Environmental Engineering Reference
In-Depth Information
TABLE 4.6
Subsurface Deformations: Forms, Occurrence, and Monitors
Movement
Occurrence
Monitoring Method
Reference
Vertical
Ground compression at
Vertical-rod extensometer
Section 4.3.2
displacement
various depths
Borros points
Cross-arm device
Mine roof deflection
Acoustical emissions device
Section 4.3.5
Lateral
Slope failure zone (
a
,
b
,
c
,
d
)
(
a
) Inclinometer for soil and rock
Section 4.3.3
displacement
masses
Wall or pile deflection (
a
)
(
b
) Deflectometers for rock
(
c
) Shear strip indicators
Weak soils under stress (
a
)
(
d
) Slope failure sensor
Fault movements (
a
,
b
,
d
)
(
e
) Acoustical emission device
Linear strain
Dam foundations or
Rock bolt extensometers
Section 4.3.4
gradients
abutments
Open or closed rock
Multiple position borehole
excavations
extensometers (rod or wire)
Differential settlement of dam
Electrical strain meters
Section 4.3.4
bodies and foundations
Piping erosion
Through, around, or beneath dams
Acoustical emission device
Section 4.3.5
Broken underground pipes
Dye or radioactive tracers
Section 8.4.1
pipe, corrugated plastic pipe, or inclinometer casing. The probes record depths by sensing
magnets or other sensor rings set in the borehole, or by a hook-type probe as described in
the Slope Indicator Catalog.
Rod Extensometers (Settlement Reference Points)
Applications
The vertical rod extensometer is a simple device for measuring deformations between the
surface and some depths in most soil types, except for very soft materials.
Instrument
An inflatable bag is set into an HX-size borehole at a depth above which the deformations
are to be monitored. The bag is connected to the surface with a 1-in.-galvanized pipe
installed inside a 3-in.-diameter pipe, which acts as a sleeve permitting freedom of move-
ment of the inner pipe. A dial gage is mounted to read relative displacement of the 1-in.-
diameter pipe and a reference point on the surface.
Installation
An HX-size hole is drilled to test depth and the 3 in. pipe installed as shown in
Figure 4.13a.
The 1 in. pipe and bag are lowered to test depth, with the pipe centered with rubber spacers
free to move in the 3 in. pipe. Cement grout is injected into the inflatable bag through the 1
in. pipe under pressures of about 50 psi to expand the bag and secure it against the sides of
the hole, below the casing. An expanded bag under surface test is illustrated in Figure 4.13b.
When the bag and pipe are filled, the grout returns to the surface. This return can be signaled
by the bursting of a short length of rubber tubing installed in the top of the grout pipe. A 10-
in.-diameter steel pipe set in a concrete pad at the ground surface, on which a dial gage is
mounted, serves as the reference measuring component (Figure 4.13c).
Piezometers are also installed (see
Section 4.4.2)
to monitor porewater pressures during
the settlement observations.
