29 Simple Retaining Walls
WHAT IS A RETAINING WALL ?
A structure introduced to achieve a step change in adjacent ground
levels, or
A structure to hold an otherwise unstable bank of ground.
(Really the same definitions)
New ground
levels
Old ground
level
This is a simple retaining wall - there are many other types . . .
basement
Observation:
Swimming Retaining
pool walls are really
Building vertical slabs,
with with special
basement configurations
and loadings.
Bridge
abutment
Some examples
FREE-STANDING CANTILEVER RETAINING WALL
In this course, we’ll confine our attention to this simple wall:
Boundary Boundary
Stem
Heel
(may be
tapered)
Toe
Base
No Heel No Toe
Where there are no
boundary constraints Where there are
boundary constraints
So how does the wall serve our purpose ?
May cause wall to So there are two
slide and/or stability problems.
overturn Resistance to sliding
and overturning are
offered by . . . .
Weight of base
Earth and stem
pressure
on stem
Passive earth
Weight of pressure on face
soil above of toe
heel
The base and stem
Friction between Soil pressure bend, so there is
base and soil acting on base also a strength
problem to consider.
So we need primary
rebar thus:
tension
tension
tension
So a design approach must address both stability
(sliding and overturning) and strength . . .
DESIGN APPROACH
Design Steps suggested:
1. Estimate horizontal soil force on stem,
2. Select trial dimensions,
3. Check for sliding and overturning,
4. Check sub-soil pressures,
5. Estimate critical bending and shear,
6. Select concrete grade and rebar cover,
7. Select rebar, observing minimum requirements.
In the following example, we assume the soil is clean sand or
gravel which is free-draining (no water pressure), backfill is
horizontal (no slope) and there is no surcharge loading . . .
1. Estimate horizontal soil force on stem
Reasonably accurate to assume soil pressure increases linearly with
depth. So lateral pressure at depth x : px = K γ x :
po = 0
Three
cases to
x
consider:
At rest: Active : Passive :
px = K γ x
Ko = 0.4-0.6 Ka = 1 - sin φ Kp = 1 + sin φ
1 + sin φ 1 - sin φ
This is the case for
our free-standing wall. So resultant force P
H acting on stem per unit
P length of wall
= 1/2 Ka γ H2
2. Select trial dimensions
Software packages are available to assist. The better packages consider
practical, as well as structural aspects. Some guidance:
>= 200 mm
Stem H
thickness =
H/8 to H/12
Depth of toe =
1.2 * stem width
Width of toe =
Base width = (0.25 to 0.4) *
0.4H to 0.7H base width
3. Check for sliding and overturning
These are stability concerns - so
Disturbing forces are augmented, and
Restoring forces are discounted.
The load factors are :
Disturbing : 1.5 Restoring : 0.8
SLIDING OVERTURNING
WA
yA
PA Ws PA ys
Pp yb
yg
Wb
Restoring force = μ Σ W + PP Restoring moment = Σ (Wy)
Disturbing force = PA Disturbing moment = PA H/3
TEST 0.8 (μ Σ W + PP) >= 1.5 PA TEST 0.8 Σ (Wy) >= 1.5 PA H/3
4. Check sub-soil pressures
This force ΣW is the
resultant of the soil
Two checks: pressures applied
upwards on the base.
(1) Ensure that
pmin is e = eccentricity from
compressive, ie centroid
that resultant e Assuming the pressure
lies in middle- pmin
is linearly distributed:
third of the
base (emax=D/6) pmax pmax , min =
ΣW / D ( 1 ± 6e / D)
. . . and
(2) Ensure that pmax < pall
5. Estimate critical bending and shear
do
STEM :
P* = V*max for
1.5 1/2 Kγ H2 design
H/3 do
M*max= P* H/3 V*max = P*
UBMD USFD
Comment:
With the dimensions commonly
encountered, shear is sometimes
a problem, so always check !
HEEL : TOE :
do do
M*max UBMD
UBMD
M*max
USFD USFD
V*max at face V*max at do from
of stem stem
6. Select concrete grade and rebar cover
Concrete Grade :
There are two exposure conditions to
consider:
1. In contact with soil, and
2. Exposure to climate.
Select the most severe.
Cover to Rebar :
Different cover is usually required for
each face. Make sure that this is
clearly shown on drawings.
7. Select rebar, observing minimum requirements
The same procedures as for design of slabs :
P* = V*max
1.5 1/2 Kγ H2 for
design
H/3
do
M*max= P* H/3
e.g. Stem: Ast = M* max / (0.8 * 0.9 * d * fsy)
Select dia. and spacing, observing Ast.min, and s max
Check that φ Vuc > V* max at do from base.
Similarly for heel and toe.
SUMMARY
• Retaining walls are used to achieve step changes in ground
levels, and must therefore resist soil pressures stably
and safely,
• Retaining walls are just like vertical slabs, with special
configurations and loadings,
• Walls may form parts of structures, or may be free-standing,
• For free-standing walls, both sliding and overturning must
be considered, using load factors of 1.5 for disturbing
forces and 0.8 for restoring forces,
