CHAPTER 3
ANALYSIS AND DESIGN OF RETAINING STRUCTURES

Learning Outcomes:

At the end of this lecture/week the student would be able to:

Describe the purpose of retaining wall and its components (C03-PO4)Understand and discuss different types of retaining earth structure (C03-PO4)

Perform analysis and design for sheet pile wall (C03-PO4)

References:

Das,B.M. (2007). Principles of Foundation Engineering.Thomson

Budhu,M. (2007). Soil Mechanics and Foundations.WILEY

Craig, R.F. (2004). Craigs Soil Mechanics. Taylor & Francis.

Thomlinson, M.J. (1995). Foundation Design & Construction. Longman Scientific and Technical

Retaining Wall

Retaining wall used to prevent the

retained material from assuming its

natural slope:

3 basic components:

Backfill material granular materialReinforcement geotextiles or metal rodsFacing

3.1Introduction

Facing unit

Earth fill

Reinforcement

Component of ERWall

3.2Application Areas (BRIDGE WORKS)

Bridge abutment

Bridge abutment,

with piles bankseat

Application Areas (BRIDGE WORKS)

Bridge abutment and

support to bankseat

Sloping bridge abutment

Reinforced embankment

in place of viaduct

ADVANTAGES:

Economic May used in poor subsoil Speed of erection high

Application Areas (BRIDGE WORKS)

Application Areas (DAMS)

Reinforced earth dam

Reinforced soil structure used to raise the height of an existing dam

Application Areas (EMBANKMENT)

Reinforced embankment

Material : Geotextile or geogrid

Geocell mattress used to increase embankment stability

Application Areas (EMBANKMENT)

Application Areas (FOUNDATION)

Geogrid reinforcement of subsoil beneath embankment

Stone columns formed from geogrid cubes

WEAK SOIL

WEAK SOIL

Sheet Pile Wall

3.2Sheet Pile

Flexible and are constructed using steel or thin concrete slabs or

wood.

Two(2) types of sheet pile :

Cantilever sheet pile = used to support height of less than 3 m

= rely on passive soil resistance for their stability

Anchored sheet pile = support deep excavation and waterfront structures

= rely on combination of anchors and passive soil resistance

Types of flexible retaining wall

Types of rigid retaining wall

Cantilever Sheet Pile

3.2.1Cantilever Sheet Pile Wall

Steel sheet piling driven into the ground for temporary works is commonly used to support the vertical sides of excavation during construction

To avoid internal proping or external anchoring, it is preferable if the wall can be designed to act in the cantilever mode

Following completion of the below ground structure and backfilling the sheet piles are usually removed

This type of wall should be limited to a maximum height 3-5 m depending on the soil type and presence of water

Contiguous or secant bored pile walls and diaphragm wall are frequently used in cantilever mode for permanent application such as for retaining structures alongside urban highways, bridge abutments and for basement walls.

Minimal vibration produced during boring so this method can be adopted for walls close to existing structures

3.2.2Analysis of Cantilever Sheet Pile Wall

Cantilever sheet pile wall are analysed by

assuming that rotation occurs at some

point O,just above the base of the wall

By assuming rotation at point O (above the

base) lateral pressure is passive behind

the wall and active in front of the wall

To simply design the passive

resistance, a force R is used at the point of

rotation O and moments about O are taken

for the active and passive thrust, Pa and

Pp

The depth is increased by 20% to 30% to

give embedment design, d

Pressure Distributions

Anchored Sheet Pile

3.2.2ANCHORED sheet pile wall

Additional support to embedded walls is provided by a row of tie-backs or props near the top wall.

Tie backs are normally high tensile steel cables or rods, anchored in the soil some distance behind the wall

Two methods to analyse anchored sheet pile wall:

a) free earth support method (frequently used)

b) fixed earth support method

The design of anchored sheet pile wall addresses:

a) embedded depth

b) Anchor load

c) maximum bending moment

The stability depends due to passive resistance developed in front

of the wall together with supporting forces in ties and props.

3.2.2Free Earth Support Method

Assumption:

Depth of embedment below excavation

level is insufficient to produce fixity at

the lower end of the wall thus base of

wall free to rotate

No passive resistance to backward

movement at bottom

Active and Passive distribution are

static

Stability depends on passive

resistance in front wall

Free Earth Support Method

Example 1: Cantilever sheet-pile wall

A cantilever sheet-pile wall is to support the side of an excavation. The depth of excavation is 3 m. The properties of soil are as follow:

c = 0, =30o, =20 kN/m3

By using FOS on shear strength = 1.4 and

FOS on embedment=1.2, determine:

The safe driving depth

Maximum moment induced in the piling

Example 2: Anchored sheet pile
(free earth support method)

The anchored sheet pile shown below to be designed by the free earth support method. The depth of excavation is 9m. The anchor will be installed at apoint 1.5m below the top of the wall. Determine the required depth of penetration and the design force for anchor.

Given: Fs=1.5 ; Fd=1.2 , FT=2.0

c = 0, =28o, =20 kN/m3

1.5 m

T

CHAPTER 3
ANALYSIS AND DESIGN OF RETAINING STRUCTURES

Learning Outcomes:

At the end of this lecture/week the student would be able to:

Understand on braced excavation (C03-PO4) Perform analysis and design forces for struts in braced excavation (C03-PO4)

Braced Excavation

3.1Introduction

Braced excavation is required when dealing with

construction of basements, bridge piers and abutments

The vertical faces of the cut need to be protected by temporary bracing system (sheet pile) to avoid failure that may

be accompanied by considerable settlement or by bearing capacity failure at nearby foundation

Interlocking sheet piles are driven to the soil before excavation. As excavation proceeds, struts and wales (horizontal steel beam) are inserted immediately after reaching the appropriate depth

3.2Wall Displacement in Braced Excavation

The wall displacements before the top struts are installed are usually very small but get larger as the excavation gets deeper

The largest wall displacement occur at the base of the excavation

Critical design element is when designing loads on struts due to different lateral load at different depth

Figure 3.1 : Distribution of displacement for Braced Excavation

WALL DISPLACEMENT

LATERAL EARTH PRESSURE DISTRIBUTION

3.3Lateral Earth Pressure

Peck (1969) suggested using design pressure envelopes for braced

cuts in sand and clay

Lateral Earth Pressure Distribution for course grained soil

Lateral Earth Pressure Distribution for fine grained soil

QUESTION

Draw the pressure diagram. Determine the forces on the struts

for the braced excavation.

(The struts are placed 3 m center to center in the plan)

SOLUTION:

CHAPTER 3
ANALYSIS AND DESIGN OF RETAINING STRUCTURES

Learning Outcomes:

At the end of this lecture/week the student would be able to:

Understand the construction methods for retaining structures(C03-PO4)

Perform analysis and design for reinforced earth structures (C03-PO4)

Understand and perform cofferdam design and analysis (C03-PO4)

3.1Construction Techniques

Hybrid Systems

Telescope Method

Sliding Method

Concertina Method

Concertina Method

Originally proposed by Vidal (1966), this method permits differential settlement within the soil mass by the face structure closing in a manner similar to a set of bellows or a concertina. This is the form of construction most frequently used with geotextiles for steep slopes.

Telescope Method

In this system the deformations within the soil mass are accommodated by the facing panels closing and moving forward an amount equivalent to the internal deformations, Vidal (1978). This is made possible by the individual facing units being held apart during construction. Failure to provide a large enough gap can result in damage to the facing panels.

Sliding Method

In the sliding method proposed by Jones (1978), differential settlement and compaction within the soil mass can be accommodated by permitting the reinforcing elements to slide vertically relative to the facing. Slideable attachments can be provided by groves, slots, vertical poles, lugs or bolts. Facings made of discrete elements, as with the telescope method, can be used as can full height facings with a range of architectural finishes.

Hybrid Systems

Reinforcement is used with conventional gravity systems to produce an

improved composite construction; an example is the tailed gabion, or

the Norwegian concrete block.

3.2Construction Tolerances and Serviceability Limit

Table 4.1: Usually accepted tolerances for faces of retaining walls and abutments.

Location of plane structure 50 mmVerticality 5 mm per metre height(i.e. 40 mm per 8 m)Bulging (vertical) or Bowing (horizontal) 20 mm in 4.5 m templateSteps at joints 10 mmAlignment along top 15 mm from reference alignment

Table 4.2 Serviceability limits.Limit on post construction internal strainsStructureStrain percentBridge abutments0.5Walls1.0

3.3Principle of Earth Reinforced Earth

Vertical stress, v = z

Lateral stress, h = Ko v

According to Jacky (1944), for both normally consolidated clays and compacted soil,

Ko = 1 Sin ,

where = the angle of friction of the soil.

ka = 1- sin

1+sin

kp = 1+ sin

1-sin

z

T

Rankine active earth wedge

Kaz

Pa = active earth force

T = tension force in reinforcement

Pa

H

H/3

45o + /2

3.4Forces in Reinforcement Strip

ER Wall with pre-cast facing unit

unit in kN

where

therefore,

where zi = depth of strip i below the

ground level

Facing unit

Soil

Reinforcement

Sh

Sv

zi

Ti

Surcharged load, q

3.4.1The maximum Tension Line

The tensile force in a reinforcement strip varies. It generally has a low (even a zero) value at the facing unit, reaches a maximum value a short distance from the facing and then tends towards zero at the un attached end.

The maximum tension line

Resistant

zone

Profile of maximum tension line

Active

zone

3.4.2Failure of a reinforcing element

a) Tensile failure (breakage)

The ultimate resistance of a reinforcing element to an axial tensile stress is equal to the ultimate axial tensile stress that the material can withstand, fy, time the cross sectional area of the rectangular strip.

where w = width of reinforcing element

t = thickness of reinforcing element

fy = yield tensile strength of tie material

Rt = fy . w . t

b) Bond Failure

For a reinforcing element the bond resistance between it and the soil will be provided by frictional resistance

Frictional resistance,

where w = width of strip

l = length of strip

Fr = 2 zi w l

Note :

Rectangular strip, Fr = 2 zi w l

Round bar (rod), Fr = zi d l ( where d = diameter of bar)

Sheet, Fr = 2 zi l

3.5Design Criteria

DESIGN PRINCIPLE

ERW

INTERNAL STABILITY

EXTERNAL STABILITY

Tensile FailureBond Failure

Sliding

Overturning

Bearing Capacity

3.5.1Internal Stability

a) FOS against breakage =

=

=

b) FOS against pull-out =

=

=

where = coefficient of friction soil tie

= 0.5 tan

OR

=

where

=

c)Determination of thickness

t design =

t corrosion = rate of corrosion x design life

(eg. 0.002mm/yr) x (50 yrs)

t required = t design + t corrosion

t use > t required

d)Determination of required length of metal strip

lr = length of metal strip within Rankines failure

wedge

le = the effective length of reinforcement

Rankine active wedge

z

H

lr

le

v

h

v

a

i

S

S

K

T

s

=

q

S

i

v

v

+

=

g

s

h

v

i

a

i

S

S

q

z

K

T

)

(

+

=

g

tie

any

in

force

aximum

m

The

tie

each

of

strength

yield

The

Sh

Sv

v

Ka

t

w

fy

s

Ti

Fr

tie

any

in

force

aximum

m

The

force

friction

aximum

m

The

Ti

Rt

Sh

Sv

v

Ka

l

w

v

s

m

s

2

Sh

Sv

v

Ka

l

w

v

s

j

s

m

tan

2

m

j

j

3

2

w

fy

Sh

Sv

v

Ka

b

FOS

s

)

(

2

45

f

+

o

2

45

f

+

o