Lect 14 lateral_earthpressures

Lateral Earth Pressures Lateral Earth Pressures Lateral Earth Pressures Retaining StructuresRetaining StructuresRetaining Structures

Lateral Earth Pressures Lateral Earth Pressures Lateral Earth Pressures Retaining StructuresRetaining StructuresRetaining StructuresRetaining StructuresRetaining StructuresRetaining Structures

Prof. Dr. Mustafa AytekinProf. Dr. Mustafa AytekinProf. Dr. Mustafa AytekinRetaining StructuresRetaining StructuresRetaining Structures

Prof. Dr. Mustafa AytekinProf. Dr. Mustafa AytekinProf. Dr. Mustafa Aytekin

SIVASIVA Copyright©2001Copyright©2001

Lateral SupportLateral Support

In geotechnical engineering, it is often necessary to prevent lateral soil movements.

Tie rod

Anchor

Sheet pile

2Cantilever retaining wall

Braced excavation Anchored sheet pile



We have to estimate the lateral soil pressureslateral soil pressureslateral soil pressures acting on these structures, to be able to design them.

S il ili3Gravity Retaining

wallSoil nailing

Reinforced earth wall


Soil NailingSoil Nailing

4


Sheet PileSheet Pile

Sheet piles marked for driving5



Sheet pile wall6



During installation Sheet pile wall

7



Reinforced earth wallsReinforced earth wallsReinforced earth walls are increasingly becoming popular.

geosynthetics

8



Crib wallsCrib wallsCrib walls have been used in Queensland.filled with

soil

Good drainage & allow plant growth.

Interlocking stretchers

and headers

Looks good.

and headers

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Earth Pressure at RestEarth Pressure at Rest

GL

In a homogeneous natural soil deposit,

Xh’v’

X

the ratio h’/ ’ is a constant known as coefficientcoefficientcoefficientthe ratio h /v is a constant known as coefficient coefficient coefficient of earth pressure at rest (Kof earth pressure at rest (Kof earth pressure at rest (K000).).).

Importantly, at K0 state, there are no lateral strains.

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Estimating K0Estimating K0

For normally consolidated clays and granular soils, K0 = 1 – sin ’0

For overconsolidated clays,

K0,overconsolidated = K0,normally consolidated OCR0.5

From elastic analysis,

K Poisson’s

10K Poisson s ratio

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Active/Passive Earth PressuresActive/Passive Earth Pressures- in granular soils

Wall moves f ilaway from soil

Wall moves AWall moves towards soil

A

smooth wall

B

smooth wall

Let’s look at the soil elements A and B during the 12wall movement.


Active Earth PressureActive Earth Pressure- in granular soils

Initially there is no lateral movement

v’ = z

A

v’h’

zInitially, there is no lateral movement.

h’ = K0 v’ = K0 zA

As the wall moves away from the soil,

v’ remains the same; and

h’ decreases till failure occurs.h dec eases t a u e occu s.

Active state13




Initially (K0 state)

Failure (Active state)

’ v

decreasing h’active earth

pressure14

p



WJM Rankine(1820-1872)

v’[h’]active

']'[ K ][ vAactiveh K

sin1 2Rankine’s coefficient of

active earth pressure15

)2/45(tansin1sin1 2

AK active earth pressure



v’

’

Failure plane is at 45 + /2 to horizontal

Ah’45 + /2

90+

v’[h’]active

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’ decreases till failure occursh decreases till failure occurs.

h’ K state

A

v’h’

zh

Active state

K0 state

Ah state

wall movement

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Active Earth PressureActive Earth Pressure- in cohesive soils

Follow the same steps asFollow the same steps as for granular soils. Only difference is that c 0.

KK 2']'[ AvAactiveh KcK 2']'[

Everything else the same as for granular soils

18as for granular soils.


ExampleExample

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What is the excavation depth without a support

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AvAactiveh KcK 2']'[

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Solved in the classroom

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Passive Earth PressurePassive Earth Pressure- in granular soils

Initially, soil is in K0 state.

As the wall moves towards the soil,

’ remains the same and

B

v’h’

v remains the same, and

h’ increases till failure occurs.Bh

Passive state

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Initially (K0 state)

Failure (Active state)

passive earth pressure

’ v

increasing ’32

increasing h



v’ [h’]passive

']'[ K ][ vPpassiveh K

sin1 2Rankine’s coefficient of passive earth pressure

33)2/45(tan

sin1sin1 2

PK passive earth pressure



v’

’

Failure plane is at 45 - /2 to horizontal

Ah’45 - /2

90+

v’ [h’]passive

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’ increases till failure occursh increases till failure occurs.

h’ Passive state

B

v’h’

h Passive state

Bh

K0 state

wall movement

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Passive Earth PressurePassive Earth Pressure- in cohesive soils

Follow the same steps asFollow the same steps as for granular soils. Only difference is that c 0.

KK 2']'[ PvPpassiveh KcK 2']'[

Everything else the same as for granular soils

36as for granular soils.


Earth Pressure DistributionEarth Pressure Distribution- in granular soils

’][h’]active

PA and PP are the l i dresultant active and

passive thrusts on the wall

[h’]passive H

P 0 5 K H2

h

PA=0.5 KAH2

PP=0.5 KPh2PP 0.5 KPh

37KAHKPh

h’h

Passive state

Active stateK0 state

Wall movement (not to scale)


Rankine’s Earth Pressure TheoryRankine s Earth Pressure Theory

AvAactiveh KcK 2']'[

PvPpassiveh KcK 2']'[

Assumes smooth wall

Applicable only on vertical walls

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Retaining Walls - ApplicationsRetaining Walls - Applications

Road

Train

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highway

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High rise buildingHigh-rise building

basement wall

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Gravity Retaining WallsGravity Retaining Walls

cement mortarl i t

cobbles

plain concrete or stone masonry

They rely on their self weight to support the backfillThey rely on their self weight to support the backfill

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Cantilever Retaining WallsCantilever Retaining Walls

Reinforced; smaller section

than gravity allswalls

They act like vertical cantilever, They act like vertical cantilever, 44fixed to the groundfixed to the ground


Design of Retaining WallDesign of Retaining Wall- in granular soils

2 2

1

2

3 3

Block no.

11

toetoe

Wi = weight of block iAnalyse the stability of this rigid body with 45

xi = horizontal distance of centroid of block i from toey y g y

vertical walls (Rankine theory valid)

Safety against sliding along the base

}{WP soil-concrete frictiontan}.{

A

iPsliding P

WPF

soil concrete friction angle 0.5 – 0.7

A

to be greater

2 2

gthan 1.5

1

2

3 3

PA

PA

H

11PP

PPS

StoeR

h

toeR

Ryy

h2 2PP= 0.5 KPh2 PA= 0.5 KAH2

Safety against overturning about toe

}{3/ WhP H/3

}{3/

A

iiPgoverturnin P

xWhPF

A

to be greater

2 2

gthan 2.0

1

2

3 3

PA

PA

H

11PP

PPS

StoeR

h

toeR

Ryy


Points to Think AboutPoints to Think About

How does the key help in improving the stability against sliding?against sliding?

Shouldn’t we design retaining walls to resist at-rest (than active) earth pressures since the thrust on the wall is greater in K0 state (K0 > KA)?

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THE END

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Lect 14 lateral_earthpressures

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