Environmental Engineering Reference
In-Depth Information
4.3.4
Linear Strain Gradients
Occurrence and Measurements
Rock Masses
Linear strains occur around the openings for tunnels, caverns, open-pit mines, or other
rock slopes as a result of excavation, or in the abutments or foundations of concrete dams
as a result of excavations and imposed loads.
In tunnels and caverns, borehole extensometers are installed to measure the tension arch
or zone of relaxation which determines the depth to which support is required, the type of
support, and the excavation methods. Deformations are also monitored during and after
construction in critical zones characterized by faulting, intense fracturing, or swelling
ground (in which horizontal stresses can be vertical stresses many times).
Earth Dams and Reinforced Earth Walls
Linear strains occurring in earth dams and reinforced earth walls are measured with elec-
trical strain meters. In earth dams, the meters are installed in trenches at various depths to
monitor strains and detect and locate cracks resulting from the differential movement of
the dam foundation or the embankment itself, and between the embankment and the
abutment (see
Figure 4.38)
.
In reinforced earth walls the meters are installed to monitor
horizontal movements in the body of the backfill.
Wire Extensometers
Wire extensometers to monitor slope movements and deformations around a tunnel open-
ing are discussed in
Section 4.2.4.
Rock-Bolt Borehole Extensometer
Application
Rock bolts (see
Figure 9.118)
are installed to monitor deflections occurring parallel to a
borehole in a rock mass. They can be installed in any orientation, and are rugged and low
in cost because they are relatively simple and can be installed rapidly. Precision is of the
order of 0.001 in.
Installation
A small-diameter borehole is drilled to a certain depth and a 1-in.-diameter steel rod is
grouted into a fixed position as shown in
Figure 4.20.
A Teflon collar is fitted on the rod as a
spacer near the tunnel face and a head is fixed to the wall, where a strain gage is mounted to
measure deflection readings. The bolts are installed at various depths to measure differential
Rock bolt
Grout
Teflon collar
Rock bolt
B
2 to 4B
2 to 4B
FIGURE 4.20
Simple rock bolt extensometer.
