accuracy±0.1% FS
Non linearity<0.5% FS
Frequency Range1650 – 2700 Hz
Nominal zero value1850 Hz
Waterproof RatingWaterproof crackmeter is rated to 16 bar (232 psi)
Diameter12.5mm to 300mm range are 13mm in diameter. 500mm range is 16mm in diameter