Field Instrumentation - Geotechnical Engineering Investigation - page 308

as a design basis, as a basis for checking design assumptions and determining the need for
additional support, or as a warning of impending failure.

Categories
Pressure and stress conditions measured in situ include:
1. Groundwater levels and pore pressures

2. Stresses in embankments, beneath loaded areas, or in structural members

3. Earth pressures against retaining structures or tunnel linings and supports

4. Residual stresses in rock masses

Summary
In situ pressures and stresses, their occurrence, and methods of measurement are summa-
rized in Table 4.7.

4.4.2 Pore-Water Pressures

Applications and Summary
Observation wells are installed to measure changes in the static water table. Piezometers
are used to monitor water levels (static, perched, artesian) in excavations, slopes, and dam
embankments, to measure excess hydrostatic pressures beneath dams and embankments,
and to aid in control of preloading operations and placement of fill over soft ground.
The types, applications, advantages, and disadvantages of piezometers are summarized
in Table 4.8. Piezometers are divided into open systems, in which measurements are made
from the surface and the water level is generally below the surface, and closed systems, in
which measurements are made remotely and the water level may be at any location.

TABLE 4.7
In Situ Pressures and Stresses: Occurrence and Measurements

Phenomenon Occurrence Measurement Reference

Pore-water pressures Natural static GWL Standpipe piezometer Section 4.4.2

Artesian conditions Casagrande piezometer

Excess pore pressures under Hydraulic piezometer

static or dynamic loads

Pneumatic piezometer
Electrical piezometer

Loads and stresses Beneath foundations Pressure cells Section 4.4.3

In embankments Load cells Section 4.4.3

In structural members Tell tales Section 4.4.3

Against retaining walls Strain gages Section 4.4.3

Tunnel linings and supports Acoustical-emissions devices Section 4.3.5

Vibrating-wire stress meter Section 4.4.3

Residual rock stresses In closed and open excavations Flat-jack test Section 4.4.4

in rock masses

Strain meters and overcoring Section 4.4.4

Borehole methods Section 4.4.4

Deformation meter
Inclusion stress plug
Strain gage (Leeman doorstopper)
Hydraulic fracturing