When grout backfill is still a fluid, it exerts an uplift force that can force even water-filled casing out of the borehole. The obvious way to counter this buoyancy - holding the casing down from the top - has some unfortunate side-effects. Held down from the top, the casing goes into compression and snakes from side to side in the borehole. Thus casing curvature is present from the start, and slight changes or errors in the positioning of the probe will produce reading errors - the larger the curvature, the larger the error.
The best way to counter buoyancy is to anchor the casing at the bottom, either by weighting it or by using a convenient casing anchor. When the bottom is anchored, the rest of the casing self-centers in the borehole and becomes very straight. The casing anchor shown here has spring loaded arms that are activated when a pin is pulled. The leaf spring in the casing anchor will expand the anchor approximately 10 inches (25cm) from tip to tip. Therefore, the casing anchor should only be utilized in boreholes that have a diameter of less than 10 inches (25cm). Once the anchor catches the side of the borehole, the anchor will continue to expand such that the distance from tip to tip is 19.5 inches (50cm). This final expansion will allow the casing to travel upwards up to 4.5 inches (11.5cm), assuming no deformation of the in-situ soils.
Casing anchors are available for 70 mm • 2.75" and 85 mm • 3.34" casing sizes. Combination anchors + grout valves are also available.