Environmental Engineering Reference
In-Depth Information
where
a
s
is the section area,
E
p
the modules of concrete (or steel, or steel and concrete) com-
posing the pile section and
R
1
and
R
2
are the deflection readings of tell tales at depths 1
and 2, respectively.
Subtracting the values for shaft friction from the total applied load provides a measure
of the load reaching the pile tip.
Vibrating-Wire Stress Meter
Purpose
Changes in
in situ
stresses occurring in rock masses during mining and tunneling opera-
tions have been measured with vibrating-wire stress meters (Sellers, 1977).
Device
A transducer consisting of a hollow steel cylinder (proving ring), across which a steel wire
is tensioned, is wedged into a 1.5- to 3-in.-diameter borehole. Rock stress release flexes the
proving ring, changing the wire's tension and natural frequency or period of vibration. An
electromagnet excites the vibrations in the wire and their frequency is measured as exca-
vation proceeds in the rock. The frequency provides a measure of the proving-ring strain
and, therefore, the stress change.
4.4.4
Residual Rock Stresses
General
Occurrence
The geologic conditions causing overstress or residual stresses in rock masses in excess of
overburden stresses are described in
Section 6.6.
In practice, they are significant in open
excavations and tunnels where their effects are mitigated with rock bolts or bracing systems.
Measurements
Measurements are made at the rock surface where disturbance and stress relief from exca-
vation may affect values obtained with strain meters, strain rosettes, or flat jacks.
Alternatively, measurements are made in boreholes. Although they are difficult to per-
form, borehole measurements provide more representative data. Borehole devices require
overcoring. Hydraulic fracturing is also performed in boreholes.
Strain Meters or Strain Rosettes
SR-4 strain gages (bonded gages) are attached to the rock wall, a zero reading is taken,
slots are cored around the gage to relieve rock stresses as shown in
Figure 4.31,
and the
gages are read again. The principal stresses can be calculated with a suitable array of
gages. Results are difficult to interpret unless rock modulus values have been measured.
Flat Jacks
Flat jack tests have been described in
Section 3.5.3.
Reference pins or strain gages are
installed on the rock face and distance measurements are accurately made (or strains are
measured by gages mounted on the jack). A slot is drilled, and the jack inserted and grouted
(Figure 4.32)
and then expanded under pressure to restore the original strain or reference-
point measurements. The restoration pressure is taken as equal to the residual rock stress.
Flat jacks as well as strain meters are effective only in tightly jointed high-quality rock.
