Environmental Engineering Reference
In-Depth Information
Instrument
Similar in principle to the water-level device, mercury pools were set in piers 20 m
apart in an interconnected array and attached to Invar rods driven to resistance in the ground.
4.2.4
Extensometers
Simple Strain Meter
Application
Joint or tension crack deformations in rock masses may be monitored with simple strain
meters; they are particularly useful on slopes.
Installation
Pins are installed in a triangular array adjacent to the crack and initial reference measure-
ments are made. Periodic measurements monitor vertical, lateral, and shear displacements
along the crack.
Simple Wire Extensometer
Application
Slope movements are monitored with simple moving wire contacts that can give warning
of large deformations, a particularly useful feature for open-pit mine and other rock slopes.
Installation
A stake is driven into the ground, upslope of the crack, and is attached to a tensioned wire
that becomes the measurement station. The wire is attached to a stake driven into an unsta-
ble area. The wire passes over guides in the measurement station to connect to a weight to
maintain wire tension. At the measurement station, trip blocks are attached to the wire that
passes along a steel scale. Displacements are measured periodically from the locations of
the trip blocks along the scale. A trip block can be placed on the scale at the location of the
desired maximum movement and can be connected to an alarm (see
Figure 4.5)
.
Electrical Crack Gage
Application
An electrical crackmeter continuously monitors rock slope joint movements when con-
nected to a data logger. The initial reading is used as a datum. Subsequent readings pro-
vide data on magnitude, rate, and acceleration of movements across the joint.
Installation
A displacement transducer is connected to anchors or steel pins set in the rock on
each side of the joint. A change in distance across the joint causes a change in the fre-
quency signal produced by the transducer when excited by the readout or data logger.
The readout device processes the signal, applies calibration factors, and displays a read-
ing in millimeters or inches.
Wire Extensometer (Convergence Meter)
Applications
Wire extensometers are used to measure the movements of soil- or rock-cut benches in the
downslope direction as indicators of potential failure or to monitor convergence in tunnels,
