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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