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PENETRATION TEST COMPARISONS: MODIFIED CALIFORNIA VERSUS STANDARD PENETRATION TEST SAMPLERS

PENETRATION TEST COMPARISONS: MODIFIED CALIFORNIA VERSUS STANDARD PENETRATION TEST SAMPLERS. Jacqueline D.J. Bott, Keith L. Knudsen & Charles R. Real California Geological Survey. Outline of talk. Why comparison is important Review of N 1,60 calculation

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PENETRATION TEST COMPARISONS: MODIFIED CALIFORNIA VERSUS STANDARD PENETRATION TEST SAMPLERS

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  1. PENETRATION TEST COMPARISONS: MODIFIED CALIFORNIA VERSUS STANDARD PENETRATION TEST SAMPLERS Jacqueline D.J. Bott, Keith L. Knudsen & Charles R. Real California Geological Survey

  2. Outline of talk • Why comparison is important • Review of N1,60 calculation • Conversion used to correct MCS blows to SPT-equivalent blow count • How did we do the comparison • Location of data • Results • Conclusions so far and future work

  3. Why? • CGS calculates N1,60 from SPT N-values for liquefaction analyses to help define Seismic Hazard Zones of Required Investigation. CGS utilizes geotechnical boring data collected from cities & counties etc. • Consultants often use MCS instead of SPT (ASTM 1526, 6066) for determining penetration resistance • Need to convert MCS blows to SPT-equivalent blow count in order to calculate N1,60

  4. Review of N1,60 calculation N1,60 = Nm.CE.CN.CR.CB.CS Where Nm = measured blows (using SPT sampler) CE = Correction for hammer energy efficiency CN = overburden correction factor (to 1 atm,) CR = correction for “short” rod length CB = Correction for borehole diameter CS = Correction for non-standard sampler

  5. Conversion to SPT-equivalent from non-standard samplers N=N’(WH/4200)(2.02-1.3752)/(OD2-ID2) (Burmister, 1948) N=N’(WH/4200)(2/OD2) (LaCroix & Horn, 1973) where N = SPT-equivalent blow count N’ = measured blow count WH = hammer mass (lbs) x fall distance (in) OD = outer diameter of non-standard sampler (in) ID = inner diameter of non-standard sampler (in)

  6. Conversion factors for MCS to SPT-equivalent blows Using CGS Definition of MCS: ID = 2.0 in (1.875 in with liners) & OD = 2.5 in. 0.77 Burmister (1948) 0.64 LaCroix & Horn (1973) Other definition of MCS: ID = 2.5 in (2.4 with liners) & OD = 3.0 in 0.65 Burmister (1948) 0.44 LaCroix & Horn (1973)

  7. How? • Compare consecutive samples (MCS & SPT) from same lithologic layer in a particular boring, that are within 5 ft of each other. • Direct comparison of two such values cancels out factors often not reported by consultants such as hammer energy, borehole diameter etc. • Only CN (and rod length for shallow samples) will be different so also compare N1,60’s

  8. Consecutive samples taken in same lithologic layer in a particular boring, separated by 5 ft or less MCS-SPT MCS-MCS SPT-SPT SM CL ML MCS MCS SPT <5 ft <5 ft <5 ft SPT SPT MCS

  9. San Francisco Bay Area Data Sets

  10. Los Angeles Basin Data Sets

  11. SPT vs SPT - SFBA Raw blows Converted to N1,60’s Shallowersample N1,60 SPT Blows SPT Blows N1,60 Deeper sample N=1121

  12. Residuals from 1:1 relation Raw blows Converted to N1,60’s Mean = -1.215 SD = 11.35 Mean = 0.424 SD = 12.32 Residuals in SPT Blows Shallower - Deeper Residuals in N1,60’s SPT-SPT

  13. SPT vs SPT - LA Basin Raw blows Converted to N1,60’s Shallowersample N1,60 SPT Blows SPT Blows N1,60 Deeper sample N=805

  14. MCS vs MCS - SFBA Raw blows Converted to N1,60’s Shallowersample N1,60 MCS Blows MCS Blows N1,60 Deeper sample N=1077

  15. Residuals from 1:1 relation Raw blows Converted to N1,60’s Mean = -0.673 SD = 11.68 Mean = 0.826 SD = 9.83 Residuals in MCS Blows Shallower - Deeper Residuals in N1,60’s MCS-MCS

  16. MCS vs MCS - LA Basin Raw blows Converted to N1,60’s Shallowersample N1,60 MCS Blows MCS Blows SPT Blows N1,60 Deeper sample N=139

  17. MCS vs SPT - SFBA Raw blows Converted to N1,60’s N1,60 N1,60 from SPT SPTsample SPT Blows MCS Blows N1,60 from MCS N1,60 MCS sample N=129

  18. Residuals from 1:1 relation Raw blows Converted to N1,60’s Mean = -7.46 SD = 14.69 Mean = -1.246 SD = 13.42 Residuals between SPT & MCS Blows Residuals in N1,60’s MCS-SPT

  19. MCS vs SPT - LA Basin Raw blows Converted to N1,60’s N1,60 from SPT SPTsample SPT Blows N1,60 from MCS MCS Blows MCS sample N=104

  20. Residuals from 1:1 relation Raw blows Converted to N1,60’s Mean = -8.73 SD = 12.51 Mean = -5.07 SD = 10.78 Residuals between SPT & MCS Blows Residuals in N1,60’s MCS-SPT

  21. MCS-SPT LS regression - SFBA 80 60 40 N160’s from SPT Blows Y=0.45x + 9.16 20 0 0 20 40 60 80 Adjusted N1,60’s from MCS Blows

  22. MCS-SPT LS regression - LA Basin 80 60 40 N160’s from SPT Blows Y=0.33x + 6.10 20 0 0 20 40 60 80 Adjusted N1,60’s from MCS Blows

  23. Conclusions so far... • There is a large scatter in blow count data - both for SPT and MCS • CGS conversion from MCS to SPT-equivalent (N1,60) gives more consistent results for SFBA than for LA Basin. Is MCS defined differently in the two locations? Is this a function of the geology? Or related to something else?

  24. Lithologies for MCS-SPT data sets SFBA LABasin SW SW CH SP CL SP CL ML SM SC SM SC ML GC,GM,GP

  25. Future work • Effect of lithology, saturation, depth, presence of gravel, etc • Investigate why residuals are not normally distributed • Survey Consultants as to how they define MCS

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