c15StabilityofEarth-RetainingStr611 Page 611 10/4/10 5:38:51 PM user-f391
/Users/user-f391/Desktop/24_09_10/JWCL339/New File
15.2
FIGURE 15.1
15. 1
QUESTIONS TO GUIDE YOUR READING
A flexible retaining wall under construction.
DE F INIT IONS OF KEY TERMS
Backfill is the soil retained by the wall.
Active earth pressure coefficient (Ka) is the ratio between the lateral and vertical principal effective
stresses at the limiting stress state when an earth-retaining structure moves away (by a small amount)
from the backfill (retained soil).
Passive earth pressure coefficient (Kp) is the ratio between the lateral and vertical principal effective
stresses at the limiting stress state when an earth-retaining structure is forced against a soil mass.
Gravity retaining wall is a massive concrete wall relying on its mass to resist the lateral forces from the
retained soil mass.
Flexible retaining wall or sheet pile wall is a long, slender wall relying on passive resistance and
anchors or props for its stability.
Mechanical stabilized earth is a gravity-type retaining wall in which the soil is reinforced by thin
reinforcing elements (steel, fabric, fibers, etc.).
15. 2
QUE ST IONS TO GU I D E YO U R REA D I N G
1. What is meant by the stability of earth-retaining structures?
2. What are the factors that lead to instability?
3. What are the main assumptions in the theory of lateral earth pressures?
4. When shall I use either Rankine’s theory or Coulomb’s theory?
5. Does Coulomb’s theory give an upper bound or a lower bound solution?
6. What is the effect of wall friction on the shape of slip planes?
7. What are the differences among a gravity wall, a cantilever wall, a cantilever sheet pile wall, and an
anchored sheet pile wall?
611
c15StabilityofEarth-RetainingStr630 Page 630 10/13/10 8:18:32 PM f-392
630
CHAPTER 15
/Users/f-392/Desktop/Nalini 23.9/ch05
STABILITY OF EARTH-RETAINING STRUCTURES
(c) Prior to wall completion, shear bands (thin zones of soil that reached critical state) in the
compacted backfill may develop.
(d) Quality of construction methods and construction loading, environmental conditions (floods,
heavy rainfall, etc.), human and animal interventions (excavation at toe, dumping of materials
on top of wall, burrowing of holes, etc.) cannot be estimated (at least, accurately) in advance.
(e) You should design the wall so that the backfill soil behaves in a ductile manner. Using f9cs or
lower values would allow the soil to respond in a ductile manner.
2. Total stress analysis in conjunction with an effective stress analysis for fine-grained soils.
1
frcs b and wall adhesion (sw 5 0.5su, but # 50 kPa for
2
active state and sw # 25 kPa for passive case).
3. Conservative values for wall friction ad .
4. A high-quality drainage system to drain water from the backfill and away from the wall.
5. For a retaining wall with a sloping back, you can use an artificial wall face of height, H, from the
heel to the soil surface (Figure 15.10) to calculate the active force. The active horizontal and vertical forces acting on the artificial wall face are Pax 5 Pa cos d and Paz 5 Pa sin d , respectively. In
calculating the active lateral earth pressure coefficient use h 5 0 in Equation (15.16).
What’s next . . . In the next three sections, we will analyze retaining walls to determine their stability. We
will consider an ESA (effective stress analysis) and a TSA (total stress analysis). We begin by considering
the possible failure modes.
15.8 T Y P E S OF R ETA I N I N G WA LLS A N D
MODE S OF FAIL UR E
There are two general classes of retaining walls. One class is rigid and consists of concrete walls relying
on gravity for stability (Figure 15.13). These are called cast-in-place (CIP) gravity and semi-gravity walls.
The other class is flexible and consists of long, slender members of either steel or concrete or wood or
plastic and relies on passive soil resistance and anchors for stability (Figure 15.14).
Backfill
Front face
Back face
Front face
Back face
Backfill
Toe
Base
Heel
Toe
(a) Gravity retaining wall
Backfill
Heel
Shear key
(b) Cantilever rigid retaining wall
Backfill
FIGURE 15.13 Types of rigid
retaining walls.
(c) Counterfort wall
(d) Buttress wall
Retaining Walls up to 1 m High---1
Retaining Walls ~ the major function of any retaining wall is to act
as on earth retaining structure for the whole or part of its height
on one face, the other being exposed to the elements. Most small
height
retaining
combination
of
walls
brick
are
built
entirely
facing
and
blockwork
of
brickwork
or
mass
or
a
concrete
backing. To reduce hydrostatic pressure on the wall from ground
water
an
adequate
drainage
system
in
the
form
of
weep
holes
should be used, alternatively subsoil drainage behind the wall could
be employed.
248
Retaining Walls up to 1 m High---2
Small Height Retaining Walls ~ retaining walls must be stable and
the usual rule of thumb for small height brick retaining walls is for
the
height
to
lie
between
2
and
4
times
the
wall
thickness.
Stability can be checked by applying the middle third rule †
249
Medium Height Retaining Walls
Retaining
Walls
medium height
up
to
6„000
retaining
high
walls
classified
as
and have the primary function
~
these
can
be
of
retaining soils at an angle in excess of the soil's natural angle of
repose. Walls within this height range are designed to provide the
necessary resistance by either their own mass or by the principles
of leverage.
Design ~ the actual design calculations are usually carried out by
a structural engineer who endeavours to ensure that:1.
Overturning of the wall does not occur.
2.
Forward sliding of the wall does not occur.
3.
Materials used are suitable and not overstressed.
4.
The subsoil is not overloaded.
5.
In clay subsoils slip circle failure does not occur.
The factors which the designer will have to take into account:1.
Nature and characteristics of the subsoil(s).
2.
Height
of
water
table
†
the
presence
of
water
can
create
hydrostatic pressure on the rear face of the wall, it can also
affect
the
shear
strength,
underside
bearing
of
capacity
reduce
the
the
foundation
of
the
subsoil
frictional
and
the
together
resistance
subsoil
with
its
between
the
and
reduce
passive pressure in front of the toe of the wall.
3.
Type of wall.
4.
Material(s) to be used in the construction of the wall.
250
the
Medium Height Retaining Walls
Earth Pressures ~ these can take one of two forms namely:1.
Active
Earth
Pressures
†
these
are
those
pressures
which
tend to move the wall at all times and consist of the wedge
of
earth
retained
plus
any
hydrostatic
pressure.
The
latter
can be reduced by including a subsoil drainage system behind
and/or through the wall.
2.
Passive
and
Earth
opposite
Pressures
force
to
~
these
any
are
a
reaction
of
an
imposed
pressure
thus
equal
giving
stability by resisting movement.
251
Medium Height Retaining Walls
Mass Retaining Walls ~ these walls rely mainly on their own mass
to overcome the tendency to slide forwards. Mass retaining walls
are not generally considered to be economic over a height of 1„800
when constructed of brick or concrete and 1„000 high in the case
of natural stonework. Any mass retaining wall can be faced with
another material but generally any applied facing will not increase
the strength of the wall and is therefore only used for aesthetic
reasons.
252
Medium Height Retaining Walls
253
Medium Height Retaining Walls
Cantilever Retaining Walls ~ these are constructed of reinforced
concrete with an economic height range of 1„200 to 6„000. They
work on the principles of leverage where the stem is designed as a
cantilever fixed at the base and base is designed as a cantilever
fixed at the stem. Several formats are possible and in most cases
a beam is placed below the base to increase the total passive
resistance
to
sliding.
Facing
materials
manner to that shown on page 253.
254
can
be
used
in
a
similar