Retaining walls - Piling Engineering - page 238

Figure 6.15 Lateral pressure due to strip load (Boussinesq equation modified by experiment). (After
Teng, 1962.)

at reasonable cost in that condition and can be removed either in short sections as
permanent work proceeds or at a stage when propping from the permanent works is
possible, they offer a valuable means of minimizing costs in certain instances.
There are various ways in which the effects of berms can be assessed, and while
little detailed research has been carried out into their behaviour, experience con-
firms that the approximate methods used in design are adequate. It would in theory
be possible to analyze the passive failure mechanism for a wall including a berm
by an iterative method involving a series of trials with an appropriate plastic fail-
ure mechanism, but this would not remove all uncertainties regarding the state
of stress in the berm, effects of disturbance, softening, etc. and would lead to a
protracted method that would be inconvenient in everyday design. For this rea-
son, several approximate methods are used in practice as indicated in the following
paragraphs.

1 An empirical method is to treat the berm as causing an increase in the effective
ground level on the passive side of the wall. In this method the height of the berm
is treated as not more than 1 3 x the berm width as shown in Figure 6.16, and

the effective ground level is then taken as half the maximum height of the berm at
the point where it contacts the wall. Any stable soil existing above the 1:3 slope
may then be treated as a surcharge acting on the effective ground surface over the
approximate width of the potential passive failure mechanism. While this method
is open to criticism on the grounds that it is highly empirical, it appears to yield
relatively satisfactory and conservative results by comparison with other methods,
both in terms of wall depth and of bending moment. Unless such a berm is wide,
it is unwise to terminate any tied wall above or near to the final excavation level
on the passive side.
2 An alternative method of treating a berm is to convert it to an effective sur-
charge acting on the potential passive failure zone. In this method the effective
self-weight of the berm is calculated and is then distributed over the approximate
width of the passive failure mechanism at the general final site excavation level (see
Figure 6.17), thus increasing the passive pressure available accordingly. In addition

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