Environmental Engineering Reference
In-Depth Information
3.3
Hydraulic Properties (Permeability)
3.3.1
Introduction
Flow-Through Geologic Materials
Definitions and Relationships
Permeability
, the capacity of a material to transmit water, is described in detail in
Section
8.3.2,
along with other aspects of subsurface flow, and is only summarized in this chapter.
Flow through a geologic medium is quantified by a material characteristic termed the
coef-
ficient of permeability k
(also known as coefficient of hydraulic conductivity), expressed in
terms of Darcy's law, valid for laminar flow in a saturated, homogeneous material, as
k
q
/
iA
(cm/sec)
(3.5)
where
q
is the quantity of flow per unit of time (cm
3
/sec),
i
the hydraulic gradient, i.e., the
head loss per length of flow
h
/
L
(a dimensional number) and,
A
the area (cm
2
).
Values for
k
are often given in units other than cm/sec. For example, 1 ft/day
0.000283
ft/day.
Secondary permeability
refers to the rate of flow through rock masses, as contrasted with
that through intact rock specimens, and is often given in Lugeon units (see
Section 3.3.4)
.
cm/sec; cm/sec
3528
Factors Affecting Flow Characteristics
Soils:
In general, gradation, density, porosity, void ratio, saturation degree, and stratifica-
tion affect
k
values in all soils. Additional significant factors are relative density in granu-
lar soils and mineralogy and secondary structure in clays.
Rocks: k
values of intact-rock relate to porosity and saturation degree.
k
values of
in situ
rock relate to fracture characteristics (concentration, opening width, nature of filling),
degree of saturation, and level and nature of imposed stress form (compressive or tensile).
Tensile stresses, for example, beneath a concrete dam can cause the opening of joints and
foliations, significantly increasing permeability.
Permeability Considerations
Determinations of k values
k
values are often estimated from charts and tables (see
Section 3.3.2)
or can be measured
in laboratory tests (see
Section 3.3.3)
or
in situ
tests (see
Section 3.3.4).
Applications
k
values as estimated or measured in the laboratory, are used for:
Flow net construction and other analytical methods to calculate flow quantities
and seepage forces.
●
Selection of groundwater control methods for surface and underground
excavations.
●
Design of dewatering systems for excavations.
●
Evaluation of capillary rise and frost susceptibility.
●
Evaluation of yield of water-supply wells.
●
