NEW DEVELOPMENTS AND IMPORTANT CONSIDERATIONS FOR STANDARD PENETRATION TESTING FOR LIQUEFACTION EVALUATIONS Jeffrey A Farrar M.S., P E 1 ABSTRACT Standard Penetration Tests (SPT) are often used for evaluating earthquake induced ground liquefaction for dam safety evaluations. A procedure for SPT testing for liquefaction potential is specified in American Society for Testing and Materials (ASTM) standard D 6066. In gravelly soils it is recommended to perform SPT and record penetration per blow. A new laser distance finder has been developed to record penetration per blow data. Automatic hammer systems are preferred for testing and because they are rate dependent corrections for rate must be made. Other factors which affect the test such as long drill rods and other mechanical and operator variables are presented. INTRODUCTION A new penetration per blow instrumentation system has been developed for use in SPT testing. The system uses a laser distance finder. Test procedures be used to perform quality SPT testing are presented in ASTM standard D 6066. The standard restricts the drilling methods that can be used and requires the drill rod energy of the hammers be known. Automatic hammers are preferred for testing because of their consistent energy delivery. SPT PENTRATION PER BLOW RECORDER Coarse grained gravelly soils are very difficult to evaluate for liquefaction triggering. SPT tests yield relatively high N values when gravels are present due to the gravel blocking advancement of the split barrel sampler. Without reliable SPT values the engineer has limited alternatives of obtaining site specific data, such as Becker Penetration Testing or Shear Wave velocity determinations for evaluating liquefaction triggering. When performing the SPT in gravelly soils it is common practice to record penetration per blow during conduct of the test. Figure 1 illustrates the use of penetration per blow data where sand blow counts can be extrapolated. Part (a) shows a smooth driving patterns that do not require corrections to N values and (b) shows strong increases in driving resistance that, along with sample recoveries, suggesting that the sampler encountered large particles. Figure 1 (b) also shows the adjusted N value, based on extrapolating the pre-obstruction driving rate. 1 U S Bureau of Reclamation, Technical Service Center, Denver CO, 303-445-2333, jfarrar@do.usbr.gov Standard Penetration Testing for Liquefaction Evaluations 1607
Manual recording of penetration per blow is problematic and difficult. In addition, it requires that the recorder be close to the drill rods during conduct of the test which is a safety hazard. Manual methods that have been used included marking of a piece of lathe taped to the drill rods or using two people to call off and record penetration by reading a graduated tape affixed to the drill rod. Figure 1. Examples of interpreting SPT blow counts on a per-inch basis. (Idriss and Boulanger, 2008). A new device to automatically record penetration per blow has been developed by the company PileTrac. The device is called a Pile Set Monitor because it was originally designed to measure a set of piles on re-strike. The device uses a laser distance finder, data acquisition system, and all weather PC tablet. Figure 2 shows the laser distance finder set up to hit a target that is placed in between the anvil drill rod connection. In this example an automatic chain cam hammer system is being used for the SPT. This new system automates the data collection and provides the user with Excel data files for later analysis. The software can display the computed N value and records of penetration per blow. Figure 3 shows a screen display of the data. 1608 Innovative Dam and Levee Design and Construction
Figure 2. Laser distance finder looking up at a target placed between the anvil and drill rods. Figure 3. Example output of the Penetration per Blow Counter showing incremental penetration. Standard Penetration Testing for Liquefaction Evaluations 1609
SPT AUTOMATIC HAMMER USE SPT automatic hammer systems are the preferred method of performing the test. ASTM D 6066 strongly recommends that these hammers be used for evaluation of liquefaction potential. This recommendation is based on an efficient energy transmission with a standard deviation of 1 to2 percent during operation as opposed to over rope and cathead method where energy can vary over ± 5 %. Energy measurements are conducted in accordance with ASTM D 4633. The chain cam hammer was first marketed by Central Mine Equipment Company. The patent has expired and other drilling companies are manufacturing machines with a similar design. The CME design was set to operate at a speed of 50 to 55 blows per minute. At that speed, the hammer is dropped 30 inches. H Bolton Seed recommended using a blow count of a rate of 20-40 blows per minute (bpm) for liquefaction investigation. Thus the hammer system must be slowed down using the flow control for the hydraulics. This hammer system is rate dependent. Figure 4 shows an example from a single drill rig, of the relationship between automatic hammer and blow count rate and drill rod energy (Farrar, 1999a). The hammer as manufactured is designed to run at 50 bpm for a 30-inch drop. Figure 4. Effect of Blow Count Rate on Drill Rod Energy using a CME Automatic Hammer (Farrar, 1999a). ASTM D 6066 requires the drill rod energy of the hammer system be known. This can be from a previous measurement or from actual measurement taken on site. The problem 1610 Innovative Dam and Levee Design and Construction
with previously measured energy occurs if the rate of the hammer has been changed. Drillers change drill rigs often and many don t convey the operating instructions for the drill. All of these errors can occur so one must be vigilant to maintain a constant method of operation. Figure 4 shows that at the design speed of about 55 blows per minute that hammer system delivered about 85 % energy. We have measured as high as 95% energy on these automatic hammers. Users are cautioned that the data in this paper are for a CME automatic hammer. Other manufactures may use different hammer components and anvil designs that may affect the energy transfer. EFFECT OF DEPTH AND DRILL ROD LENGTHS When performing tests in gravelly soils the Bureau of Reclamation uses the following refusal criteria as follows; For depths of less than 40 ft, refusal is 50 blows. For depths between 40 to 80 feet, refusal is 80 blows. For depths greater than 80 feet, refusal is 100 blows. This criterion is based on liquefaction triggering. Higher blow counts at those depths would not be liquefiable. Another criterion for refusal is if there is no advance of the sampler in 10 blows (ASTM D 1586). For short drill rod lengths less than 30 feet it was common practice to multiply blow counts by a reduction factor Cr ranging from 0.75 at 10 feet and then increasing to 1.0 at 30 foot depth. Recently, several papers have been presented showing this was not the case. The reduction is not as large as theoretical values. It s now recommended to measure drill rod energy at shallow depths to check. For depths in excess of 100 feet, research has shown that it is appropriate to reduce the drill rod energy ratio by 1 percent for every 10 feet of drill rod below100 foot in depth (Farrar, et al. 1998). FACTORS AFFECTING SPT N VALUES Several factors contribute to variability in SPT N values. The first research on drill rod energy transmission was published by Schmertmann in 1978. At the time of publication Donut hammers in addition to safety hammers and the first automatic hammers were being built. This table included estimates of energy transmission problems along with some mechanical and procedural variables. Table 1 shows the estimates based on his research, The writer has also made a table of possible test effects in a publication for Reclamation drillers (Farrar, 1999b). Table 2 is a summary table of possible affects on SPT N values which can also be affected by drilling methods and other mechanical variables. There is also a large effect on SPT N values possible drilling method and drilling disturbance in the test at the base of the drill hole. Standard Penetration Testing for Liquefaction Evaluations 1611
The US Bureau of Reclamation makes extensive use of hollow-stem auger (HSA) drilling for SPT. Since the HSA drilling is notorious for being unstable in clean sands special precautions must be taken. Methods to prevent disturbance include keeping the augers filled with water and retracting the drill tooling very slowly to prevent sand from coming up into the augers (Farrar, 1999b). Looking at Table 2, one can see there are many possible problems that could occur with the SPT. The user should be very careful when observing the conduct of the test and document all the facets of the investigation that are important and could affect the test values. CONCLUSIONS A new penetration per blow counter has been developed to automate collection of Standard Penetration Testing (SPT). In gravelly soils, measurement of penetration per blow data is recommended for liquefaction studies. For earthquake liquefaction studies, the SPT should be conducted in accordance with American Society for Testing and Materials practice D 6066. This practice allows three drilling methods and requires that the energy of the hammer system be measured. Use of an automatic hammer is strongly recommended. The readers are warned that the automatic hammer system is a rate dependent hammer and must be operated in a consistent fashion. Energy transmission of automatic hammers can vary more that 5% when the hammer rate is changed. The SPT is a highly variable test and both drilling techniques and mechanical variables can affect the test. ACKNOWLEDGEMENTS I would like to thank the U S Bureau of Reclamation for support in writing this paper. 1612 Innovative Dam and Levee Design and Construction
Table 1. Some Factors in the Variability of Standard Penetration Test N (Schmertmann 1978). Standard Penetration Testing for Liquefaction Evaluations 1613
Table 2. Summary of factors in the variability of SPT Expressed as typical N values (Farrar, 1999b) Basic Detailed Drilling method Sampler Cause 1. Use of drilling mud and fluid bypass. 2. Use of drill mud and no fluid bypass. 3. Use of clear water with or without bypass. 4. Use of hollow-stem augers with or without fluid. 5. 8-inch diameter hole compared to 4 inch. 6. Use of the larger ID barrel, without the liners 7. Use of a 3-inch OD barrel versus a 2-inch barrel Procedure 8. Use of a blow count rate of 55 bpm as opposed to 30 bpm Energy Transmission Factors. Typical Raw SPT value in Clean Sand N = 20 20 10 0-20 8-10? 0-20 8-10? 0-20 8-10? 17 8-10? 17 9 25-30 e 10 Typical Raw SPT value in Clay N=10 20 e1 10 e1 Drill Rods 9. AW rod versus NW rod 18-22 e2 8-10 e2 Hammer Operation 10. SPT at 200 ft as opposed to 50 ft 11. SPT at less than 10 ft as opposed to 50 ft with AW rods. 12 SPT at less than 10 ft as opposed to 50 ft with NW rods. 13. Three wraps versus two wraps around the cathead. 14. Using new rope as opposed to old rope. 18 e4 5 e3 30 15 25 12 22 11 19 9 1614 Innovative Dam and Levee Design and Construction
Table 2 - Summary of Factors in the Variability of SPT expressed in typical N values (Farrar, 1999b) Basic Cause Detailed Typical Raw SPT value in Clean Sand N = 20 Typical Raw SPT value in Clay N=10 Hammer Operation 15. Free fall string cut drops versus 2 wrap on cathead. 16 8 16. Use of high efficiency automatic hammer versus 2 wrap safety hammer. 14 7 17. Use of a donut hammer with large anvil as opposed to safety hammer. 24 12 18 Failure to obtain 30 inch drop height (28-in) 22 11 19 Failure to obtain a 30 inch drop height (32 in) 18 9 20 Back tapping of safety hammer 25 12 during testing e = estimated value e1 = Difference occurs in silty or clayey sands e2 = It is not known whether small drill rods are less or more efficient, with larger rods N may be less in clay due to the weight of the drill rods. e3 = N in clay may be lower due to the weight of rods e4 = Actual N value will be much higher due to higher confining pressure at great depth, e.g., the difference shown here is from energy only, and confining pressure was not considered. REFERENCES American Society for Testing and Materials, Standard D 1586-08a, Standard test method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, ASTM, West Conshohocken, PA, www.astm.org American Society for Testing and Materials, Standard D4633-10, Standard Test Method for Energy Measurement for Dynamic Penetrometers, ASTM, West Conshohocken, PA, www.astm.org Standard Penetration Testing for Liquefaction Evaluations 1615
American Society for Testing and Materials, Standard D6066-96 (2004), Practice for Determining the Normalized Penetration Resistance of Sands for Evaluation of Liquefaction Potential, ASTM, West Conshohocken, PA, www.astm.org Farrar, J.A., Nickell, J., Allen, M. G., Goble, G. and J. Berger, (1998), Energy Loss in Long Rod Penetration Testing Terminus Dam Liquefaction Investigation, Proceedings of Geotechnical Earthquake Engineering and Soil Dynamics III, August 3-6, 1998, American Society of Civil Engineers (www.asce.org ). Farrar J.A and Chitwood, D., (1999)a CME Automatic Hammer Operations Bulletin, Dam Safety Office Publication DSO 99-03, Bureau of Reclamation, Technical Service Center, Denver CO. Farrar J.A., (1999)b Standard Penetration Test Driller/Operators Guide, Dam Safety Office Publication DSO 99-17, Bureau of Reclamation, Technical Service Center, Denver CO. Idriss, I.M. and Boulanger, R.W., 2008, Soil Liquefaction During Earthquakes, Engineering Monograph MNO-12, Earthquake Engineering Research Institute, Oakland California, U.S.A. 1616 Innovative Dam and Levee Design and Construction