Lateral Earth Pressure

Lateral Earth PressuresLateral Earth PressuresLateral Earth PressuresLateral Earth Pressures

What is Lateral Earth Pressure?Designing any retaining wall requires knowledge of lateral earth pressure, the pressure developed by the backfill.

It is the force generated by the lateral earth pressure that constitutes a large part of the load that the wall must carry.

represents pressures that are “to the

side” (horizontal) rather than vertical

Lateral Support

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

Cantilever retaining wall

Braced excavation Anchored sheet pile

Tie rod

Sheet pile

Anchor

Lateral Support

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

Gravity Retaining wall

Soil nailingReinforced earth wall

Lateral Support

Reinforced earth wallsReinforced earth walls are increasingly becoming popular.

geosynthetics

Lateral Support

Crib wallsCrib walls have been used in Queensland.

Interlocking stretchers

and headers

filled with soil

Good drainage & allow plant growth.

Looks good.

Categories of Lateral Earth Pressure

Active/Passive Earth Pressures- in granular soils

smooth wall

Wall moves away from soil

Wall moves towards soil

A

B

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

Active Earth Pressure- in granular soils

A

v’

h’z

As the wall moves away from the soil,

Initially, there is no lateral movement.

v’ = z

h’ = K0 v’ = K0 z

v’ remains the same; and

h’ decreases till failure occurs.

Active stateActive state

Active Earth Pressure- in granular soils

failure envelope

v’

decreasing h’

Initially (K0 state)

Failure (Active state)

As the wall moves away from the soil,

active earth pressure

Active Earth Pressure- in granular soils

v’[h’]active

failure envelope

Rankine’s coefficient of active earth pressure

WJM Rankine(1820-1872)

Active Earth Pressure- in granular soils

v’[h’]active

failure envelope

A

v’

h’45 + /2

90+

Failure plane is at 45 + /2 to horizontal

Active Earth Pressure- in granular soils

A

v’

h’z

As the wall moves away from the soil,

h’ decreases till failure occurs.

wall movement

h’

Active state

K0 state

Passive Earth Pressure- in granular soils

B

v’

h’

Initially, soil is in K0 state.

As the wall moves towards the soil,

v’ remains the same, and

h’ increases till failure occurs.

Passive state

Passive Earth Pressure- in granular soils

failure envelope

v’

Initially (K0 state)

Failure (Active state)

As the wall moves towards the soil,

increasing h’

passive earth pressure

Passive Earth Pressure- in granular soils

v’ [h’]passive

failure envelope

Rankine’s coefficient of passive earth pressure

Passive Earth Pressure- in granular soils

v’ [h’]passive

failure envelope

A

v’

h’

90+

Failure plane is at 45 - /2 to horizontal

45 - /2

Passive Earth Pressure- in granular soils

B

v’

h’

As the wall moves towards the soil,

h’ increases till failure occurs.

wall movement

h’

K0 state

Passive state

Earth Pressure Distribution- in granular soils

[h’]passive

[h’]active

H

h

KAHKPh

PA=0.5 KAH2

PP=0.5 KPh2

PA and PP are the resultant active and passive thrusts on

the wall

Wall movement (not to scale)

h’

Passive state

Active stateK0 state

Rankine’s Earth Pressure Theory assumes:

There is no adhesion or friction between the wall and soil

Lateral pressure is limited to vertical walls

Failure (in the backfill) occurs as a sliding wedge along an assumed failure plane defined by ø.

Lateral pressure varies linearly with depth and the resultant pressure is located one-third of the height (H) above the base of the wall.

The resultant force is parallel to the backfill surface.

Rankine’s Earth Pressure TheoryRankine’s Active earth pressure coefficient

Rankine’s Passive earth pressure coefficient

The Coulomb Theory Coulomb active earth pressure coefficient:

Coulomb passive earth pressure coefficient:

The Coulomb Theory is similar to Rankine except that:

There is friction between the wall and soil and takes this into account by using a soil-wall friction angle of δ. Note that δ ranges from ø/2 to 2ø/3 and δ = 2ø/3 is commonly used.

Lateral pressure is not limited to vertical walls

The resultant force is not necessarily parallel to the backfill surface because of the soil-wall friction value δ.

Retaining Walls - Applications

Road

Train

Retaining Walls - Applications

highway

Retaining Walls - Applications

basement wall

High-rise building

Gravity Retaining Walls

cobbles

cement mortarplain concrete or stone masonry

They rely on their self weight to support the backfill

They rely on their self weight to support the backfill

Cantilever Retaining Walls

They act like vertical cantilever, fixed to the ground

They act like vertical cantilever, fixed to the ground

Reinforced; smaller section

than gravity walls