Retaining Structures - Elements of Soil Mechanics - page 238

limit state is applied to assess the rotational stability using this theoretical pressure distribution. The
method generates two equations containing the unknowns d and d 0 , which are solved by repeated itera-
tion until the correct values are obtained.
Example 8.3 illustrates the design of a cantilever sheet pile wall using the GEO limit state.

Traditional methods

Various traditional methods of design exist and some are still in use. Each involves the determination of
an overall factor of safety for passive resistance, F p , based on different lateral earth pressure distributions.
The methods are described in detail by Padfield and Mair ( 1984) .

(1) Gross pressure method
The method is also referred to as the CP2 method, after the Institution of Structural Engineers' original
Code of Practice published in 1951. It is very unlikely that the full passive resistance for the soil in
front of the wall will be developed. Common practice is to divide the total theoretical value of thrust
K p γ d 2 /2 by a factor of safety, traditionally taken as F p = 2.0. The effective passive resistance in front
of the wall is therefore assumed to have a magnitude of K p γ d 2 /4 and is of trapezoidal distribution, the
centre of pressure of this trapezium lying between d/2 and d/3 above the base of the pile (for ease
of calculation the value is generally taken as d/3). It is common to use lower values of F p for low values
of φ ′ . Padfield and Mair ( 1984 ) recommend the values given in Table 8.1.

Table 8.1 Acceptable F p values for ranges of φ ′ (Padfield and Mair, 1984) .

φ ′ F p

(degrees)
> 30

2.0

20-30 1.5-2.0

< 20

1.5

Calculations are considerably simplified if it is assumed that the passive resistance on the back of
the wall, P p1 , acts as a concentrated load, R, on the foot of the pile, leading to the pressure distribu-
tion shown in Fig. 8.12d , from which d can be obtained by taking moments of thrusts about the base
of the pile. The value of d obtained by this method is more nearly the value of d 0 in Fig. 8.12a , the
customary practice being to increase the value of d by 20% to allow for this effect.
(2) Net available passive resistance method
The method is also referred to as the Burland, Potts and Walsh method after Burland et al. ( 1981) .
They advocate a modified pressure distribution (Fig. 8.12e ) with the effect that the factor of safety is
applied to the net available passive resistance.
(3) Net total pressure method
This was advocated by British Steel in the British Steel piling handbook (1997), where the net horizontal
pressure distribution is used (Fig. 8.12f ). The pressure distribution is derived by subtracting the active
earth and water pressures from the passive earth and water pressures.

8.6.2 Dealing with passive earth pressure

There is a question over how passive pressure should be treated when using the Eurocode 7 GEO limit
state, as it could be regarded as either a favourable action or as a resistance .
i.e. to establish the design passive resistance we have either: