Geology Reference
In-Depth Information
Foliation is a characteristic feature of metamorphic rocks. Along a fault,
relative movement of rock mass takes place while long discontinuities
extending over several km are termed as lineaments. Fracturing is caused by
tectonic stresses, residual stresses, contraction-cooling and desiccation,
unloading and weathering.
Fractures and other discontinuities are the most important geological
structures from the hydro-geological point of view as they facilitate storage
and movement of fluids through them. On the other hand, some discontinuities,
e.g. faults and dykes may also act as barriers to groundwater flow.
A number of factors including stress, fracture geometry and temperature
etc. control the groundwater flow through fractures. For example, fracture
aperture and flow rate are directly interrelated, non-parallelism of walls and
wall roughness leads to friction losses, hydraulic conductivity of fractures is
inversely related to normal stresses and depth because normal stress tends to
close the fractures and reduce the hydraulic conductivity. Fractures parallel
to the maximum compressive stress tend to be open, whereas those
perpendicular to this direction tend to be closed.
Experience in the hard rock areas of Scandinavian countries and in India
indicate that tensional fractures are more open and transmissive than shear
fractures, as the latter are held closed by a component of normal stress. This
hydro-tectonic model was originally suggested by Larsson.
As fracture geometry and their hydraulic properties greatly depend on
stress distribution, palaeo-stress and in situ stress analysis is important. Studies
in the hard rock areas of Norway have also indicated the importance of post-
glacial isostatic rebound on well yields (Banks et al., 1996).
In addition to stress distribution, interconnectivity of fractures, fracture
aperture and other fracture properties are also important from permeability
point of view. Fracture permeability reduces with increasing temperature. As
temperature increases with depth, thermal expansion in rocks takes place,
which leads to reduction in fracture aperture and corresponding decrease in
permeability. Further, the fracture permeability also decreases by cementation,
filling and weathering etc.
Sheeting joints developed in granitic rocks due to weathering and unloading
impart higher hydraulic conductivity horizontally. However at deeper level
vertical fractures may be the main cause of groundwater flow where
K
h
/
K
v
may be less than one.
In view of the above, from hydrogeological point of view, including
movement of contaminants, it is extremely important to study the structure
of rock mass and quantify the pattern and nature of fractures etc.
Fractures, lineaments and faults can be studied in the field outcrops, and
by aerial photographs, remote sensing as well as from bore hole surveys.
Fractures have certain orientation, spacing, aperture, length etc. which are
studied in rock outcrops in the field. Several sets of fractures are often
