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.
Casing anchors are available for 70 mm • 2.75" and 85 mm • 3.34" casing sizes. Combination anchors + grout valves are also available.
Grout Valve
A grout valve provides a way to deliver grout backfill when there is no room in the borehole for an external grout pipe. The grout valve is a one-way valve installed in the bottom of the casing. After the casing is in place, a grout pipe is lowered through the casing to mate with the valve and deliver the grout.
The gasket-type grout valve, shown at right, provides a straight-pipe connector with a rubber gasket at its base. The pipe extends one foot into the casing. An open-ended grout pipe slides over the connector pipe to rest on the gasket. Grout is then pumped through the valve. When grouting is finished, the remaining grout in the pipe is flushed with water (just enough to displace the grout in the pipe) and then withdrawn from the casing.
Grout valve are easy to use, but require a slightly deeper borehole and grout that is carefully mixed so that it does not have lumps that could be caught in the valve mechanism.
本文转自：China Industry News
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