Environmental Engineering Reference
In-Depth Information
Hoek and Bray (1981) suggest the use of the equation:
Qln(2mL D)
2LH
/
ke
(5.3)
p
where m
(ke/kp)
1
⁄
2
kp
equivalent permeability parallel to the hole
ke
equivalent permeability normal to the hole
D
diameter of hole.
They suggest that for most applications ke/kp is about 10
6
. This implies no fractures
parallel to the hole and in most rocks would not be a reasonable approximation.
If it is assumed that ke/kp
10, and water pressure testing in 5 m lengths of NMLC
hole (75 mm diameter), then 1 Lugeon is equivalent to ke
10
7
m/sec.
1.6
For ke/kp
1, i.e. homogeneous, isotropic conditions, m
1 and 1 Lugeon is equiva-
10
7
m/sec. The Moye (1967) formula gives similar results to this
homogeneous isotropic case.
lent to ke
1.3
5.14.2
Test methods
There are two common methods of water pressure testing in a drill hole (
Figure 5.30
). The
“down-stage” (or “single-packer”) method is recommended and involves isolating and testing
successively the bottom sections of the drill hole. This method enables progressive assessment
of permeability and allows later stabilisation of the wall of the hole by casing or grouting if
caving occurs. A disadvantage of down-stage packer testing is that it disrupts drilling progress,
but this is far outweighed by its advantages over the alternative method discussed below.
The alternative method is to complete the drilling of the hole and water test in sections by
sealing the hole above and below the test area (the “double-packer” method). This method
has the advantage of convenience in that all water testing is carried out at one time but
results can be affected by:
-
Damage to the sides of the hole by drill rods and casing;
-
Possible leakage from the test section past the lower packer, which cannot be detected, and
-
Sections of the hole which have been stabilised by cement or by casing to enable deeper
drilling cannot be tested. Commonly these sections will be highly permeable.
The purpose of the test is to estimate the potential of water to pass through rock
defects. The use of drilling mud to stabilise the hole can block these defects and make the
results of water pressure testing meaningless. For best results the drilling fluid used in
holes where permeability testing is required should be water. If necessary a small amount
of soluble oil appears to improve drilling efficiency without affecting permeability.
There are several proprietary chemical drilling muds such as “Revert” which are
reputed to break down and dissolve when treated with “Fastbreak”. Testing experience
indicates that this procedure is only partly effective and that holes drilled with Revert give
lower indicated permeabilities than holes in the same situation drilled with water.
5.14.3
Selection of test section
To maximise the information on rock mass permeability the investigation drill hole
should be oriented to intersect as many joints as practicable (see
Figure 5.31
)
.
Every effort should be made to test the total length of hole in rock. It is preferable to over-
lap sections and thus have two tests over a short length than to miss some length of hole.
The upper limit which can be tested is the highest level at which a packer can be satis-
factorily sealed, often in distinctly weathered rock. Location of the packer within casing
above the rock does not seal the hole as water may leak past the casing.
