Environmental Engineering Reference
In-Depth Information
TABLE 4.3
(
Continued
)
Method/Instrument
Applications
Subsurface Deformations
Inclinometer
Measure lateral deflections. Used behind walls, in lateral pile-load tests, for
measuring deflections beneath loaded areas over soft soils, and to locate
the failure surface in a slope and monitor slope movements
Deflectometers
Used in rock as permanent installation to monitor movements perpendicular
to the borehole in rock slopes, open-pit mines, and fault zones
Shear-strip
Used to locate failure surface in earth mass and to send an
indicators
alarm when failure occurs
Borehole
Installed singly or in series (MPBX) in boreholes to monitor
extensometers
deflections occurring parallel to hole. Used to monitor slopes in rock,
tunnels, and caverns. Installed in any orientation
Subsurface Deformations
Electrical strain meters
Installed below the surface in earth dams to monitor longitudinal strains
between embankment and abutment and to locate transverse cracks
Acoustical emissions device
Detect and monitor subaudible noise in soil and rock resulting from distress
caused by slope movements and mine collapse, and along faults. Also
used to locate leakage paths in dams
In Situ Pressures and Stresses
Piezometers
Monitor pore-water pressures in slopes, dewatered excavations, beneath
embankments, in dams, beneath buildings, and during preloading.
Various systems available. Application depends on soil or rock
conditions, response time required, and necessity for remote readout
and recording
Stress or pressure cells
Measure stresses behind walls, in tunnel linings, beneath foundations during
load test, and in embankments
Load cells
Measure loads in anchors, wall braces, and tunnel lining
Tell tales
Measure deflections at various depths in a pile during load test. Used to
compute side friction and end bearing
Strain gages
Measure strains in piles during load test, bracing for retaining structures,
earth and rock anchors, and steel storage tank walls during hydrostatic
testing
Strain meters
Purposes similar to tiltmeters above, but meters tire encased so as not to be
susceptible to short circuits, and are welded to the structure so as to not be
subject to long-term creep of a cementing agent
Stress meters
Installed in borehole to measure stress changes during tunneling and mining
operations
Residual Rock Stresses
Shallow-Depth Methods
Strain meters or rosettes
Stresses a short distance behind the wall remain unknown and excavation
for test relieves some residual stress
Flat Jacks
Relatively low costs. Used in good-quality rock
Deep Methods
Permits deep measurement of residual stresses by borehole
Borehole devices
overcoring techniques. Any borehole orientation is possible,
Deformation gage
but installation and overcoring are difficult operations.
Inclusion stress meter
Practical depth limit is about 10 to 15 m
Strain gage
Hydraulic fracturing
Allows very deep measurement, about 300 to 1500 m
Boreholes are limited to vertical or near-vertical. Technique is in
development stages
