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Overcoming buoyancy during inclinometer casing installation

By John Dunnicliff, geotechnical instrumentation consultant, and P Erik Mikkelsen, geoengineering and instrumentation consultant.

The authors have become aware that some installers of inclinometer casing overcome buoyancy during installation by using the drill head or a weight to restrain the top from moving upward. This is not an acceptable procedure.

The first parts of this article are applicable to the use of ABS (acrylonitrile/butadiene/styrene) casing. At the end of this article the buoyancy issue is discussed when PVC (polyvinylchloride) casing is used.

When grout is used to fill the annular space between the casing and the wall of the borehole there will be a buoyancy force, acting on the bottom cap, to push the casing upward.

This will happen even when the casing is filled with water, because the density of grout is significantly greater than water. If restraint is provided at the top of the casing, the buoyancy force will set up a compressive force in the casing and cause the casing to support itself from side-to-side (snake) within the borehole.

This problem is particularly severe for deep installations, where the buoyancy force is largest and where sections of the borehole diameter may be enlarged. The combination of compressive force and eccentric loading may produce excessive bending moments in the casing.

This problem is aggravated if snap-together joints are used rather than cemented or riveted joints or couplings. Kinking may occur at snaptogether joints.

At a recent 105m deep installation with snap-together joints, the inclinometer probe could be made to stick in the 85mm od casing at 35m depth.

Another drawback with 'snaked' casing is the potential for reading errors due to variation in depth control of the probe.

Any change or error in the positioning of the probe will produce reading errors - the larger the curvature, the larger the error. For example, if the change of inclination between adjacent reading increments is 2degrees and the probe is positioned 25mm from the correct depth, the resulting error in measured displacement would be 1mm.

The density of typical grout used for this purpose is between 1,200kg/m 3and 1,450kg/m 3, so that the buoyancy force on a water-filled 70mm OD casing is between 0. 7kg and 1. 6kg metre of depth. For an 85mm OD casing it is between 1. 2kg/m and 2. 5kg/m. It can be seen from these figures that the buoyancy force can be large.

This article is a combination of two articles written for the September and December 2000 issues of Geotechnical News, reprinted here with the kind permission of BiTech Publications, Richmond, British Columbia, Canada.

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