SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

1. PART 2 – SITES EARTH SPILLWAY EVALUATION B. Earth Spillway Integrity Analysisb. Characterization of geologic materials: iii. Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

2. Objectives • State the common range of the Kh parameter for cohesive soils • Identify plot of rate of headcut migration versus Kh parameter and explain sensitivity of plot to values of Kh • Using example soil data, assign a value to the Kh parameter accurately • Identify tests used to characterize soils for estimating the Kh parameter

3. Phase III Erosion Process • Phase 3 is the headcut advance that occurs after establishment in Phase IIWhere dx/dt = rate of headcut advance A = hydraulic attackA0 = Attack threshold (no movement below value)C = Proportionality Coefficient = -0.79 x ln(Kh) + 3.04 for Kh < 18.2 and C = 0.75 for Kh > 18.2

4. Other Terms in dx/dt equation for Phase III

5. Headcut - classical

6. EXPANDING HEADCUT Rate of migration of headcut primarily a function of soil Kh value

7. HEADCUT MIGRATION RATE dX/dt

8. HEADCUT MIGRATION dx/dt = C (A - Ao) dX/dt= rate of headcut migration, C = material-dependent advance rate coefficient, A = hydraulic attack, and Ao = material-dependent threshold.

10. Chapter 52, Appendix B • First, determine if material is soil or rock • ASTM D2488 – provides limited guidance. • Refer to Chapter 52 for Information on Rocks

11. Typical values for the KhFactor 0.01 Soil 0.2 Weathered Rock 0.5 Soft or Jointed Rock 10 Hard Rock

12. Background • Ms is the primary term that affects the value of Kh for soil materials. The other terms are more important for rock. An exception might be blocky clays.

13. Available Tools For Estimating Kh • NEH 628, Chapter 52, Appendix B • Soil Catalog on SITES CD

14. Chapter 52, Appendix B - Soils • If the PI greater than 10 – Cohesive • If the PI is < 10, soil is Cohesionless

15. Headcut Migration • The rate of headcut migration is very sensitive to values of the Kh factor in the lower ranges of the value. • Equations for headcut advance put into spreadsheet to evaluate sensitivity

16. Demonstration of Influence of Kh on predicted rate of migration of headcut • Examples developed from equations for headcut migration shown in Chapter 51, Part 628, NRCS • First example is for relatively low unit discharge of 20 cubic feet per second per foot width of spillway – Condition for critical depth of 2 feet 2 feet

17. Soil Weathered Rock Poor Quality Rock 0.2

18. Available Tools For Estimating Kh • Spreadsheet • Uses soil properties from field and laboratory tests or table estimates • NEH 628, Chapter 52, Appendix B • Soil Catalogue

19. Examples of Catalog Soils • Catalogue Soils are on SITES CD

20. Soil Catalog • Existing catalog for soils (assuming that Kh value is < 0.2) is ten samples • Ten soils in catalog have Kh values from 0.01 to 0.17. • Catalog listings with Kh values of 0.2 and above are weathered shale – these have Kh values of 0.2 – 0.5. Five Listings

21. Soil Catalogue • Soil descriptions are very brief • Some wording confusing – example index value based on laboratory strength notation for nonplastic ML soil from Oklahoma • Discussion on methods used for back-computation of Kh value for soils

22. No soils in range of 0.05 to 0.10 No soils in range of 0.12 to 0.16

23. Kh – 0.05 Twin Caney 17-34, KS CL, firm, soil fill;headcut

24. Kh – 0.05 East Fork Pond River 7B, KY Debris fill; spillway exit – (probably CL)

25. Kh – 0.10 East Fork Pond River 9A, KY CL, firm, soil cover in headcut; upper material only considered in Kh determination

26. Kh – 0.16 Misteguay 4, MI CL, stiff, glacial till; headcut

27. Material Strength Number, Ms • For Cohesive Soils • Ms = 0.78 (UCS)1.09, where UCS is unconfined compressive strength – Use Table 52.3 • Needlessly complicated – use simpler methods (later)

28. Confusion on terms related to unconfined compressive strength • The distinction is important because it is a factor of 2 P A t qu

29. Confusion on terms related to unconfined compressive strength • When an unconfined compression test is performed in a laboratory, the value usually reported is the unconfined compressive stress applied to the sample at failure • When a field vane shear test is performed, the value reported is usually the shear strength of the soil (1/2 the qu strength).

30. Conditions for unconfined compressive strength • Should unconfined compression tests be routinely performed at the insitu water content or should the samples be saturated prior to testing? Values will depend strongly on this factor. The unconfined strength of a saturated sample could be ½ that of a sample tested at 90 percent of saturation.

31. Conditions for unconfined compressive strength • Unconfined compression tests are inadvisably performed on low PI soils. High strengths may be measured from negative pore pressures that occur during the test. Soils with PI’s less than 15 should be tested in a submersed condition to avoid this problem

32. Table 52-3

33. Relation of consistency to unconfined compressive strength • Table 5 in Chapter 52 shows different range of unconfined strengths for consistency than other standard geotechnical references • See Table from Terzaghi as Example

34. Table 52-3

35. Confusion on units related to unconfined compressive strength • Table 52-3 employs units of kilopascals (kPa) while Table 5 in Appendix B uses units of mega Pascals (mPa). • While this is only a conversion factor of 1,000, confusion could still result, particularly when laboratories report results in pounds per square foot or pounds per square inch

36. Undrained Shear Strength • Only applicable for quick loading of slowly permeable saturated soils

37. Undrained Shear Strength • Mistake to perform tests on more permeable material such as silts • Unrealistically high strengths on these materials from negative pore pressures during load application

38. Very Stiff Clays with PI > 12, Overconsolidated – Undrained Shear Strength > 2,000 psf 0.33 0.67 1.0 0 Liquidity Index < 0 2,000 psf Undrained Strength, c = qu/2, psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30

39. Stiff Clays with PI > 12 overconsolidated Undrained Shear Strength = 1,000 - 2,000 psf 0 0.33 0.67 1.0 Liquidity Index = 0 to 0.33 2,000 psf 1,000 psf Undrained Strength, c = qu/2, psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30

40. Medium Clays with PI > 12, Slightly Overconsolidated – Undrained Shear Strength = 500-1,000 psf 1.0 0.33 0.67 0 Liquidity Index = 0.33 to 0.67 500psf 1,000 psf Undrained Strength, c = qu/2, psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30

41. Soft Clays with PI > 12Normally Consolidated Undrained Shear Strength = 250-500 psf 1.0 0 0.67 0.33 Liquidity Index = 0.67 to 1.0 500psf 250psf Undrained Strength, c = qu/2, psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30

42. Very Soft Clays with PI > 12, Under Consolidated Undrained Shear Strength < 250 psf 1.0 0 0.33 0.67 Liquidity Index > 1.0 250psf Undrained Strength, c = qu/2, psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30

43. Distinguish between Unconfined Compressive Strength (qu) and Undrained Shear Strength (su or c)

44. Torvane measures Undrained Shear Strength

45. Pocket Penetrometer measures unconfined compressive strength (qu)

46. Unconfined compression test Load s3 = 0 Strain rate = 1-2%/min.

48. Unconfined compression test • su = “undrained shear strength” (c value) • qu = “unconfined compressive strength” t = c = su = ½*qu t c = su s s1 @ failure = q u s3 = 0

49. Factors in erodibility of overconsolidated clay soils • What is importance of blocky structure and slickensides? • Is there available case history data?

50. Factors in erodibility of overconsolidated clay soils • What is importance of blocky structure and slickensides? • Is there available case history data?