Geotechnical Parameters for Retaining Wall Design

Transcription

1 11 th October 2012 Geotechnical Parameters for Retaining Wall Design Tanya Kouzmin 1

2 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

3 Limit State Design AS Earth Retaining Structures Has been with us for 10 years!! 3

4 Retaining Walls 4

5 Gravity Wall Failure Mechanisms Must ALL be checked for drained and undrained conditions 5

6 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

7 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

8 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

9 Friction angle of cohesive materials It all starts with Friction Angle -When is φ NOT 30 0? 9

10 Soil SANDS CLAYS Typical Range of Friction Angle Friction Angle o Loose o Medium o Dense (For Silty Sand reduce the above by 3 0 ) SC (Tertiary) CL to CI (Tertiary /Devonian Clays CI-CH (Silurian) CH (Basaltic) Residual Friction Angle

11 Friction Angle vs SPT 11

12 Relative Density vs SPT 12

13 Relative Density vs DCP and PSP 13

14 Ultimate Limit States ALL Soils Q: What causes the ultimate limit states? A: Movement of the wall 14

15 Slip Lines 15

16 Required Horizontal Movement 16

17 Wall Movements 17

18 Wall Movements 18

19 Wall Movements 19

20 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

21 At Rest Coefficient K 0 of Clays 21

22 Download This 22

23 Rankine Earth Pressure Coefficients 23

24 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

25 Rankine Bell Active Earth Pressure that includes effective cohesion c 25

26 Coulomb Earth Pressure Coefficients 26

27 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

28 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

29 Comparing K a and K p values 29

30 Comparing P a values 30

31 Coulomb Passive Earth Pressure Coefficient 31

32 Sokolovski Passive Pressure Coefficients 32

33 Sokolovski Passive Pressure Coefficients 33

34 Typical Values for vertical wall and horizontal backslope 34

35 Slope Geometry AGS Journal Dec

36 Braced excavations in Sand 36

37 Braced excavations in Clay Short term 37

38 Braced excavations in Clay Long Term 38

39 Wall Friction δ 39

40 Failure Surface 40

41 Trial Wedge 41

42 Trial Wedge 42

43 Trial Wedge 43

44 Trial Wedge 44

45 Terraced walls get the spacing right Where is the critical failure surface? 45

46 Gravity Wall Failures 46

47 Gravity Wall Failures 47

48 Compaction Stresses 48

49 Permeability of Backfill Material 49

50 Permeability of Backfill Material 50

51 Expansive Clay Behind From Robert Lytton s presentation 51

52 Expansive Clay Behind From Robert Lytton s presentation 52

53 Expansive Clay Behind From Robert Lytton s presentation 53

54 Expansive Clay Behind From Robert Lytton s presentation 54

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56 Post and Waler cantilever Walls with backfill 56

57 The End thank you for your attention Tanya Kouzmin 57