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There are a wide range of options of access tubing and magnetic target types to suit individual application.

Type

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”) 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
  • Two types available - 33mm OD x 25mm (1”) ID with flush threads or 70mm flush coupled QJ inclinometer casing.
  • Telescopic section
  • Recommended where settlement is expected to exceeded 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. It is often misconstrued that the legs are used to “key” into the surrounding soil and   while the legs may provide some "coupling" effect, their real purpose is to hold the magnet in place until the borehole is grouted.
  • 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 requires 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.

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.