Planning the Magnetic Extensometer Installation
While the IPX is a very reliable and precise measurement device, the success of its deployment relies on the correct installation of the magnetic extensometer.
Follow the manufacturers guidelines for installing the magnetic extensometer.
Special considerations are outlined below:
Access Tube
It is important—both for ease of installing the IPX, and for general performance—that the magnetic extensometer access tube is installed as straight as possible. This can be achieved by using a heavy length of chain, attached to a rope, to weigh the access tube down from the bottom during grouting.
Some conditions may increase the likelihood of installations that are not straight. These include deep installations, installations where multiple grout stages are required, or installations with short distances between magnets.
All internal joints should be smoothed with a pipe deburring tool to reduce damage to cables upon insertion and retraction of the IPX system
Magnetic Targets
The datum magnet should be installed no less than 1m from the base of the access tube.
The datum magnet should notbe fixed to the access pipe, but rather held in place using O-rings or paper tape.
Protecting the Top
The magnetic extensometer must be suitably protected from damage at the surface.
Protective Cap
A protective cap allows the IPX to be buried under fill. This minimises the risk of damage caused by construction activities and removes unwanted obstacles at the surface.
Hard Barricade
Where the IPX is being modified during construction, it is important to properly protect it during its operational lifetime. We recommend a hard barricade to minimise the risk of damage caused by plant operations.
Grouting Best Practices
Grout serves to fill the annulus of the borehole, support the access casing, and mechanically couple the target magnets to the surrounding material.
Key Requirements:
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The grout mix must be sufficiently soft such that axial compressions and extensions of the ground are able to be accurately reflected in movement of the target magnet
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Grout that is too firm is likely to decouple from the surrounding soil, and result in a stiff column that is not representative of the surrounding geotechnics
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Grouts intended only for piezometric or lateral deformation monitoring are likely to be much too stiff for extensometer applications
General application of a Standard (e.g., ISO, DIN, BS) to determine grout ratios is unlikely to produce acceptable results, especially where these standards are intended for general geotechnical use, and not application specific.
Best Practices:
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Modulus of deformation, rather than strength, should be the key metric in judging a mix
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Variations in cements, bentonites, mix strategies and application techniques all have an impact on resultant grout
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It is therefore highly unlikely that a priori mix specifications will produce intended results. Assessment must be made in situ
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If in doubt, softer mixes are always preferable
Recommended Resource: Mikkelsen, P. Erik. "Cement-bentonite grout backfill for borehole instruments." Geotechnical News 20.4 (2002): 38-42.