11 th October 2012 Geotechnical Parameters for Retaining Wall Design Tanya Kouzmin 1
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 2
Limit State Design AS 4678-2002 Earth Retaining Structures Has been with us for 10 years!! 3
Retaining Walls 4
Gravity Wall Failure Mechanisms Must ALL be checked for drained and undrained conditions 5
Getting Started Do we use drained or undrained analysis? oundrained (clays only): C u and K a = K p = 1 odrained (all soils) : K a, K o K p based on effective soil friction angle (and effective cohesion) Which is more conservative? Why would we use it? 6
Getting Started 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 = 2C u /γ Drained generally more critical for clays Q: But we always design for sand, or φ= 30 0! A: How much sand is there in Melbourne? 7
What affects the earth pressure parameters? SOIL FRICTION ANGLE GEOMETRY Back slope and Front slope Surcharge Failure Surface Wall Angle WALL FRICTION FORMULA USED 8
Friction angle of cohesive materials It all starts with Friction Angle -When is φ NOT 30 0? 9
Soil SANDS CLAYS Typical Range of Friction Angle Friction Angle o Loose 29 0-33 0 o Medium 34 0-38 0 o Dense 39 0-43 0 (For Silty Sand reduce the above by 3 0 ) SC (Tertiary) 29 0-31 0 CL to CI (Tertiary /Devonian Clays 26 0-38 0 CI-CH (Silurian) 22 0-25 0 CH (Basaltic) 18 0-21 0 Residual Friction Angle 8 0 18 0 10
Friction Angle vs SPT 11
Relative Density vs SPT 12
Relative Density vs DCP and PSP 13
Ultimate Limit States ALL Soils Q: What causes the ultimate limit states? A: Movement of the wall 14
Slip Lines 15
Required Horizontal Movement 16
Wall Movements 17
Wall Movements 18
Wall Movements 19
At Rest Coefficient K 0 K o = 1-sin φ for no back-slope K o = (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) K o, oc = (1 -sin φ')ocr 0.5 20
At Rest Coefficient K 0 of Clays 21
Download This http://www.cedd.gov.hk/eng/publications/geo/doc/manu_eg1.pdf 22
Rankine Earth Pressure Coefficients 23
Rankine Active Earth Pressure Coefficient Rankinetheory 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 24
Rankine Bell Active Earth Pressure that includes effective cohesion c 25
Coulomb Earth Pressure Coefficients 26
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. 27
Passive Earth Pressure Coefficients Rankinetheory usually under-estimatespassive earth pressure because the direction of the thrust is incorrectly assumed in the theory Coulomb theory significantly over-estimatesthe passive earth pressure the error is due to the assumption of a planar surface, and increases rapidly with increasing value of wall friction δ. 28
Comparing K a and K p values 29
Comparing P a values 30
Coulomb Passive Earth Pressure Coefficient 31
Sokolovski Passive Pressure Coefficients 32
Sokolovski Passive Pressure Coefficients 33
Typical Values for vertical wall and horizontal backslope 34
Slope Geometry AGS Journal Dec 1999 35
Braced excavations in Sand 36
Braced excavations in Clay Short term 37
Braced excavations in Clay Long Term 38
Wall Friction δ 39
Failure Surface 40
Trial Wedge 41
Trial Wedge 42
Trial Wedge 43
Trial Wedge 44
Terraced walls get the spacing right Where is the critical failure surface? 45
Gravity Wall Failures 46
Gravity Wall Failures 47
Compaction Stresses 48
Permeability of Backfill Material 49
Permeability of Backfill Material 50
Expansive Clay Behind From Robert Lytton s presentation 51
Expansive Clay Behind From Robert Lytton s presentation 52
Expansive Clay Behind From Robert Lytton s presentation 53
Expansive Clay Behind From Robert Lytton s presentation 54
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Post and Waler cantilever Walls with backfill 56
The End thank you for your attention Tanya Kouzmin 57