Geology Reference
In-Depth Information
3
a
a
K s =
KK
K
(11)
f
m
m
s
12
s
where s is fracture spacing. Usually K m is very low except when rock is
porous and/or fractures are filled with impervious material.
The range of hydraulic conductivity of fractured rocks from some areas
in Europe and USA is given in Fig. 4.
Figure 4. Range of hydraulic conductivity ( K ) and permeability ( k ) in
some crystalline rocks estimated from in situ borehole tests (based on data
from Brace, 1984; Black, 1987). 1. Granite batholith, Monticello, SC, USA;
2. Altnabreac, Scotland; 3. Carynnen, Cornwall, UK; 4. HDR, Cornwall,
UK; 5. Deep drilling in northern Switzerland; 6. Four sites in Sweden.
Relationship of Permeability with Depth
The decrease in permeability with depth in fractured rocks is usually attributed
to reduction in fracture aperture and fracture spacing due to increased stress.
Although, a decrease in permeability with increasing depth is demonstrated
from several places, this decrease may not be systematic, especially at greater
depths (>50 m). The permeability can also vary several order of magnitude
at the same depth (Fig. 5). Higher permeabilities at shallow depths (<50 m)
can be attributed to greater influence of surficial phenomena like weathering
etc. and development of sheeting joints due to unloading. Further, fractures
at the same depth below the ground surface but with different orientations
may be subjected to different stresses and, therefore, may have different
permeability.
Sheeting joints developed at shallow depths in granitic rock terrains are
more open thereby imparting higher hydraulic conductivity horizontally which
imparts a greater heterogeneity. However, at deeper levels vertical fractures
are the main cause of permeability. Lee and Farmer (1993) have suggested
values of K h / K v ranging from 1 to 10.
 
Search WWH ::




Custom Search