Environmental Engineering Reference
In-Depth Information
strains and to determine the extent of the zone in which significant strains occur, which is a
function of the tunnel diameter.
Multiple-Position Borehole Extensometer (MPBX): Rod Type
Application
The rod-type MPBX is installed to monitor deflections occurring parallel to a borehole at
a number of positions in a single hole. Vertical overhead installation is difficult and lengths
are limited to about 50 ft. Sensitivity is of the order of 0.001 in.
Installation
The ends of up to six metal rods are cemented into a borehole at various distances from
the excavation face and the rods are attached to a measuring platform fixed to the exca-
vation wall as shown in
Figure 4.21a.
Multiple-Position Borehole Extensometer (MPBX): Wire Type
Application
The wire-type MPBX is used like the rod type but can be installed to far greater depths.
Installation
Up to eight stainless-steel tension wires are installed in a borehole and anchored by grout-
ing or by some other means, to the side of the hole at various positions along the hole as
shown in Figure 4.21b. A measuring head is threaded over the wires and installed in the
collar of the hole, and each wire is connected to a measuring element in the head. The meas-
uring element can be a steel rod running in a track, pulling against a tension spring or push-
ing against a compression spring, which is read by dial gages; it can be an electric sensor,
such as an LVDT, linear potentiometer, or strain gage connected to an electrical readout.
Calibration is difficult and allowances are made for wire stretch, friction between the
wires and the borehole, and temperature. Wire-type MPBXs can be read to the nearest
0.001 in., but because of calibration difficulties, the deeper the installation, the greater
should be the changes in readings before they are judged as significant.
Horizontal Strain Meters and Inclinometers
Application
Linear strains occurring during slope movements or fault displacements can be monitored
with electrical strain meters or inclinometers. When used in dam embankments, they pro-
vide a plot of the vertical deformation that is occurring. They can be used in nets, con-
nected to a data acquisition system that continuously monitors movements, and set as an
early warning system when a preset value is exceeded.
Instrument
Linear potentiometers are mounted in a slip-jointed polyvinyl chloride (PVC) pipe. The
Slope Indicator horizontal in-place inclinometer consists of a string of inclinometer sen-
sors deployed in inclinometer casing as discussed in
Section 4.3.3.
Performance
Installed in shallow trenches, the inclinometers are read and continuously recorded at
remote receiving stations, and can be set in series to extend over long distances. Changes
of several inches can be measured over gage lengths of several feet with a sensitivity of
10
3
times the movement range.
