Introduction to Magnetic Extensometers
Manual Magnetic Extensometers were originally development at the UK’s Building Research Establishment (BRE) by Burland et al (1972) and are designed to measure manually settlement or heave of soft ground under the influence of loading or unloading due to the construction of embankments, fills, buildings, foundations, and structures.
Until recently they have been the main tool for Geotechnical Monitoring but now there are automated options. IPI-X and IPX.
MANUAL SYSTEM – GXM
For the manual GXM system, pictured above, a Reed Switch probe is lowered inside an access tube to detect and measure the position of magnetic anchors located around the access tube placed at various depths along the borehole or within the fill.
AUTOMATIC SYSTEMS
IPX (In-Place Extensometer)
Automation is provided by a series of in-situ positional sensors which detect and measure the position of magnetic anchors located around the access tube placed at various depths along the borehole or within the fill. Data acquisition can be automated to provide near real time monitoring.
IPI-X (In-Place Inclinometer-Extensometer)
As for the IPX where In-Place Inclinometers (IPI) are combined with the in-situ positional sensors to provide a full 3D profile for tilt and settlement. Data acquisition can be automated to provide near real time monitoring.
SYSTEM CONFIGURATIONS
For both the manual and automated systems Magnetic Extensometers are supplied in various configurations of the access casing, spider, plate and datum magnets to suit varying installation environments and techniques. The ones supplied by Geosense are listed in the table below:
GXM-100 – comprises of a series of 3 or 6 leaf spider magnetic targets positioned on the outside of 33mm (1”) or 42mm (1¼”) flush jointed access casing.
GXM-100P – 3 or 6 leaf spider magnetic targets positioned on the outside of 33mm (1”) flush jointed access casing for installation in a borehole prior to fill after which a series of settlement plates and telescopic joints are added.
GXM-100T – 6 leaf spider magnetic targets positioned on the outside of 33mm (1”) flush jointed access casing together with telescopic joints to accommodate higher settlements than the GXM100
GXM-200 – 3 or 6 leaf spider magnetic targets positioned on the outside of a flush jointed inclinometer casing.
GXM-200C –for large fill projects such as earth fill dams where cross arm magnetic targets are used which extend into the fill area by up to 2 metres thus providing a more representative sample of the fill material.
GXM-200P – 3 or 6 leaf spider magnetic targets positioned on the outside of flush jointed inclinometer casing for installation in a borehole prior to fill after which a series of settlement plates and telescopic joints are added.
GXM-200PT – 6 leaf spider magnetic targets positioned on the outside of a flush jointed inclinometer casing together with telescopic joints to accommodate higher settlements than the GXM200.
GXM-300 – 33mm (1”) flush threaded access casing inside corrugated tubing for applications where high settlement is expected. The corrugated pipe de-couples the access casing from the surrounding ground and the couplers act as magnetic targets.
GXM-300i – 70mm flush coupled inclinometer casing inside corrugated tubing for applications where high settlement is expected. The corrugated pipe de-couples the access casing from the surrounding ground and the couplers act as magnetic targets.
Components
Access Tubing
Three types available – 33mm OD x 25mm (1”) ID, 42mm x 38mm (1¼”) ID with flush threads or 70mm flush coupled QJ inclinometer casing.
Telescopic section
Recommended where settlement is expected to exceed 2-3%. Typically, this could occur at interface zones, typically between soft ground materials and fill material. Where these are used only 6 leaf spiders can be installed
Spider Magnets (targets)
For borehole installations the magnetic targets are located in position by means of leaf spring legs attached to the targets which are known as “Spider Magnets” and are available in either three or six leaf versions.
3 leaf Spider
3 Leaf Spider Magnets are the easiest to install as they can simply be placed on the outside of the access tubing and pushed down to the required depth using a special installation tool or lengths of piezometer pipe.
6 Leaf Spider
The installation of ‘6 leaf’ spider magnets require the legs to be compressed for installation and released when the magnet is in position at its specified depth. This is done by a simple chain wrapped around the legs and held in position by a trigger pin which has a pull rope attached to it. The targets are secured in place on the access tubing by a biodegradable tape such as masking tape that prevents it being pulled up the casing when pulling the rope on the trigger pin.
Plate magnet
They are used in fill to provide a large surface area contact to the surrounding soil.
Cross-arm magnet
Used which extend into the fill area by up to 2 metres thus providing a more representative sample of the fill material.
Datum Magnet
Fixed in place near the bottom section of the access tubing in stable ground, ‘beyond the zone of expected movement’ and is the point which all other targets are referenced to.
It is often misconstrued that the legs are used to “key” into the surrounding soil. While the legs may provide some “coupling” effect, their real purpose is to hold the magnet in place until the borehole is grouted
Installation
Target identification
It is very important that each target is labelled and given a unique identification. Where 6 leaf spiders are used the pull ropes should also be labelled.
Grout
In soft ground applications, a ratio (by weight) of 6.6 water : 1 cement : 0.4 Bentonite is a good option which will provide a 28 day compressive strength of about 25kPa
In a hard clay a ratio (by weight) of 2.5 water: 1 cement : 0.3 Bentonite is a good option which provides a 28 day compressive strength of about 690kPa.
See further details in Grout for Geotechnical Instruments blog.
Base Reading
It is essential to take a base reading of the datum magnet once the installation is complete.
Survey
If it is not possible to have the datum magnet in stable ground, it is VITAL that the top of the access tube should be surveyed each time the instrument is read.
Operation
Troubleshooting
Unexpected Readings
These could be caused by:
The top of the access casing being cut without a new survey being done
The datum is not in stable ground and the top of the casing has not been surveyed
Debris in the access casing causing a problem with the reed switch probe
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