Civil Engineering Reference
In-Depth Information
For some projects, temporary retaining walls may be constructed of sheeting (such as
sheet piles) that are supported by horizontal braces, also known as
struts.
Near or at the top
of the temporary retaining wall, the struts restrict movement of the retaining wall and prevent
the development of the active wedge. Because of this inability of the retaining wall to deform
at the top, earth pressures near the top of the wall are in excess of the active (
k
A
) pressures. At
the bottom of the wall, the soil is usually able to deform into the excavation, which results in
a reduction in earth pressure. Thus the earth pressures at the bottom of the excavation tend to
be constant or even decrease, as shown in Fig. 10.10.
The earth pressure distributions shown in Fig. 10.10 were developed from actual measure-
ments of the forces in struts during the construction of braced excavations. In Fig. 10.10,
case
a
shows the earth pressure distribution for braced excavations in sand and cases
b
and
c
show the earth pressure distribution for clays. In Fig. 10.10, the distance
H
represents the
depth of the excavation (i.e., the height of the exposed wall surface). The earth pressure dis-
tribution is applied over the exposed height
H
of the wall surface with the earth pressures
transferred from the wall sheeting to the struts (the struts are labeled with forces
F
1
,
F
2
, etc.).
Any surcharge pressures, such as surcharge pressures on the ground surface adjacent to
the excavation, must be added to the pressure distributions shown in Fig. 10.10. In addition,
if the sand deposit has a groundwater table that is above the level of the bottom of the excava-
tion, then water pressures must be added to the case
a
pressure distribution shown in Fig. 10.10.
Because the excavations are temporary (i.e., short-term condition), the undrained shear
strength (
s
u
c
) is used for the analysis of the earth pressure distributions for clay. The
earth pressure distributions for clay (i.e., cases
b
and
c
) are not valid for permanent walls
or for walls where the groundwater table is above the bottom of the excavation.
10.6.2 Earthquake Analysis
Since temporary retaining walls are usually only in service for a short time, the possibility
of earthquake effects is typically ignored. However, in active seismic zones or if the con-
sequence of failure could be catastrophic, it may be prudent to perform an earthquake
analysis. Depending on whether the wall is considered to be yielding or restrained, the
analysis would be based on the data in Sec. 10.2 or Sec. 10.5. Weakening of the soil dur-
ing the design earthquake and its effects on the temporary retaining wall should also be
included in the analysis.
10.7 PROBLEMS
The problems have been divided into basic categories as indicated below.
Pseudostatic Method
10.1
Using the retaining wall shown in Fig. 10.4, assume
H
4 m, the thickness of the
reinforced concrete wall stem
0.4 m, the reinforced concrete wall footing is 3 m wide by
0.5 m thick, the ground surface in front of the wall is level with the top of the wall footing,
and the unit weight of concrete
23.5 kN
m
3
. The wall backfill will consist of sand having
32
and
t
20 kN
m
3
. Also assume that there is sand in front of the footing with these
same soil properties. The friction angle between the bottom of the footing and the bearing
soil
38
. For the condition of a level backfill and neglecting the wall friction on the
backside of the wall and the front side of the footing, determine the resultant normal force
N
and the distance of
N
from the toe of the footing, the maximum bearing pressure
q
and
the minimum bearing pressure
q
exerted by the retaining wall foundation, factor of safety
