Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.77
weight to resist failure from overturning and sliding. Counterfort walls consist of
a footing, a wall stem, and intermittent vertical ribs (called counterforts) that tie
the footing and wall stem together. Crib walls consist of interlocking concrete members
that form cells which are then filled with compacted soil.
Granular soils (sands or gravels) are the standard recommendation for backfill
material. There are several reasons for this recommendation:
1. Predictable Behavior. Import granular backfill generally has a more predictable
behavior in terms of earth pressure exerted on the wall. If silts or clays are used
as backfill material, expansive soil-related forces could be generated by these
soil types.
2. Drainage System. To prevent the build-up of hydrostatic water pressure on the
retaining wall, a drainage system is often constructed at the heel of the wall.
This system will be more effective if highly permeable granular soil is used as
backfill.
3. Frost Action. In cold climates, the formation of ice lenses in the backfill soil
can cause so much lateral movement that the retaining wall will become unusable.
Backfill soil consisting of granular soil and the installation of a drainage
system at the heel of the wall will help to protect the wall from frost action.
6.7.1 Retaining Wall Analyses
Figure 6.34 shows various types of retaining walls and the soil pressures acting on
the walls. Three types of soil pressures act on a retaining wall: (1) active earth
pressure, which is exerted on the back side of the wall, (2) passive earth pressure,
which acts on the front of the retaining wall footing, and (3) bearing pressure,
which acts on the bottom of the retaining wall footing. These three pressures are
individually discussed below.
Active Earth Pressure. In order to calculate the active earth pressure resultant
force (P A), in kN per linear meter of wall or pounds per linear foot of wall, the
following equation is used for granular backfill:
1 2
PA � ⁄2kA�tH (6.29)
where k A � active earth pressure coefficient
� t � total unit weight of the granular backfill
H � height over which the active earth pressure acts as defined in Fig. 6.34a
In its simplest form, the active earth pressure coefficient (k A) is equal to:
2 1
kA � tan (45� � ⁄2�) (6.30)
where � � friction angle of the granular backfill. Equation (6.30) is known as the
active Rankine state, after the British engineer Rankine, who in 1857 obtained this
relationship. Equation (6.30) is valid only for the simple case of a retaining wall
that has a vertical rear face, no friction between the rear wall face and backfill soil,
and the backfill ground surface is horizontal. For retaining walls that do not meet
these requirements, the active earth pressure coefficient (k A) for Eq. (6.29) is often
determined using the Coulomb equation (see Fig. 6.35). Often the wall friction is
neglected (� � 0�), but if it is included in the analysis, typical values are � � 3 ⁄4�