SOIL AND GROUNDWATER - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 57

the free water table or interfaces between fluids (multiple fluid flow, such as fresh
and salt water). The Hele Shaw model is an ideal method to visualise effects. It is,
however, not suitable to simulate transport by flow in heterogeneous fields.
A typical example is the free surface flow through the model (Fig 4.4). The
outcome is the well known Dupuit formula for flow through earthen dams ( B is the
length of the dam)

Q = Bk ( H 1 2 - H 2 2 )/2 L (4.15)

This formula includes the aspect of vertical flow components and the seepage
face (see Fig 4.4).

Heterogeneity and anisotropy
Since the soil structure and constitution may vary in space, the corresponding
permeability will vary, which is referred to as heterogeneity. Different values for
permeability are given in Table 2.6.
When the soil structure shows a different pattern in horizontal and vertical
direction, the flow for similar gradients is not the same in different directions. This
is called anisotropy. For horizontally sedimented soils in delta areas, anisotropy is
given by the ratio of horizontal and vertical permeability, e.g. k h /k v = 3 . In
heterogeneous substrata the groundwater flow and corresponding groundwater
pressures are not easily obtained. User-friendly computer programs are available to
determine the groundwater flow pattern. Comprehensive computer programs
provide modules to calculate the groundwater pressures as a part of a total analysis.

C BOILING , HEAVE AND PIPING

Boiling
Granular soils subjected to upward vertical flow can become a mud (heavy
fluid). This process of fluidisation is used in laboratory tests to make low-density
sand. When fluidised, the sand lost it strength and one refers to its constitution by
the quick or boiling condition. It is also called liquefaction, which is further
outlined in Chapter 16. The vertical equilibrium of such soil is described by

( ' u )

v v

' + w (4.16)

z z

' is the submerged weight of the soil. Using (4.7) and (4.8) the pore
pressure gradient can be expressed by

Here,

u

( i ) (4.17)

w

z

Here, i = h / z is the potential gradient. From (4.16) and (4,17) one finds for the

effective stress w is the seepage force per volume and it

causes the effective stress to vanish at any value of z , when i = i c =

v ' = ( ' i w ) z . The factor i

'/

w , the so-