22
E10037-180424
PROBEX
m
= Poisson's ratio of calibration tube material
E
m
= Modulus of elasticity of calibration tube material
The PROBEX is supplied with a standard, steel calibration tube having the following
characteristics:
nominal I.D.: 76.2 mm
nominal O.D.: 101.6 mm
E
m
(modulus of elasticity): 207 x 10
6
kPa
m
(Poisson's ratio): 0.30
Using these values in the previous equation and assuming a dilatable length of the PROBEX
membrane equal to 457 mm, one obtains the following "b" value:
b
= 86.6 x 10
-6
cm
3
/kPa
Important considerations on volume correction
The volume correction factor
‘c’
enters in the equation used to calculate the modulus of
deformation of the rock. A high level of confidence in the modulus calculated therefore
depends on the exactness of the
‘c’
value and of its representativeness of the actual intrinsic
deformation of the PROBEX during the tests. Normally, the factor
‘c’
obtained from the last
of the series of calibrations is used for data reduction. The correction factor
‘c’
is calculated
typically between 15 and 30 MPa. The stiffer the rock is the greater the influence of
correction factor
‘c’
. It is also recommended to run a volume calibration after a test if the
operator has any reason to believe that the factor
‘c’
might have changed.
Volume of the probe at rest (Vo)
Vo
is theoretically
1950 cm
3
considering the theoretical length (45.7 cm) and diameter (7.37
cm) of the membrane. For a more representative result of
Vo
the user can either 1) measure
directly the length of the dilatable portion (length between the steel ends) and the mean
diameter on the specific membrane used during testing, or 2) use the following formulae
according to the ASTM D4719 standard:
Vo = (pi*D
2
/4) L - Vi
Where:
-
L
is the length of the membrane
-
D
is the inside diameter of the calibration tube
-
Vi
is the resulting intercept of a straight line extension of the volume
calibration curve. Refer to the ASTM D4719 standard for more details.
In softer material, the yielding pressure can be determined and, as for a soil pressuremeter,
a limit pressure P
L
can be estimated.
Determination of Radial Expansion of the membrane
The volume reading can be converted into radial expansion
using cylinder volume equation :
V = (pi x D
2
/ 4) x L
With:
V =
Volume in cc of the membrane
L
= Length of the membrane (nominal value = 45.7 cm)
D
= Diameter of the membrane (nominal value = 7.37 cm)
Injected
volume (cm
3
)
Diameter -
approx. (cm) *
0
73.7
100
75.6
200
77.4
300
79.2
400
80.9
500
82.6
600
84.2
676
85.5
* Depending on Vo
