Craig's Soil Mechanics 7th Edition
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Craig's Soil Mechanics 7th Edition

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�1 ¼ �k1h1 and �2 ¼ �k2h2

At the common point B, h1 ¼ h2; therefore


¼ �2

Differentiating with respect to s, the direction along the boundary:




¼ 1




¼ v2s


For continuity of flow across the boundary the normal components of discharge
velocity must be equal, i.e.

v1n ¼ v2n

Figure 2.14 Transfer condition.

54 Seepage





¼ 1



Hence it follows that


¼ k1


Equation 2.26 specifies the change in direction of the flow line passing through point B.
This equation must be satisfied on the boundary by every flow line crossing the boundary.
Equation 2.13 can be written as

� ¼ �n



�q ¼ �n


If �q and �h are each to have the same values on both sides of the boundary then



� �

k1 ¼ �n

� �


and it is clear that curvilinear squares are possible only in one soil. If



� �





� �

¼ k1


If the permeability ratio is less than 1⁄10 it is unlikely that the part of the flow net in
the soil of higher permeability needs to be considered.


This problem is an example of unconfined seepage, one boundary of the flow region
being a phreatic surface on which the pressure is atmospheric. In section the phreatic
surface constitutes the top flow line and its position must be estimated before the flow
net can be drawn.

Seepage through embankment dams 55

Consider the case of a homogeneous isotropic embankment dam on an imper-
meable foundation, as shown in Figure 2.15. The impermeable boundary BA is a flow
line and CD is the required top flow line. At every point on the upstream slope BC
the total head is constant (u/�w and z varying from point to point but their sum
remaining constant); therefore, BC is an equipotential. If the downstream water
level is taken as datum then the total head on equipotential BC is equal to h, the
difference between the upstream and downstream water levels. The discharge sur-
face AD, for the case shown in Figure 2.15 only, is the equipotential for zero total
head. At every point on the top flow line the pressure is zero (atmospheric), so total
head is equal to elevation head and there must be equal vertical intervals �z
between the points of intersection between successive equipotentials and the top
flow line.
A suitable filter must always be constructed at the discharge surface in an embank-

ment dam. The function of the filter is to keep the seepage entirely within the dam;
water seeping out onto the downstream slope would result in the gradual erosion of
the slope. A horizontal underfilter is shown in Figure 2.15. Other possible forms of
filter are illustrated in Figure 2.19(a) and (b); in these two cases the discharge surface
AD is neither a flow line nor an equipotential since there are components of discharge
velocity both normal and tangential to AD.
The boundary conditions of the flow region ABCD in Figure 2.15 can be written as


Equipotential BC: � ¼ �kh
Equipotential AD: � ¼ 0
Flow line CD: ¼ q (also, � ¼ �kz)
Flow line BA: ¼ 0

The conformal transformation r ¼ w2
Complex variable theory can be used to obtain a solution to the embankment dam
problem. Let the complex number w ¼ �þ i be an analytic function of r ¼ xþ iz.
Consider the function

r ¼ w2

Figure 2.15 Homogeneous embankment dam section.

56 Seepage


ðxþ izÞ ¼ ð�þ i Þ2
¼ ð�2 þ 2i� � 2Þ

Equating real and imaginary parts:

x ¼ �2 � 2 ð2:28Þ
z ¼ 2� ð2:29Þ

Equations 2.28 and 2.29 govern the transformation of points between the r and w planes.
Consider the transformation of the straight lines ¼ n, where n ¼ 0, 1, 2, 3 (Figure

2.16(a)). From Equation 2.29

� ¼ z

and Equation 2.28 becomes

x ¼ z

� n2 ð2:30Þ

Equation 2.30 represents a family of confocal parabolas. For positive values of z the
parabolas for the specified values of n are plotted in Figure 2.16(b).
Consider also the transformation of the straight lines � ¼ m, where

m ¼ 0, 1, 2, . . . , 6 (Figure 2.16(a)). From Equation 2.29

 ¼ z

and Equation 2.28 becomes

x ¼ m2 � z


Figure 2.16 Conformal transformation r ¼ w2: (a) w plane and (b) r plane.

Seepage through embankment dams 57

Equation 2.31 represents a family of confocal parabolas conjugate with the parabolas
represented by Equation 2.30. For positive values of z the parabolas for the specified
values of m are plotted in Figure 2.16(b). The two families of parabolas satisfy the
requirements of a flow net.

Application to embankment dam section

The flow region in the w plane satisfying the boundary conditions for the section
(Figure 2.15) is shown in Figure 2.17(a). In this case the transformation function

r ¼ Cw2

will be used, where C is a constant. Equations 2.28 and 2.29 then become

x ¼ Cð�2 � 2Þ
z ¼ 2C�

The equation of the top flow line can be derived by substituting the conditions

 ¼ q
� ¼ �kz

Figure 2.17 Transformation for embankment dam section: (a) w plane and (b) r plane.

58 Seepage


z ¼ �2Ckzq
; C ¼ � 1



x ¼ � 1

ðk2z2 � q2Þ

x ¼ 1


� k

� �

The curve represented by Equation 2.32 is referred to as Kozeny’s basic parabola and
is shown in Figure 2.17(b), the origin and focus both being at A.
When z ¼ 0 the value of x is given by

x0 ¼ q

; q ¼ 2kx0 ð2:33Þ

where 2x0 is the directrix distance of the basic parabola. When x ¼ 0 the value of z is
given by

z0 ¼ q
¼ 2x0

Substituting Equation 2.33 in Equation 2.32 yields

x ¼ x0 � z


The basic parabola can be drawn using Equation 2.34, provided the coordinates of one
point on the parabola are known initially.
An inconsistency arises due to the fact that the conformal transformation of the

straight line � ¼ �kh (representing the upstream equipotential) is a parabola, whereas
the upstream equipotential in the embankment dam section is the upstream slope.
Based on an extensive study of the problem, Casagrande [2] recommended that the
initial point of the basic parabola should be taken at G (Figure 2.18) where
GC ¼ 0:3HC. The coordinates of point G, substituted in Equation 2.34, enable the
value of x0 to be determined; the basic parabola can then be plotted. The top flow line
must intersect the upstream slope at right angles; a correction CJ must therefore be
made (using personal judgement) to the basic parabola. The flow net can then be
completed as shown in Figure 2.18.
If the discharge surface AD is not horizontal, as in the cases shown in Figure 2.19, a

further correction KD to the basic parabola is required. The angle � is used to describe
the direction of the discharge surface relative to AB. The correction can be made with

Seepage through embankment dams 59

the aid of values of the ratio MD/MA ¼ �a/a, given by Casagrande for the range of
values of � (Table 2.4).

Seepage control in embankment dams

The design of an embankment dam section and, where possible, the choice of soils are
aimed at reducing or eliminating the detrimental effects of seeping water. Where high
hydraulic gradients exist there is a possibility that the seeping water may cause internal
erosion within the dam, especially if the soil is poorly compacted. Erosion can work its
way back into the embankment, creating voids in the form of channels or ‘pipes’, and
thus impairing the stability of the dam. This form of erosion is referred to as piping.

Figure 2.18 Flow net for embankment dam section.

Figure 2.19 Downstream correction to basic parabola.

Table 2.4 Downstream correction to basic parabola. Reproduced from
A. Casagrande (1940) ‘Seepage through dams’, in Contributions to Soil