Civil Engineering Reference
In-Depth Information
TABLE 14.9
Retaining Walls
Topic
Discussion
Introduction
The main provisions in the
International Building Code
(2012) for retaining walls
are Sec. 1610 (Soil Lateral Loads) and Sec. 1807 (Foundation Walls, Retaining
Walls and Embedded Posts and Poles).
General
regulations
General provisions concerning retaining walls are presented in Sec. 1807 of the
International Building Code
(2012), as follows:
“Section 1807.2.1 General. Retaining walls shall be designed to ensure stability
against overturning, sliding, excessive foundation pressure and water uplift.
Where a keyway is extended below the wall base with the intent to engage passive
pressure and enhance sliding stability, lateral soil pressures on both sides of the
keyway shall be considered in the sliding analysis.
Section 1807.2.2 Design lateral soil loads. Retaining walls shall be designed
for the lateral soil loads set forth in Section 1610.
Section 1807.2.3 Safety factor. Retaining walls shall be designed to resist the
lateral action of soil to produce sliding and overturning with a minimum safety
factor of 1.5 in each case. The load combinations of Section 1605 shall not apply
to this requirement. Instead, design shall be based on 0.7 times nominal earth-
quake loads, 1.0 times other nominal loads, and investigation with one or more
variable loads set to zero. The safety factor against lateral sliding shall be taken
as the available soil resistance at the base of the retaining wall foundation divided
by the net lateral force applied to the retaining wall. Exception: Where earth-
quake loads are included, the minimum safety factor for retaining wall sliding and
overturning shall be 1.1.”
Regulations
for lateral
loads
Concerning lateral soil loads on retaining walls, the
International Building Code
(2012) states:
“Section 1610.1 General. Foundation walls and retaining walls shall be
deigned to resist lateral soil loads. Soil loads specific in Table 1610.1 [see Table
14.10] shall be used as the minimum design lateral soil loads unless determined
otherwise by a geotechnical investigation in accordance with Section 1803.
Foundation walls and other walls in which horizontal movement is restricted at
the top shall be designed for at-rest pressures. Retaining walls free to move and
rotate at the top shall be permitted to be designed for active pressure. Design
lateral pressure from surcharge loads shall be added to the lateral earth pressure
load. Design lateral pressure shall be increased if soils at the site are expansive.
Foundation walls shall be designed to support the weight of the full hydrostatic
pressure of undrained backfill unless a drainage system is installed in accordance
with Sections 1805.4.2 and 1805.4.3. Exception: Foundation walls extending not
more than 8 feet (2438 mm) below grade and laterally supported by the top by
flexible diaphragms shall be permitted to be designed for active pressure.”
5.
Frost action.
If freezing temperatures prevail, the backfill soil can be susceptible
to frost action, where ice lenses will form parallel to the wall. Backfill soil consisting
of clean granular soil and the installation of a drainage system at the heel of the wall
will be much more effective in preventing frost action than using clayey backfill.
6.
Seismic loads.
Clayey soil can trap groundwater behind the wall leading to a
weakening of the soil during the earthquake. The weakened soil could lead to higher
earth pressures than as originally designed, resulting in excessive wall movement
and/or bearing capacity failures.
