Environmental Engineering Reference
In-Depth Information
Settlement Plates
Application
Ground surface settlements under fills and surcharges are monitored with settlement plates.
Procedure
Plates, at least 3 ft (90 cm) square, made of heavy plywood or steel, are placed carefully on pre-
pared ground. A 1 in. rod is attached to the plate and its elevation is determined by survey.
As fill is placed, readings of attachments are taken, and when necessary, additional rod
sections are attached. A relatively large number of plates should be installed on a given
site, not only to provide data on differential movements, but also because some will invari-
ably be disturbed or destroyed during filling operations.
Data Reduction
Plots of movements provide data on the rate and magnitude of settlement when refer-
enced to an immovable bench mark.
Terrestrial Photography
Applications
Slopes, retaining structures, buildings, and other structures experiencing movements may
be monitored on a broad basis with terrestrial photography.
Procedure
A bench-mark reference point is established and photographs are taken periodically from
the same location, elevation, and orientation with a high-resolution camera and film. After
each photo, the camera is moved a short distance laterally (about 1 m) to obtain at least a
60% overlap of adjacent photos. Sequential photos provide pairs suitable for stereoscopic
viewing when enlarged. Phototheodolites provide a sophisticated system.
Data Obtained
The direction and relative magnitude of movements are detected by comparing sequential
photos. The precision is not equal to that of the standard survey techniques, but it may be ade-
quate for detecting changes in the locations of survey reference points and providing for
approximate measurements if a reference scale is included in the photo. Stereo-terrestrial
photographs are extremely useful for mapping joints and other discontinuities on rock slopes.
4.2.3
Tiltmeters
Electrical Tiltmeter
Application
Tilting or rotational movements of retaining walls, structures, and rock benches are mon-
itored with electrical tiltmeters.
Instrument
The portable tiltmeters utilize a closed-loop force-balanced servo-accelerator to measure
tilt. Accuracy is reported to be equivalent to a surface displacement of 0.06 in. of a rock
mass rotating on an axis 100 ft beneath the surface, or a displacement of 200
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in. over the
4-in. length of the instrument.
