Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 7 RETAINING WALL
7.4.2.3 Effects of Wall Friction
The magnitude and direction of the developed wall friction depends on the relative movement
between the wall and the soil. In the active case, the maximum value of wall friction develops only
when the soil wedge moves significantly downwards relative to the rear face of the wall. In some
cases, wall friction cannot develop. These include cases where the wall moves down with the soil,
such as a gravity wall on a yielding foundation or a sheet pile wall with inclined anchors, and cases
where the failure surface forms away from the wall, such as in cantilever and counterfort walls.
The maximum values of wall friction may be takes as follows :
Timber, steel, precast concrete wall
Cast in-situ concrete wall
δ max. = Ø’/2
δ max. = 2 Ø’/3
Considerable structural movements may be necessary, however, to mobilize maximum wall friction,
for which the soil in the passive zone needs to move upwards relative to the structure. Generally,
maximum wall friction is only mobilized where the wall tends to move downwards, for example, if a
wall is founded on compressible soil, or for sheet piled walls with inclined tensioned members.
Some guidance on the proportion of maximum wall friction which may develop in various cases is
given below (Teng)
δ = 20 0 concrete or brick walls
= 15 0 uncoated sheetpile
= 0 0 if wall tends to move downward together with the soil
= 0 0 sheetpiling with small penertration or penetrated into soft or loose soil
= 0 0 if backfill is subjected to vibratiion
In general, the effects of wall friction on Rankine and Coulomb methods of earth pressure
computation are as follows:
a) The Rankine method cannot take account of wall friction. Accordingly, K a is overestimated
slightly and K p is under-estimated, thereby making the Rankine method conservative for
most applications.
b) The Coulomb theory can take account of wall friction, but the results are unreliable for
passive earth pressures for wall friction angle values greater than φ′/3 because the failure
surface is assumed to be a plane. The failure wedges assumed in the Coulomb analysis take
the form of straight lines as shown in Figure 7.8. However, this contrasted with the curved
shapes of failure surface observed in many model tests. This assumption resulted in K a
being underestimated slightly and K p being overestimated very significantly for large values
of φ′.
In general, the effect of wall friction is to reduce active pressure. It is small and often disregarded.
However, wall friction increases the value of K p significantly and thus could yield lateral earth
pressure that could be very large and could be unsafe as passive earth pressure forces are generally
resisting forces in stability analysis
March 2009 7-9