Groves Sess4 102309


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Groves Sess4 102309

  1. 1. An Overview of the Statistical Reliability of Levee Design Data SAME 2009 Water Conference Sacramento, California Chris Groves, P.E. Shannon & Wilson, Inc.
  2. 2. Early levees were constructed by farmers.
  3. 3. Standard practice for levee design is EM 1110-2-1913
  4. 4. EM recommends boring explorations on 200- to 1000- foot centers.
  5. 5. Explorations could easily miss a critical subsurface feature.
  6. 6. For an agriculture levee the risk may be acceptable.
  7. 7. Some agricultural levees later become urban levees.
  8. 8. Hydrology and Hydraulics have been expressed as a frequency or probability of exceedance
  9. 9. Geotechnical issues are deterministic. The public believes we are dealing with certainty. What is the probability of a geotechnical failure when the factor of safety is 1.4?
  10. 10. Alluvial deposits are sorted by water and as a result can vary significantly in a short distance.
  11. 11. Geomorphology can help
  12. 12. Geophysics can help
  13. 13. The variability of soil properties has been addressed with the Taylor method of analysis.
  14. 14. Stastical Analysis can address site variability. <ul><li>Procedure </li></ul><ul><li>Characterize site conditions at numerous locations </li></ul><ul><li>Analyze levee at each location for seepage and stability </li></ul><ul><li>Sort and plot the factor of safety in a cumulative distribution </li></ul>
  15. 15. The method looks at an entire levee reach not a single location.
  16. 16. The method assumes that the soil profile can be accurately determined at each CPT or boring
  17. 17. Levee geometry is also required for each section
  18. 18. The method assumes the analysis parameters can be determined at each section CPT data reduction
  19. 19. Seepage Analysis
  20. 20. Stability Analysis
  21. 21. Plot of Factor of Safety Versus Cumulative Probability Distribution
  22. 22. A seepage and stability berm may be added
  23. 23. Reanalyze with levee improvements
  24. 24. This method may give an estimate of the hazard due to high variability along a reach Less Site Variability
  25. 25. Summary <ul><li>Levees are constructed on alluvial soil which is often highly variable. </li></ul><ul><li>Widely spaced subsurface explorations leave a hazard of poor unknown conditions unidentified. </li></ul><ul><li>Statistical analysis can indicate the potential for unknown hazard. </li></ul><ul><li>The resulting combined probability of a flood and a very weak levee section may be very small. </li></ul><ul><li>Analysis can be conducted for 50 yr., 100 yr, and 200 yr. and the combined probability ploted. </li></ul><ul><li>Reanalyzing with proposed levee improvements can be an indicator of the reduced hazard and the corresponding reduction in risk. </li></ul>
  26. 26. What are the limitations? <ul><li>Layer assumptions </li></ul><ul><li>Soil parameters </li></ul><ul><li>Slope of line may be a better indicator than the FS </li></ul><ul><li>Does not address reoccurring seepage </li></ul><ul><li>Does not address animal burrows </li></ul>
  27. 27. Further Research <ul><li>Model repair options </li></ul><ul><li>Study slope of the cumulative probability line </li></ul><ul><li>Look at combined probability of FS<1 vs. 50-yr, 100-yr and 200-yr floods </li></ul>