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26 Chapter Two

2.6 Hydraulic properties of fractured rocks

By adopting the Darcian approach to the analysis
of groundwater flow described in Section 2.3, it is
implicit that the physical assemblage of grains that
comprise the porous material are considered as a rep-
resentative continuum, and that macroscopic laws,
such as Darcy’s law, provide macroscopically averaged
descriptions of the microscopic behaviour. In other
words, Darcy’s law describes groundwater flow as
a flux through a porous material that is imagined to
Fig. 2.7 Four possible combinations of heterogeneity and
have continuous, smoothly varying properties. In
anisotropy describing the hydraulic conductivity of a porous
reality, intergranular and fractured porous materials
material. After Freeze and Cherry (1979).
are highly heterogeneous when examined at a scale
similar to the spacing of the dominant pore size. The
where K , K and K are the hydraulic conductivity consequence of this is that Darcy’s law can be used
x y z
values in the x, y and z directions. successfully, but only at a scale large enough to con-
tain a representative assemblage of pores. This is the
continuum scale. At subcontinuum scales, the local
2.5 Aquifers, aquitards and aquicludes pore network geometry strongly influences flow and
the transport of contaminants. This is particularly rel-
Natural variations in the permeability and ease of evant in fractured rocks where the dimension of the
transmission of groundwater in different geolo- fracture spacing can impart a continuum scale that
gical materials lead to the recognition of aquifers, exceeds the size of many practical problems.
aquitards and aquicludes. An aquifer is a layer or lay- In fractured material such as carbonate and crystal-
ered sequence of rock or sediment comprising one line rocks and fissured clay sediments such as glacial
or more geological formations that contains water tills, the conceptual model of groundwater flow can
and is able to transmit significant quantities of water either be grossly simplified or a detailed description
under an ordinary hydraulic gradient. Aquifers there- of the aquifer properties attempted as depicted in
fore have sufficient permeability to transmit ground- Fig. 2.8. With the exception of conduit flow in karst
water that can be exploited economically from wells aquifers, fracture flow models generally assume that
or springs. Good aquifers are usually developed in both fracture apertures and flow velocities are small
sands, gravels, solutionally weathered limestones and such that Darcy’s law applies and flow is laminar
fractured sandstones. (Box 2.2). In the example of the equivalent porous
The term aquitard is used to describe a formation material shown in Fig. 2.8b, the primary and sec-
of lower permeability that may transmit quantities of ondary porosity and hydraulic conductivity distribu-
water that are significant in terms of regional ground- tions are represented as the equivalent or effective
water flow, but from which negligible supplies of hydraulic properties of a continuous porous material.
groundwater can be obtained. Examples of aquitards A drawback with this approach is that it is often dif-
include fluvial and glacio-fluvial silts and sandy clays, ficult to determine the size of the representative
sedimentary rocks with few fractures and fractured elemental volume of material from which to define
crystalline rock. the effective hydraulic property values. Hence, the
An aquiclude is a saturated geological unit of equivalent porous material approach may adequately
such low permeability that it is incapable of transmit- represent the behaviour of a regional flow system but
ting significant quantities of water under ordinary is likely to reproduce local conditions poorly.
hydraulic gradients and can act as a barrier to regional More advanced approaches, such as the discrete
groundwater flow. Aquiclude rocks include clays, fracture and dual-porosity models shown in Figs 2.8c
shales and metamorphic rocks. and 2.8d, represent groundwater movement through