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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