Environmental Engineering Reference
In-Depth Information
Applicability:
Although used in all soil types and soft rocks, the Menard pressuremeter
is most useful in materials for which undisturbed sampling is difficult or not possible,
such as sands, residual soils, glacial till, and soft rock (in a smooth borehole).
Procedure:
The apparatus is illustrated in
Figure 3.86.
A cylindrical flexible probe is low-
ered into an NX-size borehole to the test depth, and increments of pressure are applied to
the probe by gas, while radial expansion of the borehole is measured in terms of volume
changes. The test can be performed up to failure of the surrounding materials; the limit of
the radial pressure is in the range of 25 to 50 tsf.
Test Data:
The volume of the expanded probe is plotted vs. the “corrected” pressure,
which is equal to the gage pressure at the surface minus the probe inflation pressure plus
the piezoelectric head between the probe and the gage, as shown in
Figure 3.87.
The quan-
tities obtained from the test include:
P
i
in the initial or seating pressure at the beginning of the elastic stress stage, gen-
erally considered equal to
P
o
, the at-rest horizontal stress,
P
f
the creep pressure
at the end of the elastic stress stage,
●
P
L
the limiting pressure, or ultimate pressure; the failure pressure,
●
E
c
the compression modulus; obtained from the slope of the compression curve
between
P
i
and
P
L
Equation 3.73,
●
E
r
the rebound modulus, obtained from the slope of the rebound curve.
●
Computing the compression modulus E
c
:
E
c
K
(d
p
/d
v
) (psi, kg/cm
2
)
(3.75)
where
K
V
m
) is a constant of the pressuremeter accounting for borehole
diameter, probe size, and Poisson's ratio (usually taken as 0.3-0.4),
V
o
the initial hole vol-
2(1
ν
)(
V
o
Pressure gage
Relief
valve
Gas line
Pressure
volumeter
Compressed gas
Manometer
Water
line
Concentric
tubing
NX diameter core hole
Exterior guard cell
FIGURE 3.86
Schematic of pressuremeter
equipment. (From Dixon, S. J.,
Determination of the In Situ Modulus of
Deformation of Rock,
ASTM Special
Technical Publication 477, American
Society for Testing and Materials,
Philadelphia, PA, 1970. Reprinted
With permission of the American
Society for Testing and Materials.)
Zone of core hole
under stress
Zone of core hole
under measurement
Interior measuring cell
Core hole expansion device (probe)
