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Geotechnical Parameters
for Retaining Wall Design
11th October 2012
Tanya Kouzmin
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Most geotechnical failures are of retaining walls
Are failure caused by WRONG calculations?
Not usually – calculation methods are given in every textbook.
The main contributions are WRONG geotechnical parameters and WATER
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AS 4678-2002
Earth Retaining Structures
Has been with us for 10 years!!
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Limit State Design
Retaining Walls
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Gravity Wall Failure Mechanisms
Must ALL be checked for drained and
undrained conditions
Do we use drained or undrained analysis?
o Undrained (clays only): Cu and Ka = Kp = 1
o Drained (all soils) : Ka, Ko Kp based on effective soil
friction angle (and effective cohesion)
Which is more conservative?
Why would we use it?
Getting Started
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All retaining walls must work for both drained and
undrained conditions of the surrounding soil!!!!!
�Undrained (short term) generally not problematic
because clay can stand on it’s own
�Unsupported height = 2Cu/γ
�Drained generally more critical for clays
Q: But we always design for sand, or φ = 300 !
A: How much sand is there in Melbourne?
Getting Started
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• SOIL FRICTION ANGLE
• GEOMETRY
�Back slope and Front slope
�Surcharge
�Failure Surface
�Wall Angle
• WALL FRICTION
• FORMULA USED
What affects the earth pressure parameters ?
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It all starts with Friction Angle - When is φ NOT 300 ?
Friction angle of cohesive materials
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Soil Friction Angle
SANDS
o Loose 290 - 330
o Medium 340 - 380
o Dense 390 - 430
(For Silty Sand reduce the above by 30)
CLAYS
SC (Tertiary) 290 - 310
CL to CI (Tertiary /Devonian Clays 260 - 380
CI-CH (Silurian) 220 - 250
CH (Basaltic) 180 - 210
Residual Friction Angle 80 – 180
Typical Range of Friction Angle
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Friction Angle vs SPT
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Relative Density vs SPT
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Relative Density vs DCP and PSP
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Q: What causes the
ultimate limit states?
A: Movement of the
wall
Ultimate Limit States – ALL Soils
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Slip Lines
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Required Horizontal Movement
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Wall Movements
Wall Movements
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Wall Movements
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Ko = 1- sin φ for no back- slope
Ko = (1- sin φ) x (1+β) for back slope β
The above values apply only for normally consolidated soil.
For over-consolidated soil (residual soil or compacted fill)
Ko, oc = (1 - sin φ')OCR0.5
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At Rest Coefficient K0
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At Rest Coefficient K0 of Clays
Download This
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http://www.cedd.gov.hk/eng/publications/geo/doc/manu_eg1.pdf
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Rankine Earth Pressure Coefficients
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RankineActive Earth Pressure
Coefficient
Rankine theory is based on
a limit equilibrium state in
the soil. It does not apply
when
• backfill slope is >φ/2
• wall is restrained at top
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Rankine –Bell Active Earth Pressure that
includes effective cohesion c’
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Coulomb Earth Pressure Coefficients
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Coulomb Active Earth Pressure Coefficient
Coulomb theory
considers shear stresses
along the edges of the
failure wedge. A value
for wall friction must be
given.
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Passive Earth Pressure Coefficients
Rankine theory usually under-estimates passive earth pressure
because the direction of the thrust is incorrectly assumed in
the theory
Coulomb theory significantly over-estimates the passive earth
pressure – the error is due to the assumption of a planar
surface, and increases rapidly with increasing value of wall
friction δ.
Comparing Ka and Kp values
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Comparing Pa values
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Coulomb Passive Earth Pressure Coefficient
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Sokolovski Passive Pressure Coefficients
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Sokolovski Passive Pressure Coefficients
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Typical Values for vertical wall and horizontal backslope
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AGS Journal Dec 1999
Slope Geometry
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Braced excavations in Sand
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Braced excavations
in ClayShort term
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Braced excavations
in Clay Long Term
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Wall Friction δ
Failure Surface
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Trial Wedge
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Trial Wedge
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Trial Wedge
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Trial Wedge
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Terraced walls – get the spacing right
Where is the
critical failure
surface?
Gravity Wall Failures
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Gravity Wall Failures
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Compaction Stresses
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Permeability of Backfill Material
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Permeability of Backfill Material
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Expansive Clay Behind
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From Robert Lytton’s presentation
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Expansive Clay Behind
From Robert Lytton’s presentation
Expansive Clay Behind
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From Robert Lytton’s presentation
Expansive Clay Behind
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From Robert Lytton’s presentation
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Post and Waler
cantilever Walls with backfill
The End
thank you for your attention
Tanya Kouzmin
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