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) 1) for Bedrock SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

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

3. Headcut Erodibility IndexNational Engineering Handbook 210-VI-NEH Chapter 52 • Field Procedures Guide for the Headcut Erodibility Index • Appendix 52B - Headcut Erodibility Index Flow Chart • Appendix 52C – Field Data Sheets

4. SITES Program • Construct Profile along Auxiliary Spillway • Assign material parameters to each major horizon in spillway • Apply design hydrograph – duration and unit q values • Sequence of computations involving Phase II and III predict erosion of spillway

5. SPILLWAY SURFACE , ft ELEVATION STATION , ft

6. SPILLWAY SURFACE , ft ELEVATION STATION , ft

7. 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

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

9. Soil Weathered Rock Poor Quality Rock 0.2

10. Demonstration of Influence of Kh on predicted rate of migration of headcut • Second example is for moderately high unit discharge of 50 cubic feet per second per foot width of spillway – Condition for critical depth of 4+ feet 4 feet

11. Soil Rock Weathered Rock

12. Poor Quality Rock Weathered Rock Soil

13. OVERVIEW • What’s needed • For every geologic unit present • Through invert elevation at valley floor • How to get it

14. Headcut Erodibility Index Kh = Ms x Kb x Kd x Js • Ms = material strength number • Kb = block or particle size number • Kd = discontinuity/inter-particle bond shear strength number • Js = relative ground structure number How well they’re stuck together How easy the chunks are to rip up How big the chunks are How strong it is Kh = x x x x

16. Alabama Schist 2005

17. Headcut Erodibility Index Kh = Msx Kb x Kd x JsMs= material strength number Unconfined compressive strength of the materials themselves • Bedrock • Core for ASTM D 2938 Unconfined (Uniaxial) Compressive Strength of Intact Rock Core x

18. Rock “Ms” Determination • Best determined by field estimates • Practical • Lab test expensive & rarely justified • Guidance in Table 52-4 • 210-VI-NEH, Chapter 52 • Other – rebound hammer method • ASTM D 5873 • ASTM STP 984

19. Rebound Hammer Method

20. psi 181-725 87-181 psi 725-1,812 1,812-7,250 7,250-14,500 14,500-36,250 > 36,250

21. Catalogue Kh = 0.5

22. Catalogue Kh = 2.0

23. Catalogue Kh = 10

24. Catalogue Kh = 13

25. Catalogue Kh = 20

26. Catalogue Kh = 550