Environmental Engineering Reference
In-Depth Information
measurements of strain vs. depth beyond the upper and lower plates, and concrete fac-
ing is poured over the rock mass to provide flat-bearing surfaces. As load is applied, the
extensometers sense compression strains that are recorded on electronic readouts.
See also ASTM D4394 and D4395 (Plate load tests).
Computing Moduli
Theoretical basis is the Boussinesq solution. The Young's modulus
E
r
and surface dis-
placement
y
are related to the applied load
P
(Jaeger, 1972) as
y
P
(1
ν
)
2
/
π
E
r
r
(3.59)
where
r
is the plate radius.
Moduli
E
i
,
E
se
, and
E
t
are determined from the stress-strain curve. A recovery modulus
E
sr
can be taken from the unload portion of the curve to provide a measure of rock elas-
ticity. If
E
t
E
sr
the material is perfectly elastic. A value of
E
se
much lower than
E
t
usually
indicates the closure of fractures and plastic deformation.
Test Limitations
Rock-mass response is affected by disturbance during excavation and surface preparation
for testing. It is also affected by the relatively small stressed zone, often to the degree that
results are not representative of response under construction loading, although the results
will usually be conservative (Rocha, 1970).
Radial Jacking and Pressure Tunnel Tests
Performance
Radial jacking test
: The pressure to cause deformation of the wall of an adit excavated into
the rock mass is applied mechanically. Displacements are measured with extensometers or
other strain-measuring devices.
Pressure tunnel test:
A portion of the adit is sealed with concrete and water is pumped in
under high pressure to cause deformation. Displacements are measured with extensome-
ters or some other strain-measuring device. Flow into the rock mass may cause large errors
from pressure drop.
Computing Moduli
Analysis is based on the Boussinesq stress distribution and theories of stresses and deflec-
tions about a circular hole, or a hole cut in a plate (Wallace et al., 1970; Misterek, 1970).
Comments
Although both tests are relatively costly to perform, larger areas are stressed than is pos-
sible with plate-jack tests, which allows a better assessment of rock-mass deformation.
Measurements of differential wall movements permit the assessment of the anisotropic
properties of the rock mass. Excavation, however, results in straining of the rock mass,
which affects its properties.
Flat-Jack Tests
Performance
A slot is cut into rock with a circular diamond saw or by a series of line-drilled holes.
A steel flat jack Freyssinet jack
(Figure 3.72)
is grouted into the slot to ensure uniform
