SPT Energy Measurements with the Pile Driving Analyzer

1. SPT Energy Measurements with the Pile Driving Analyzer Laura Krusinski, P.E. Senior Geotechnical Engineer

3. Usage Policy Questions How do we use N-values obtained from an automatic hammer in design computations? How do we handle field data obtained from different hammer systems? How do we handle consultant/contracted drill rigs? How can inspectors determine energy delivery & mechanical issues from assembly inside the guide tube? What goes on the field logs? (N-confusion) What should the final logs look like?

4. MaineDOT

5. Advantages of safety hammer-rope-cathead SPT testing method N60 values do not have to be corrected whereas the N60 is the standard To use correlations, LRFD Eq. 10.4.6.2.4 – 2 requires correction to reflect greater energy N60 = (ER/60%) x N N60 = SPT blow count corrected for hammer efficiency ER = Hammer efficiency. The following values maybe assumed if specific data is not available ER = 60 % for conventional drop hammers using rope & cathead ER = 80% for automatic trip hammers N = uncorrected SPT blow count

6. Advantages of safety hammer-rope-cathead SPT testing method Rope/cathead SPT testing permits real time logging of N values – one less opportunity for error in design due to incorrect conversions of raw N-values to Ncorrected. Eliminates confusion of Contractors or other users of logs as to type of N-value on the logs

7. Advantages of safety rope-cathead SPT testing method Continued use of Terzaghi & Peck 1948 correlations of N60 to relative density without confusion (inspectors can assign density descriptors to soil units on the field logs per MaineDOT Key to Soil Descriptions) Use of the safety hammer-rope-cathead method is in strict conformance with ASTM 1586, which calls for raising a 140# hammer 30” and letting it fall unimpeded

8. Advantages of rope & cathead SPT testing method The most commonly used soil parameter, N60,, is used for correlations and estimates of internal friction angle, bearing capacity, pile capacity, settlement, etc.

9. Disadvantages of Automatic Hammer SPT testing Opportunity for error in design due to incorrect conversions of raw N-values to Ncorrected. Use of uncorrected N-values with auto hammers (~80% efficiency) would result in over-conservative designs – economy is lost. Lack of knowledge about policies and practices for use of automatic hammers

10. Advantages of automatic hammers Reduced driller labor Reduced driller fatigue Reduced safety issues More consistent energy delivery More reliable field data Permitted in ASTM 1586

11. Policies for Automatic Hammer Use Information gathering Draft of the “Proposed Test Method for Energy Measurement for the SPT Test” for AASHTO Minnesota DOT “SPT Energy Measurements with the PDA” Noticed significantly different blow counts on projects where drill rigs w/different SPT hammer systems were sampling in the same soil conditions Performed tests on 6 drill rigs with PDA Value in standardizing all SPT data by calibrating each hammer so that each would provide an energy transfer/efficiency of 60%

12. Minnesota DOT Data

13. Policies for Automatic Hammer Use Information gathering (cont’d) Oregon DOT “Standard Penetrations Test Energy Measurements” Importance of “normalizing” N-values for variety of SPT hammers Found cost of testing was more than offset by the reduction in conservatism, created by utilizing lower N-values from auto hammers

14. Oregon DOT Data

15. Utah DOT Data

16. ASTM D-4633-05 Adopted in 1986 Withdrawn in 1995 Approved on November 1, 2005 and re-instated

17. Outcome CME 340 automatic hammer installed on the drill rig in January 2007 5-Part Policy for Automatic Hammer Use established Annual “calibration” or SPT “energy measurements” per ASTM D 4633-05

18. SPT Energy Measurement Testing August 2007 Test Site selected: Fryeburg, Maine GRL quote - $3,750/day of testing

19. ASTM D 4633-05 “Perform measurements for at least 3 depths of quality data, with 5 depths preferred” AASHTO – “For a test to be satisfactory, an N-value of at least 10 must be obtained at each depth and a total of 50 measurements must be recorded for a particular sequence of depths Our objective – identify site with enough overburden to perform 5 SPT tests at 5 ft intervals with N=10

20. ASTM D 4633-05 “Tests should be limited to moderate N-values ranges between 5 and 50” “Energy evaluation of the hammer system is more reliable when the length (L) is 9 m (30 ft) to 12 m (40 ft) or more” Le = length between the location of the transducers on the instrumented subassembly and the bottom of the sampler

21. SPT Energy Testing “Moderate N-values” Mostly sands (SPT test not meaningful in soft clays or silts) No influence of gravel Need full 24” recovery

22. NWJ instrumented rod section 2 accelerometers 2 strain gages Velocity & force measurements

23. Pile Driving Analyzer

24. Force - Velocity Method Energy transfer computed by the PDA via processing force and velocity measurements a – t0 of energy transfer b – time of maximum energy transfer to the rods b EMX = ? F (t) V (t) dt a

25. ASTM D 4633-05

26. Automatic Hammer Use Policy Part 1. Calibration factor of 0.77 established per ASTM D-4633 for one year Part 2. Hammer type for SPT testing, whether it is safety w/rope & cathead or automatic hammer, is the decision of the Engineer on a project-by-project basis. Part 3. Field logs show uncorrected N & indicate hammer type

27. Field Log – uncorrected N-values

28. Automatic Hammer Use Policy (continued) Part 4. Final LOGDRAFT logs to show both Nraw and N60 based on calibration factor

29. Final Log - shows Nuncorrected & N60

30. Automatic Hammer Use Policy(continued) Part 5. Contracted automatic hammers and spooling winches on State Projects are required to be calibrated annually

31. The End