Environmental Engineering Reference
In-Depth Information
5.14
IN SITU
PERMEABILITY TESTS IN ROCK
5.14.1
Lugeon value and equivalent rock mass permeability
The permeability of the rock mass can be determined by either constant head tests or
falling head tests. A section of a drill hole is isolated using a sealing packer and water is
added to maintain a constant head, or the rate of fall in water level is measured after a
slug of water is added to the hole. Both methods suffer from the potential effects of smear
and clogging of defects, but by careful flushing of the hole before testing, reasonable val-
ues can be obtained. In falling head tests the additional pressure which can be added to
the test section is limited by the level of the test section and the practicability of extending
the pipe imposing the head above ground level.
The most common and effective method of measuring rock mass permeability is the
water pressure test (also known as the Lugeon or “packer” test). The test consists of iso-
lating a section of drill hole and pumping water under pressure into that section until the
flow rate for any given pressure is constant (i.e. it is a constant head test). The use of suc-
cessive rising and falling test pressures establishes the relationship between the volume of
water accepted into the section and the pressure, to provide an estimation of permeabil-
ity, and indicate water flow mechanisms.
As rock substance is generally almost impermeable, the permeability determined in this
test represents an indication of the number, continuity and opening of the rock defects
which intersect the wall of the borehole in the test section.
Results are expressed in Lugeon (uL) units. A Lugeon is defined as the water loss of
1 litre/minute per metre length of test section at an effective pressure of 1 MPa.
Indicative rock permeabilities are:
Lugeon
Range
Condition
1
Low
Joints tight
1-5
Low/Mod
Small joint openings
5-50
Mod/High
Some open joints
50
High
Many open joints
There is no unique relationship between Lugeon value and equivalent rock mass per-
meability (ke). Moye (1967) recommended use of the equation:
QC
LH
ke
(5.1)
where ke
the equivalent coefficient of permeability (m/sec)
the flow rate (m
3
/sec)
Q
L
the length of the test section (m)
H
Net head above the static water table at the centre of the test section (m)
1
ln(L/2r)
2
C
(5.2)
p
r
the radius of the hole (m).
This is based on the assumption of radial laminar flow in a homogeneous isotropic rock
mass, a condition seldom, if ever, achieved.
