Inclinometer Casing Accessories
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.
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 is shown at right. You can see the gasket at the base of a smaller diameter pipe. A larger diameter grout pipe slides over the smaller pipe to rest on the gasket. Grout can then be 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the remaining grout in the pipe is flushed with water (just enough to displace the grout in the pipe) and then withdrawn from the casing.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 valves are easy to use, but require a slightly deeper borehole. Also grout must be mixed carefully so that it does not have lumps that could be caught in the valve mechanism.
The grout pipe is nominal 1.25" pipe. A 1.25" schedule 40 pipe has an ID of 1.35" (35mm). The smaller diameter pipe, which is part of the grout valve, is 3/4" pipe. It has an OD of 1.07" (27 mm).