Introduction to function of retaining wall and different types of retaining walls

1. RETAINING WALLS

2. RETAINING WALL
Basic function – to retain
soil at a slope which is
greater than it would
naturally assume, usually
at a vertical or near
vertical position

3. Terminology of Retaining Wall

4. Angle of Repose
The natural slope taken up by any soil is called its angle
of repose and is measured in relation ship to the
horizontal
It is the wedge of soil resting on this upper plane of the
angle of repose which a retaining wall has to support

5. ANGLE OF REPOSE
 The angle of repose or the critical angle of
repose, of a granular material is the steepest
angle of descent or dip relative to the horizontal
plane to which a material can be piled without
slumping. The angle of repose can range from 0°
to 90°
 At this angle, the material on the slope face is on
the verge of sliding.

7. The design of retaining wall is basically concerned with the
lateral pressures of the retained soil and any subsoil water
Greater the angle of repose of a material, the less is the
pressure exerted

8. Increased pressures must be allowed for when
a. there is a surcharge or
b. when there are buildings or traffic carrying roads near the
top of the wall

9. Design of retaining wall
 Retaining walls have primary function of retaining soils
at an angle in excess of the soil’s nature angle of
repose.
 Walls within the design height range are designed to
provide the necessary resistance by either their own
mass or by the principles of leverage.
 Design consideration:
1. Overturning of the wall does not occur
2. Forward sliding does not occur
3. Materials used are suitable
4. The subsoil is not overloaded

10. Earth Pressures on Retaining wall
 Pressure at rest
 Active earth pressure
 Passive earth pressure
Pressure at Rest
 This is the case when wall
has a considerable rigidity.
 Basement walls generally
fall in this category.

11. Active Earth Pressure
 If a retaining wall is allowed to move away from the
soil accompanied by a lateral soil expansion, the
earth pressure decreases with the increasing
expansion.
 A shear failure of the soil is resulted with any further
expansion and a sliding wedge tends to move
forward and downward. The earth pressure
associated with this state of failure is the minimum
pressure and is known as active earth pressure.

12. Passive Earth Pressure
 If a retaining wall is allowed to move towards the soil
accompanied by a lateral soil compression, the earth
pressure increase with the increasing compression in the
soil.

13. Factors which designer need to take account
 Nature and characteristics of the subsoil's
 Height of water table – the presence of water can create
hydrostatic pressure, affect bearing capacity of the
subsoil together with its shear strength, reduce the
frictional resistance between the underside of the
foundation
 Type of wall
 Materials to be used in the construction

14. Forces acting on Retaining Wall

15. FORCES ACTING ON A RETAINING WALL
The designer is mainly concerned with the effect of two forms
of earth pressure- active & passive

17. STABILITY OF RETAINING WALLS
The overall stability of a retaining wall is governed by the
action and reaction of a number of loads

18. Active pressure is exerted by the retained material &
water pressure on the back of the wall
Passive pressures are the induced loads at the toe
and the friction between the underside of the base
and the soil
Ground water behind a retaining wall can have
adverse effects upon the design and stability of the
retaining wall

19. Typical dimensions of different types of
retaining wall

20. Types of walls
• Mass retaining walls / Gravity Retaining Wall
• Cantilever walls
• Counterfort retaining walls
• Precast concrete retaining walls
• Precast concrete crib-retaining walls

21. Counterfort
Mass retaining
Wall / Gravity
RW
T-Shaped RW
L-Shaped RW
Backfill
Backfill
Counterfort RW
Buttress
Backfill
Buttress RW

22. Mass retaining walls
 Sometimes called gravity walls
and rely upon their own mass
therefore, is rather massive in
size.
 Mass itself, together with the
friction on the underside of the
base to overcome the tendency
to slide or overturn
 Generally only economic up to
1.8 m
 Mass walls can be constructed of
semi-engineering quality bricks
bedded in a 1:3 cement mortar or
of mass concrete

23.  Natural stone is suitable for small walls up to 1m high but
generally it is used as a facing material for walls over 1
m, and occasionally constructed in plain concrete
 The thickness of wall is also governed by need to
eliminate or limit the resulting tensile stress to its
permissible limit .
 Plain concrete gravity walls are not used for heights
exceeding about 3m, for obvious economic reasons.
 Stress developed is very low.
 These walls are so proportioned that no tension is
developed anywhere and the resultant of forces remain
within the middle third of the base.

24. Typical example of mass retaining
walls
BRICK MASS RETAINING WALL

25. Typical example of mass retaining
walls
MASS CONCRETE RETAINING WALL
WITH STONE FACINGS

26. Brick retaining
wall
Stone retaining wall

27. Semi-Gravity Walls
 Semi-gravity walls resist
external forces by the
combined action of self
weight, weight of soil
above footing and the
flexural resistance of the
wall components.
 Concrete cantilever wall is
an example and consists
of a reinforced concrete
stem and a base footing.
 These walls are non-
proprietary.

28. Cantilever walls
 Usually of reinforced concrete
and work on the principle of
leverage where the stem is
designed as a cantilever fixed at
the base and the base is
designed as a cantilever fixed at
the stem
 Economic height range of 1.2 m
to 6 m using pre-stressing
techniques
 Any durable facing material can
be applied to the surface to
improve appearance of the wall

29.  Two basic forms:-
• A base with a large heel
• A cantilever with a large toe
Cantilever L
Cantilever T

30.  The structure consists of vertical stem , and a base slab,
made up of two distinct regions, viz., a heel slab and a
toe slab
 “Stem” acts as a vertical cantilever under the lateral
earth pressure
 “Heel slab” acts as a horizontal
cantilever under the action of weight
of the retained earth (minus soil
pressure acting upwards from below)
 “Toe slab ” acts as a cantilever under
the action of resulting soil pressure
acting upward.
T- Shaped Cantilever walls

31. L- Shaped Cantilever walls
 It resists the horizontal earth pressure as well as
other vertical pressure by way of bending of various
components acting as cantilevers.

32. Shear Key for
additional stability

37. Counterfort retaining walls
 Can be constructed of reinforced or prestressed
concrete
 Suitable for over 7 m
 Stem and Heel slab are strengthened by providing
counterforts at some suitable intervals.
 The stability of the wall is maintained essentially by the
weight of the earth on the heel slab plus the self weight
of the structure.
 Counterfort wall are placed at regular intervals of
about1/3 to ½ of the wall height, interconnecting the
stem with the heel slab
 The counterforts are concealed within the retained earth
on the rear side of the wall.

38.  For large heights, in a cantilever retaining
wall, the bending moments developed in the
stem, heel slab and toe slab become very
large and require large thickness.
 The bending moments can be considerably
reduced by introducing transverse supports,
called counterforts.
 The counterforts subdivide the
vertical slab (stem) into rectangular
panels and support them on two
sides(suspender-style), and
themselves behave essentially as
vertical cantilever beams of T-
section and varying depth.

39. Butress Retaining Wall
Counterfort Retaining Wall

42. Precast concrete retaining wall
 Manufactured from high-grade pre cast concrete on the
cantilever principle.
 Can be erected on a foundation as permanent
retaining wall or be free standing to act as dividing wall
between heaped materials which it can increase three
times the storage volume for any given area
 Other advantages- reduction in time by eliminating
curing period, cost of formwork, time to erect and
dismantle the temporary forms
 Lifting holes are provided which can be utilized for
fixing if required

43. Application

45. Precast concrete retaining walls

46. Pre cast concrete crib-retaining walls
 Designed on the principle of mass retaining walls
 A system of pre cast concrete or treated timber
components comprising headers and stretchers which
interlock to form a 3 dimensional framework or crib of pre
cast concrete timber units within which soil is retained
 Constructed with a face batter between 1:6 and 1:8
 Subsoil drainage is not required since the open face
provides adequate drainage.

48. PRE CAST CONCRETE CRIB
RETAINING WALL

49. BASEMENT WALLS

50. REVETMENTS
GABIONS RIP RAP

64. Active System with the mesh
anchored on the rock facing.
Passive System with simple
drapery system.

65. Provisions for Joints in the Construction
of Walls
Cast concrete retaining walls may be constructed with any
or all of the following joints:
Construction Joints:
These are vertical or horizontal joints that are used
between two successive pours of concrete. Keys are used
to increase the shear resistance at the joint. If keys are not
used, the surface of the first pour is cleaned and
roughened before the next placement of concrete. Keys are
almost always formed in the base to give the stem added
sliding resistance. The base is formed first, and the stem
constructed afterwards

66. Contraction joint
These are vertical joints or grooves formed or cut into the
wall that allows the concrete to shrink without noticeable
harm. Contraction joints are usually about 0.25 inches wide
and about ½ to ¾ inch deep, and are provided at intervals
of not exceeding 30 feet.
Expansion Joints:
Vertical expansion joints are incorporated into the wall to
account for expansion due to temperature changes. These
joints may be filled with flexible joint fillers. Greased steel
dowels are often cast horizontally into the wall to tie
adjacent sections together. Expansion joints should be
located at intervals up to 90 feet.

67. Backfill Drainage of Retaining Walls
One area that can be commonly overlooked, or at least
underestimated, is the necessity to drain the backfill of
rainwater and/or groundwater. Hydrostatic pressure can
cause or induce retaining wall failure, or at least damage.
Drainage of water as a result of rainfall or other wet
conditions is very important to the stability of a retaining
wall. Without proper drainage the backfill can become
saturated, which has the dual impact of increasing the
pressure on the wall and lessening the resistance of the
backfill material to sliding. Granular backfill material
offers the benefits of good drainage, easy compaction,
and increased sliding resistance.

68. Drainage systems usually utilize weep holes and
drainage lines.
Weep holes actually penetrate the retaining wall and drain
the area immediately behind the wall. Weep holes should
have a minimum diameter so as to permit free drainage; for
large walls, 4 inch weep holes are common. Adequate
spacing between weep holes allows uniform drainage from
behind the wall. Weep holes should always have some kind
of filter material between the wall and the backfill to prevent
fines migration, weep hole clogging, and loss of backfill and
caving.
Drainage lines are often perforated and wrapped in geo
textile or buried in a granular filter bed, and serve to carry
water to the weep holes from areas deeper within the backfill.

69. Sand + Stone Filter
Weepers
Or
Weep Holes
Drainage Pipes f 100-200 mm @ 2.5 to 4
m Perforated
Pipe
Suited for short walls

72. Sliding Failure
Overturning
Failure
Bearing capacity
Failure
Shallow shear
Failure
Deep shear
Failure
Five Modes of Failure

73. The design of retaining wall
must ensure there is no
1.Failure due to overturning
2.Failure due to sliding
3.Failure due to bending
The resultant thrust on the soil
should be in the middle third of
the base

74. Sliding Failure
Sliding failure is nothing but sliding of wall away from backfill
when there is shearing failure at the base of wall
The Factor of safety against sliding is:-
R Of resultant Vertical & horizontal components -
:
RV& RH of weight of wall & earth pressure
µ = coefficient of friction = tan δ

75. Overturning failure
Overturning failure is rotation of wall about its toe due to
exceeding of moment caused due to overturning forces
to resisting forces
-
: The Factor of safety against overturning is given by
= sum of resisting moment about toe
= sum of overturning moment about toe

76. Bearing capacity failure
The pressure exerted by resultant vertical force at toe of wall
must not exceed the allowable bearing capacity of the soil ,
the pressure distribution is assumed to be linear
The maximum pressure is given by :
:The Factor of safety against bearing failure is

77. Shallow shear failure

78. Deep shear failure