Environmental Engineering Reference
In-Depth Information
Measure frequency
of vibration of wire
f
ยต
w
=
k
(
f
'
2
-
f
o
'
2
)
Electrical cable
Fill surface level
during installation
Remote
readout
Vibrating wire
Exciter and pickup
magnets
Diaphragm
Porous tip
FIGURE 4.29
Electrical-transducer-type piezometer.
Clean sand
4.4.3
Loads and Stresses
Applications and Summary
Loads and stresses are measured to check design assumptions, provide early warning
against failure, and to permit the installation of remedial measures to provide additional
support wherever necessary. Measurements are made primarily during construction. The
various devices and their applications are summarized in
Table 4.9.
Strain Gages
Purpose
Strain gages are used to measure deformations of structural members to provide the basis
for estimating stresses and loads. There are two general types: electrical (see
Section 4.1.4)
and mechanical.
Resistance Strain Gages (see
Figures 4.1
and
4.2)
Bonded gages
are used primarily for short-term surface measurements. Installation requires
meticulous care and skill during the preparation of the receiving surface and the cement-
ing of the gage in place. Moisture sensitivity is high and adequate field waterproofing is
TABLE 4.9
Load or Stress vs. Device Suitable for Measurements
Condition
Load or Stress to be Measured
Device Suitable
Soil formations
Stress distribution beneath foundations or in
Pressure cell
embankments
Earth pressure distributions behind retaining structures
Pressure cell
Rock formations
Stress distribution beneath foundations or in abutment
Carlson strain meter
walls, during or after construction
Borehole stress meter
Structural members
Stress distribution in piles to differentiate between end
Tell tale
bearing and shaft friction during load test
Strain gage
Stresses on retaining structure bracing, earth and rock
Strain gage
anchors, steel storage-tank walls
Stresses on tunnel linings
Pressure cell
Loads on tunnel lining elements, earth and rock
Load cell
anchors, pile tips
