CE 632 Retaining Wall Design Part-1 PPT

E-632 Foundation Analysis and

Foundation Analysis Analysis and Design: Dr Dr.. Amit Prashant

Conventional Ret eta ainin Wal allls 

Gravity Retaining Structures 



Semi-gravity Retaining Structures 



Stability depends on the self weight of the wall

Minimum amount of reinforcement may be used in the wall to reduce the size of wall

Cantilever Retaining Walls  

Reinforced concrete is used in wall design with thin stem and slab base Relatively economical for design

Foundation Analysis and Design: Dr. Amit Prashant

Conventional Retainin Walls 

Counterfort/Buttressed Retaining walls , used at some interval to tie the base slab and stem in order to reduce the shear force and bending moment for more economical desi n


Retainin Wall Desi n: Pro ortionin







First, approximate dimensions are retaining wall. Then, stability of wall is checked for . Section is changed if its undesirable from the stability or view.

Toe

Heel


Retainin Wall Desi n: Pro ortionin

0.3 m min

0.3 m min


Earth Pressure on Retainin Wall Earth pressure may be section going through the heel of wall. This is under the constraint that Heel is proportioned in such a way that line AC makes an an le less than or e ual to η with vertical.


Earth Pressure on Retainin Wall

Pa (Rankine)

Pa (Coulomb)


E uivalent Fluid Method 

Along line AB

P

=

1

K H ′

2

Pv

=

1

K v H ′2

The units of Kh and Kv are the same as (Ph /H2)

Terzaghi and Peck have produced semi-empirical charts for Kh and Kv for eren ypes o so s as s e n e a e e ow



Retaining walls with backfill slope of finite distance


Earth Pressure Retaining walls with backfill slope of finite


Earth Pressure on Retaining walls with backfill slope of finite distance


Stabilit of retainin wall

OVERTURNING about its toe.

BEARING CAPACITY failure of supporting base

SLIDING

Excessive SETTLEMENT may occur if weak soil la er is located below the foundation within 1.5 times foundation width.


Stabilit of retainin wall Deep seated shear failure may occur if there is a weak soil layer below the foundation within a depth of about 1.5 times width of foundation. The failure surface may be assumed to have c lindrical sha e and critical failure surface for sliding may be determined through analysis.

For back fill with its slope less than 10º, the critical sliding surface may be assumed to pass through heel of the retaining wall.


Check A ainst OVERTURNNG


Check A ainst OVERTURNNG The wall must be safe against overturning about the toe

FOS =

∑

FOS =

R

Resisting Moment

O

Overturning Moment

Pav .B + P

.

∑ W .x i

−P .

i

FOS = 1.5, if wind/seismic forces are considered

≥2

Location of Resultant force from toe can determined as

( P + ∑ W ) .x = ∑ M − ∑ M av

i

x

=

R

− Pav + ∑ W i R

O

O

In the design of cantilever retaining wall it is preferred that the stem center is


Check a ainst SLIDING F ∑ FOS =

R

≥ 1.75

S

F FS

= R.tan δ + c

In most cases passive earth pressure is ignored while calculating FOS against sliding

B+P

= Pah = 1.5, w n se sm c forces are considered

Base friction and adhesion may be taken by the following assumption

δb

= ⎛⎜ ⎝2

cb

=⎜ ⎝2

1

to

to

⎞ .φ ′ ⎟ 3⎠ 2

2

.c′ ⎟ 3⎠

2


Alternatives for Im rovin FOS a ainst Slidin

Use base key to increase the passive sliding

Use a Dead man anchor at the stem to transfer a part of sliding force to it. Increase the width of base s a pre era y on ee s e


Check for BEARING CAPACITY failure R =

+

Pav

∑ =

Eccentricity: Q

=

1+ ⎜ B ⎝

qmin

=

− ⎜ B ⎝ 1

+ ( Pah − PP )

− ∑ M O Pav + ∑ W i

=

qmax

Wi

2

6e

B e B

R

e=

⎟= ⎠

av

⎟= ⎠

av

2

−x i

B i

B

⎜1 + ⎝ ⎜ − ⎝ 1

6e

B e B

⎟ ⎠ ⎟ ⎠

, min , i.e. tensile force. This is not desirable and re-proportioning is required

2


Check for BEARING CAPACITY failure Bearing capacity of soil can be calculated using general bearing capacity equation.

qu

= c.N c .sc .dc .ic + q.N q .sq .d q .iq + 0.5γ .B.Nγ .sγ .dγ .iγ

Following consideration have to made during the analysis 

The eccentricity of load on the foundation can be incorporated using effective area method. The bearin ca acit is calculated assumin the width of foundation as B'

B′ = B − 2e 

Inclination of resultant force has to taken into account

tan

actor o sa ety aga nst bearing capacity:

=

Pah Pav

− PP

+

FOS =

W i u

q

granular soil [ 23 for for cohesive soils


Wall Joints 

Construction Joints: Vertical or horizontal joints are placed between two successive pour of concrete. To increase shear resistance at the joints, .



Contraction Joint: These are vertical joints placed in the wall (from top of base concrete to shrink without noticeable harm. The groove may be 6-8 mm wide, 12-16 , spacing.


Wall Joints 

Expansion Joint: These vertical joints are provided in large retaining and they are usually extended from top to bottom of the wall. These joints may be filled with flexible joint fillers. Horizontal reinforcing steel . , the current thinking is that the large resistance to expansion/contraction on the back face of wall from lateral pressure + the friction resistance of , .


Wall Draina e 

Accumulation of rain water in the back fill results in its saturation, and thus a considerable increase in the earth pressure acting on the wall. s may even ua y ea o uns a e con ons. wo o e op ons o take care of this problem are the following:  

Provision of weep holes w/o geo-textile on the back-face of wall Perforated pipe draining system with filter

Weep hole

Filter material

Filter material Perforated pipe


Wall Draina e 

Weep Holes: They should have a minimum diameter of 10 cm and be adequately spaced epen ng on e ac ma er a . eotextile material or a thin layer of some other filter may be used on the back face of wall fill material entering the weep holes and eventually clogging them.

Inclined drains

Combination of inclined and horizontal drain for

Top drains for clay backfills


Wall Draina e 

Perforated Pipes: These are provided horizontally along the back face of . should satisfy the following requirements.  The soil to be protected should note wash into the filter

D15( Filter )

<5

( Backfill )

85

permeability.

D D15( Backfill )

>4


Wall Settlements 

Settlement of soil below the wall 



. Consolidation settlement in cohesive soil.

Differential settlement 

Heel settlement is larger when there is substantial increase in



Toe settlements are produced by lateral earth pressure. To minimize toe settlements, ground may be strengthened using sand , , , . Differential settlements along the length of wall may produce cracks in the wall. This can be watched during construction itself an preemp ve ac on may e a en suc as ensur ng proper compaction of the ground.




Design of Cantilever Retaining Wall

CE 632 Retaining Wall Design Part-1 PPT

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