Installation procedures tend to be application specific. Here are
some general concerns.
Boreholes Most rod extensometers are designed for installation in a 75 to
83 mm (3.0 to 3.25 in) diameter drill hole. Some reference heads re-
quire enlarged borehole collars.
Boreholes should be free-draining, if possible. Holes must be clean
and free of debris before installation. Use either a diamond drill or a
percussive drill.
Boreholes should be parallel with the direction of expected move-
ment, since the extensometer measures only axial movements. In a
tunnel, for example, the extensometer is typically perpendicular to
the tunnel wall. In a fault zone, the extensometer is placed to mea-
sure separation. In a shear zone, the extensometer is placed at a
narrow angle to measure displacement.
Spacing of anchors is determined mainly by geological factors and
the size and geometry of the mass being instrumented. Diamond
drill cores, if available, may reveal planes of weakness along which
failure may occur. The anchor spacing should be adjusted to accom-
modate such planes.
It is useful to have one of the anchors positioned in stable ground so
that the movements of the other anchors can be measured relative to
this stationary anchor. In a tunnel, for example, the deepest anchor
should be at least three tunnel diameters away from the surface of
the tunnel opening.
Twisting of Rods Do not twist rods during installation. It may help to color code rods
and put corresponding colored marks around the borehole collar.
Grouting The specified grout is usually a neat cement mix with a water/ce-
ment ratio of not more than 1:1. A fluidifier may be used to facili-
tate pumping through the small-diameter grout tube.
Other additives are not usually required. Grout pressure must be
controlled to avoid collapse of the sleeves protecting extensometer
rods.
When grouting through long lengths of polyethylene tubing, wet
down the walls of the tube by pumping a small quantity of water
ahead of the grout. This will help minimize friction.
Borehole Extensometers
Rembco Geotechnical Contractors specializes in
slope stabilization,
soil nail wall,
foundation repair
and support,
grouting,
shoring,
micropile
construction,
rock drilling, and we remediate
sinkholes. We use a
problem-solving approach in the geotechnical portions of buildings, bridges, water plants, sewage plants, tunnels,
installing caisson bottom, and roads. We are experienced in these applications:
micro piles,
sinkhole repair,
permeation and
pressure
grouting,
soil nails,
cement grouting,
chemical grouting,
micropiles,
compaction grouting, and
rock anchors.
Rembco techniques apply when adding new structures: especially
rock anchor,
microfine cement,
soil nailing, and
micropile
installations for foundation support. Our specialties are
soil nails,
rock anchor,
chemical and
compaction grouting,
mini piles,
sinkhole repair,
micropiles, and
soil nailing.
