Environmental Engineering Reference
In-Depth Information
The constant
c
depends on the position, shape, and size of the area under pressure as
well as the location of the measured points, and has the dimension of length. It may be
determined by model tests in large plaster blocks for a given jack and strain gage arrange-
ment.
Borehole Jack Tests
Apparatus
The dilatometer (
Figure 3.73)
and the Goodman jack (Goodman et al., 1968), devices similar
to the pressuremeter (see
Section 3.5.4)
,
are lowered into boreholes to measure moduli
in situ
.
Hydraulic pressure is applied between a metal jacket and a deformable rubber or
metallic jacket in the apparatus which presses against the borehole walls. Linear variable
differential transducers (LDVTs) in the instrument allow the measurement of four diam-
eters, 45° apart, to account for lateral rock anisotropy. Designed to operate underwater
in NX-size boreholes, Interfel dilatometers upto 6-in. in diameter are available.
Dilatometers measure Young's modulus up to about 25 GPa and the Goodman jack up
to about 50 GPa.
Computing Moduli
Rocha (1970) gives the following expression for computing
E
r
:
E
r
2
r
(1
ν
/
∆
)
P
(3.61)
where 2
r
is the hole diameter,
ν
the Poisson's ratio, and
∆
the deformation when pressure
P
is applied
Methods Compared
A series of tests was performed in jointed granite for the Tokyo Electric Power Company
using a number of methods (Hibino et al, 1977) including
in situ
triaxial testing, and the
results were compared. The
in situ
triaxial compression test was performed on a 1m
3
block
of rock carved out of the rock mass but with its base remaining intact with the mass. The
block was surrounded with 1500-ton-capacity flat jacks to provide confinement, the space
between the rock and the jacks was backfilled with concrete, and loads were applied to the
rock by the jacks.
Moduli values measured by the various tests are summarized in
Figure 3.74.
A wide
range in values is apparent. The largest values were obtained from laboratory sonic col-
umn and uniaxial compression tests, which would not be expected to be representative of
mass conditions. The larger plate-jack test resulted in less deflection (higher modulus)
than did the smaller plate and would be considered as more representative of deformation
characteristics, as would the water chamber test.
Modulus and Rock-Quality Relationships
Selection of Design Moduli
Deformation is related to rock-mass quality, which, because of fabric, discontinuities, and
decomposition, can be extremely variable. Most practical problems involve poor-quality
rock, which is the most difficult to assess.
The various test methods, in general, individually often do not yield representative val-
ues for
E
r
. All static methods disturb the rock surface during test setup, and most stress a
