August 07, 2019
Extensometer or extension meters are extensively used to measure the change in the length of an object. They are designed for strain measurements or to carry out tensile tests.
Geotechnical instrumentation and monitoring is a vast field that includes several monitoring instruments such as piezometers, tiltmeters, strain gauges, beam sensors, extensometers, etc.
Since extensometers are one of the most significant monitoring instruments, let us give you a better insight into its working principle, construction, specifications, and application areas.
An extensometer is a high-precision instrument specially designed for geologists or civil engineers to measure the elongation of a material under stress. This instrument is perfect for tensile tests.
It can also determine yield strength, tensile strength, yield point elongation, strain-hardening exponent, and strain ratio.
Besides this, extension meters have a huge scope of work in the geotechnical field. They are available in various types and sizes depending upon application area.
Usually, the extensometers are classified as contact, non-contact, laser, and video extensometers.
However, the working principle of extension meters depends on their type as well as the application area. Encardio Rite deals with a wide range of extensometers, including magnetic, electrical, mechanical, and soil extensometers. All of these extension meters have different working principles.
The magnetic extensometer system is designed to measure settlement or heave of the soft ground under the influence of loading or unloading due to the construction of embankments, fills, buildings, and structures.
Lateral movement at any level within a soil mass may be assessed by monitoring the location of magnetic targets positioned over a near-horizontal access tube. The lateral ground movement may be in abutments, foundations or embankments, and a consolidation-induced settlement in embankments and foundations.
The electrical borehole extensometer incorporates a vibrating wire transducer for measurement of displacement. A groutable reinforced bar anchor (1) is attached to fibreglass (2.a) or AISI 410 stainless steel (2.b) connecting rods of appropriate length, as specified in the design.
Fibreglass connecting rods are supplied from the factory in single lengths. The connecting rods are enclosed inside heavy-duty protective tubing to allow for free movement. The displacement sensor is coupled to the connecting rod using link plate and lock nut.
The borehole extensometer measures the extension (displacement) which takes place with time in a borehole or several boring holes in a rock mass. It essentially consists of one or more anchors and a reference plate. The anchor or anchors are set in the same borehole or different boreholes drilled adjacent to the first borehole.
They help to accurately measure the distance between the various anchors with respect to the reference plate and monitor their relative displacement with time. It is usually assumed that the deepest anchor is in the stable ground and so any change in the anchor spacing gives information about the settlement of foundation taking place.
The soil extensometer uses a vibrating wire sensor for monitoring displacement. The system consists of a sensor assembly with flanges that are mounted with adaptors, adjustment unit, sockets and extension rods between two anchors to monitor the horizontal movement of surrounding soil.
The system is enclosed in telescopic PVC tubing with proper ‘O’ ring seals to eliminate friction between the rods and surrounding soil and to prevent any ingress of water.
The extensometer system is supplied with different gauge lengths (gauge length is the distance between two anchors).
Several extensometer units can be connected in series to measure incremental displacements over large distances. The movement of relative position between two anchor channels at the ends of the soil extensometer is representative of the mass movement.
The relative movement between the anchors causes a change in the output of the vibrating wire sensor. This output can be measured by the Encardio Rite model EDI-51V portable read-out unit/datalogger or monitored by a remote model EDAS-10 data acquisition system.
The initial reading is taken as the datum. The difference between subsequent readings and initial readings gives the magnitude of the movement.
Figure 1: Typical installation of MPBX
Encardio Rite deals with some of the most reliable, high-precision, and robust extensometers in the industry.
The Encardio Rite Model EDS-63U-D mechanical borehole extensometer is ideally suited for upward, downward or inclined boreholes. It is a precision instrument designed to measure deformation of rock mass and adjacent or surrounding soil.
Together with anchor bolt load cell and tape extensometer, it is essential equipment for investigation and monitoring of foundations, slopes & embankments and for studying the behaviour of rock around underground cavities, tunnels and mines.
The groutable reinforced bar anchor (1) is attached to SS 410 stainless steel (φ 8 mm) or fibreglass (φ 6 mm) connecting rods (2) of appropriate length, as specified in the design.
The anchors are inserted into borehole along with their respective connecting rods and fixed in position by cement grout. The connecting rods are protected from the cement grout by enclosing in heavy-duty protective tube (3), thus allowing for free movement and displacement.
The extension head consists of a reference head/flanged housing (6) and a protective cover (13). The flanged housing is grouted concentric with the borehole. The reference head has four grouting anchors (11) for fastening the extensometer head to the surface of the borehole.
The protective tube (3), grout tube (5) and vent tube (4) are secured to the reference head/flanged housing with rubber washers (8) and hollow bolts (9) to make the system leak proof. A reference plate (12) is used to measure the displacement of the anchor from the reference button (10) fixed on the connecting rod (2). Encardio Rite uses the convention that depth of anchor is calculated as the distance from the mouth of the borehole to the near end of the anchor.
The measurement with Model EDS-63U-D borehole extensometer is economical, very reliable and is preferred at locations where access to the mouth of the borehole is easily available.
Displacement readings are taken by measuring the depth of the near end of the connecting rod from a reference plate provided at the mouth of the borehole. A digital calliper/micrometre depth gage with a resolution of 0.01 mm is used to take the readings.
The Model EDS-63U-D borehole extensometer with up to three positions can be installed in a 3” φ NX (76 mm φ) borehole; with up to six positions, it can be installed in a 100 mm φ borehole.
It is not possible to install expandable anchors effectively at locations where the rock formation is soft. Groutable anchor is ideally suited for such applications. In a borehole, it is usual to use a 20 mm φ, 500 mm long groutable anchor.
Don’t forget to check out this video on Model EDS-63U-D mechanical borehole extensometer:
The Encardio Rite Model EDS-64U-D single position mechanical borehole extensometer is suitable for 50 mm upward, downward or inclined boreholes. It is a precision mechanical instrument designed to measure the deformation of rock mass and adjacent or surrounding soil.
The construction, working principle, and specifications of Model EDS-64U-D Mechanical Borehole Extensometer System remain like that of Model EDS-63U-D Mechanical Borehole Extensometer System except that it is suitable for 50 mm upward, downward or inclined boreholes.
The Encardio Rite Model EDS-70V multi-point borehole extensometer is a precision instrument designed to help civil engineers and geologists in the measurement of deformation of rock mass & adjacent or surrounding soil.
Together with anchor bolt load cell and tape extensometer, it is essential equipment for investigation & monitoring of foundations, slopes & embankments and for studying the behaviour of rock around underground cavities, tunnels and mines. The borehole extensometer is an important instrument especially in the investigation of underground cavities & landslides.
The Model EDS-70V borehole extensometer incorporates a vibrating wire transducer for measurement of displacement. A groutable reinforced bar anchor (1) is attached to fibreglass (2.a) or AISI 410 stainless steel (2.b) connecting rods of appropriate length, as specified in the design.
EDS-70V can be ordered from the factory for monitoring settlement at up to six depths. It is suitable for a 3” Ф (~76 mm) borehole for up to 1~3 points & 4” Ф (~102 mm) borehole for 4~6 points.
The diameter at the mouth of the borehole is increased to 90 mm and 125 mm to house the head assembly. Spacer adjustment provided optionally for movement more than the range in case of heave/settlement.
Model EDI-51V portable readout logger is used to measure the displacement of Model EDE-VXX vibrating wire linear displacement transducers. The readings can also be monitored remotely using Model EDAS-10 data acquisition system.
Model EDS-70M version of this borehole extensometer is available for taking the reading with a mechanical dial gauge. This model is supplied without the vibrating wire sensors.
At a later stage, this extensometer can be converted to take reading electronically by installing vibrating wire displacement sensors.
Model EDE-VXX vibrating wire linear displacement transducer incorporates a vibrating wire sensor having a stroke of 50 mm (EDE-V05), 100 mm (EDE-V10) or 150 mm (EDE-V15). It converts the mechanical displacement to an electrical output in frequency form. This frequency output can be read or logged by Encardio Rite model EDI-51V remote digital readout unit or Encardio Rite model EDAS-10.
The Encardio Rite Model EDS-71V electrical borehole extensometer is a precision instrument designed to help civil engineers and geologists in the measurement of deformation of rock mass and adjacent or surrounding soil.
The construction, operating principle, applications, and specifications of Model EDS-71V borehole extensometer are similar to that of the Model EDS-70V/EDS-70P Electrical Borehole Extensometer.
The Encardio Rite Model EDS-91V/H magnetic extensometer is used to measure vertical settlement or lateral movement depending on the application. Settlement or heave at various levels within a soil mass may be assessed by monitoring location of magnetic targets which have been positioned over a near-vertical access tube.
Two types of access tubing with telescopic coupling are available for the installation. For monitoring settlement only, regular access tubing is used. For monitoring lateral displacement along with the settlement, inclinometer casing with telescopic coupling is used.
For vertical installation in an earth fill, 3 m long tubes are connected, one by one, with telescopic couplings to keep pace with the fill operation. The magnets are located centrally on the lower diameter tube.
For horizontal installations, similar telescopic access tubes are laid at the base of the trench, with magnet plates embedded in the trench floor. For monitoring a dead-end hole, a pull cable with optional reel, cable return pipe and a dead-end pulley assembly is available. Alternatively, for small length holes, a push rod may be used.
EDS-91/1.1:PVC access tubing, 26 mm i.d., and 32.8 mm o.d., 3 m long.
EDS-91/1.2:Extension/compression PVC access tubing for above, around 35.5 mm i.d. and 41.5 mm o.d., 1 m long.
EDS-91/1.3: ABS grooved tubing, 58 mm i.d., 70 mm o.d., length 3 m. Weight around 3 kg.
EDS-91/1.4:ABS grooved telescopic coupling (allows 150 mm compression). Weight around 0.5 kg
EDS-91/1.5: Return pipe same as EDS-91/1.1
The probe, 22 mm Φ x 150 mm long has a reed switch encapsulated inside it in silicon rubber for protection against shock, corrosion and ingress of water. It is connected to the signal receiver consisting of a reel with a battery pack, an on-off switch, buzzer, LED by a flat measuring tape.
The measuring tape is virtually non-expandable, insulated flat, and 10 mm wide x 2 mm thick. The probe is available in tape lengths of:
Length ‘L’ (m):30, 50, 100, 150, 200, 300
Length ‘L’ (ft):50, 100, 150, 300, 500
The moisture-resistant electronics and standard 9 V PP-3 size battery are housed in a hub on the cable reel. The hub can be easily removed to replace the battery or check the electronics without disassembling the entire cable reel.
EDS-91/2.2: For centrally locating probe in larger diameter access tubing, a 100 mm long adapter of diameter 6 mm less than that of the access tubing is available.
EDS-91/2.3:This is an alternative for EDS-91/4.2. It is an inclinometer type adapter with two fixed wheels and spring-loaded wheels with a longitudinal distance of 30 cm.
EDS-91/3.1: Plate magnet of 300 mm square size with centre hole to slide over the access tubing.
EDS-91/3.2:Ring magnet 57 mm Φ x 20 mm width for EDS-91/1.1 and 92 mm Φ x 20 mm for EDS-91/1.3 for the datum. Or (Ring magnet to fixed permanently on the tubing for datum)
EDS-91/3.3: Spider magnet with 6 leaves for springing out in correct position.
EDS-91/3.4:Spider magnet with 3 leaves for pushing down over access tube.
A dead-end pulley assembly is provided for horizontal installation for monitoring a dead-end hole.
In case of installations required for monitoring horizontal movement, a pull cable reel with cable return pipe is provided. It assists in moving the probe backwards or forward through access tubing.
The Encardio Rite model EDS-92 extensometer is used for monitoring displacement between two surfaces that may shift with respect to each other with time. The soil extensometer finds major application in the measurement of lateral strains and settlement in or beneath earth/rock fill embankments.
It is used in the measurement of foundation movements and controlling of natural and cut slopes. The displacement of retaining walls, bridge piers and abutments can also be monitored by the soil extensometer.
It also measures displacement across construction joints in concrete, and across joints/faults in the rock. The extensometers are usually installed in approximately 500 mm wide x 600 mm deep trenches. The extensometers are usually installed in approximately 500 mm wide x 600 mm deep trenches.
The Encardio Rite Model EMA-11 measuring anchor (rock bolt extensometer), is a combination of rock bolt and an extensometer. EMA-11 Measuring anchor is used to determine the load exerted on rock bolts.
It is a precision instrument designed to help civil engineers and geologists to evaluate anchor system forces and their distribution within the bolt and hence its safety and effectiveness.
The principal function of measuring anchor is to determine the depth where force is exerted on the bolt due to loosening effects of rock.
Model EMA-11 measuring anchor consists of a steel hollow anchor/rock bolt with a precise 4-point extensometer integrated into it. The measuring bars inside the hollow bolt have anchor points at each quarter of the total length so that the extension of the rock bolt and the corresponding forces can be determined.
Model EMA-11 measuring anchor has a breaking force of up to 250 kN. The length offered ranges from 3 m to 6 m.
The change in length due to compression or extension between individual measuring bars are taken by measuring the depth of the near end of the respective measuring bars from a reference plate provided at the mouth of the borehole. A digital calliper/micrometre depth gage with a resolution of 0.01 mm is used to take the readings.
An electrical head assembly consisting of four potentiometric sensors is optionally available for electrical output. Encardio Rite model EDI-53P readout/data logger or EDAS-10 data acquisition system are available for monitoring measuring anchor at the site.
This was all about the extensometers from its types, construction to operating principle, and specifications. Though we tried to cover everything about extensometers, do comment below if we missed out something. Also, do let us know your queries or suggestions.
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