The Effect of the Hammer Energy Efficiency Ratio on SPT-Based Liquefaction Evaluation
Troy Hull, P.E., G.E., M. ASCE1; Jason Butler-Brown, P.E., M. ASCE 2; and Travis
Willis, E.I. 3 1 Principal Geotechnical Engineer, Earth Engineers, Inc., 713 Northeast 152nd Avenue, Vancouver, Washington 98684; [email protected] 2 Engineer - Geotechnical, City of Portland Office, Bureau of Development Services, 1900 SW Fourth Avenue, Suite 5000, Portland, Oregon 97201; [email protected]
3 Geotechnical Engineering Associate, Earth Engineers, Inc., 713 Northeast 152nd Avenue, Vancouver, Washington 98684; [email protected] ABSTRACT: The Standard Penetration Test (SPT) has long been used by the geotechnical community to evaluate the factor of safety (FOS) against liquefaction and to estimate liquefaction induced soil strength loss, vertical settlement and lateral spread. There are other investigative methods of evaluating liquefaction risk, the most common being the cone penetration test (CPT). However, the SPT method is often the first choice for geotechnical professionals because the testing is economical, the equipment readily available, and it returns physical samples that can be tested in the laboratory. The public and governing jurisdictions trust that the geotechnical community is using field-testing methods that provide relatively accurate soil strength data, which is critical to liquefaction evaluation. Because most jurisdictions in the United States do not require the use of calibrated SPT hammers, the majority of SPT liquefaction analyses are based on uncalibrated SPT hammer data. There appears to be a perception by governing jurisdictions that it is acceptable for geotechnical engineers to rely on their engineering judgment to determine SPT hammer energy. However, numerous past studies demonstrate that SPT hammer energy varies widely depending upon the hammer type, manufacturer and lifting mechanism. The variance in energy can be as much as 200 to 250 percent, which is too large to reasonably estimate using engineering judgment alone. This paper focuses on the influence of hammer energy on the SPT liquefaction analyses results. We examine the variability within the results as a function of measured hammer energy. Finally, the paper encourages public agencies to require the use of calibrated SPT hammers in the evaluation of liquefaction triggering, strength loss, dynamic settlement and lateral spread hazards. It is our hope that this practice will be adopted in local and state building codes.
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INTRODUCTION There are scores of published technical papers that discuss the liquefaction phenomenon. Of significant debate recently are the types of soil conditions that are potentially liquefiable, as well as the triggering level of liquefaction. However, the focus of this paper is the evaluation of a common method of characterizing the soil strength—the Standard Penetration Test—not whether a particular soil type (i.e. silt or clay) liquefies. Soil strength is an important engineering property to focus on because it directly impacts the determination of liquefaction potential. As a general characterization, a weaker soil is more likely to be liquefiable while a stronger soil has less risk of liquefaction. If the soil strength data collected by the geotechnical engineer is not accurate, then it impairs the engineer’s ability to determine whether or not a site may liquefy. Having a clear understanding of the liquefaction susceptibility of a particular site is important to protecting the public and property. It also has great economical impact. Misidentifying a site as being liquefiable—when it is not—can lead to unnecessarily higher construction cost. Conversely, classifying a liquefiable site as being non-liquefiable can lead to unanticipated building damage and risk to life-safety. There are a number of field investigation methods to assist the engineer in evaluating the factor of safety (FOS) against liquefaction and estimating liquefaction induced soil strength loss, vertical settlement and lateral spread. Four in-situ test methods are recommended for use (Seed et al., 2003): the Standard Penetration Test (SPT), Cone Penetration Test (CPT), in-situ shear wave velocity measurement (Vs), and the Becker Penetration Test (BPT). Because SPT testing is economical, the equipment is readily available, and the testing returns soil samples that can then be tested in the laboratory, it is often the first choice for geotechnical professionals when they plan their subsurface investigation. The Standard Penetration Test (ASTM D1586, revised 1999) consists of driving a standard split-tube sampler a distance of 18-inches into the soil at the bottom of a borehole and counting the number of blows required to drive the device three consecutive 6-inch intervals (18 inches total). The sampler is driven with a 140-pound weight falling a distance of 30 inches. The number of blows required to drive the initial 6 inches is typically recorded but ignored as this interval seats the sampler below disturbed soil at the bottom of the borehole. The actual number of blows required to drive the sampler the final 12 inches is the standard penetration resistance, or N value. The N value is not to be confused with the N1 and (N1)60 values, which are discussed later in more detail. The lifting and dropping mechanisms used to drive an SPT hammer include automatic trip, safety rope and pulley, donut rope and pulley, and wire winch line. Ideally, with all of these methods, the 140-pound SPT hammer falling 30 inches would transfer 350 foot-pounds of energy to the sampler (i.e. 2.5 feet times 140 pounds). In actuality, essentially no SPT hammers generate the full 350 foot-pounds
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of energy due to energy loss. Various studies have shown that the actual SPT hammer energy is typically much less—as low as about 20 to 30 percent of the theoretical hammer energy. The N values are used to determine the resistance to liquefaction in the SPT-based liquefaction analysis. Therefore, it is extremely important to understand the actual SPT hammer energy when measuring N values in the field. The liquefaction evaluation procedure consists of calculating (1) the earthquake load, or Cyclic Stress Ratio (CSR), and (2) the soil resistance load, or Cyclic Resistance Ratio (CRR). The ratio of the resistance load (CRR) divided by the earthquake load (CSR) then provides a factor of safety against liquefaction, which is used by the geotechnical engineer to evaluate whether a site is liquefiable or not. The CRR value is calculated based on the N values obtained in the field. Early studies of hammer energy variability (i.e. Kovacs et al., 1983) eventually resulted in the recommendation to account for SPT hammer energy efficiency when evaluating liquefaction potential (Seed et al., 1985). NCEER (1997) provided a method to account for SPT hammer energy variation as well as other SPT factors including effective overburden stress, equipment, and procedural effects. N values corrected for energy, effective overburden stress, equipment and procedural effects are referred to as (N1)60 values. The equation for (N1)60 is as follows: (N1)60 = N * CN * CR * CS * CB * CE (1) where N = blow counts obtained in the field CN = correction for overburden effects CR = correction for “short” rod length CS = correction for non-standardized SPT sampler CB = correction for borehole diameter CE = correction for hammer energy efficiency CN, CR, CS and CB are correction factors that can be found in the NCEER (1997) proceedings and are not presented in detail in this paper. CE is the correction to account for rod energy and is obtained by dividing the hammer energy ratio, ER, by 0.60. The hammer energy ratio can only be known if the SPT hammer is calibrated. If the SPT hammer is not calibrated, then the hammer energy is unknown and it must be assumed based on the hammer type. Numerous studies (i.e. Batchelor et al., 1995; Davidson et al., 1999; Sjoblom et al., 2002) have measured the actual variability in energy within SPT hammers, focusing on differences between manufacturers, models, and duration of use. These studies have defined the expected maximum and minimum energy efficiency ratios. Table 1 below outlines the SPT hammer energy efficiency ratio ranges for 4 studies that we considered. It should be noted that the Utah and Seattle studies do not necessarily represent the maximum potential range of hammer energy efficiency variation. The
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energy efficiency ranges reported in these 2 reports and summarized in the table below are the ranges for a limited number of hammers tested.
Table 1. SPT Hammer Energy Efficiency Variance Data
SOURCE SPT HAMMER TYPE SPT HAMMER ENERGY EFFICIENCY RATIO (ER)
RANGE
LiquefyPro, Version 5.5b computer program input options
Automatic trip Safety rope and pulley Donut rope and pulley Winch wire line
0.54 – 0.96 0.36 – 0.70 0.27 – 0.60
no data presented
Seed et al., 2003
Automatic trip Safety rope and pulley Donut rope and pulley Winch wire line
0.5 – 0.8 0.4 – 0.7 0.3 – 0.6
no data presented
Utah DOT Study (Sjoblom et al., 2002)
Automatic trip Safety rope and pulley Donut rope and pulley Winch wire line
0.46 – 0.95 0.55 – 0.76
no data presented 0.50
Seattle ASCE Study (Batchelor et al., 1995)
Automatic trip Safety rope and pulley Donut rope and pulley Winch wire line
0.91 – 0.99 0.64 – 0.75
no data presented 0.25
As is demonstrated in Table 1 above, the SPT hammer energy efficiency ratio can vary by up to a factor of about 2. So the geotechnical engineer who is using an uncalibrated SPT hammer for their site investigation must assume a hammer energy efficiency ratio with an expected wide range of potential error. There is no dispute in the geotechnical community that hammer energies vary from drill rig to drill rig. Even though hammer energy variance has been acknowledged by the geotechnical engineers that use it in their liquefaction analyses and by the jurisdictions that review the geotechnical analyses prior to approving developments, very little has been done by governing jurisdictions (i.e. states, counties, cities and departments of transportation) to require calibrated hammers in an effort to limit the potential for inaccurate assessment of liquefaction potential. An informal survey of jurisdictions across the United States (U.S.) revealed only one that currently requires SPT hammer calibration—Minnesota Department of Transportation. The common response from jurisdictions when questioned about why they don’t require calibrated SPT data was that it was their assumption that geotechnical engineers should use their engineering judgment to determine the appropriate SPT hammer energy efficiency ratio. To evaluate how the variance in hammer energy ratio impacts the FOS against liquefaction, we performed a parametric study as presented below.
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PARAMETRIC STUDY LiquefyPro, Version 5.5b computer software by CivilTech Software of Bellevue, Washington was used to demonstrate the variability in calculating the FOS against liquefaction due to SPT hammer energy variance. As discussed earlier in this paper, the procedure for calculating the FOS against liquefaction consists of calculating the earthquake load (CSR) and the soil resistance load (CRR). The CRR values are dependent upon the SPT (N1)60 values (Eq. (1) above). The FOS against liquefaction is determined by dividing the CRR by the CSR. In this study, a soil layer was deemed liquefiable if it had a FOS less than or equal to 1.3. This is supported by the recommendation in California Division of Mines and Geology’s Special Publication 117 which is widely used as a guideline for liquefaction evaluation (California Division of Mines and Geology and others, 1997). There are an infinite number of soil and groundwater conditions that could be modeled for liquefaction potential. For the purposes of this limited study, we selected 2 models. Model 1 represents a relatively thin liquefiable layer of 5 feet while Model 2 represents a relatively thick liquefiable layer of 15 feet. For both models, the soil properties and SPT N values were held constant. The soil models are presented in Figure 1 below.
Model 1:
Ground Surface
Model 2:
Ground Surface
Silty Sand, Non-liquefiable
(above water table)
Silty Sand,
Non-liquefiable (above water table)
Silty Sand, Potentially Liquefiable
(below water table) Bedrock
Silty Sand,
Potentially Liquefiable (below water table)
Soil Properties from 0 to 15 Feet: Total Unit Weight = 110 pcf
Fines Content = 35% N value = 10
Bedrock
Soil Properties from 0 to 25 Feet: Total Unit Weight = 110 pcf
Fines Content = 35% N value = 10
FIG. 1. Summary of Soil Conditions for Models 1 and 2.
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The analysis is based on the earthquake magnitude and peak ground acceleration. These values were selected following a review of the 2002 US Geological Survey (USGS) probabilistic seismic hazard deaggregation data for major cities in the U.S. (see Table 2 below). The data was generated using the 2002 Interactive Deaggregation Tool from the USGS website (http://eqint.cr.usgs.gov/ deaggint/2002/index.php). The following input parameters were held constant: frequency = 0 hertz (i.e. PGA) and return period = 2475 years (i.e. 2 percent probability of exceedance in 50 years). The return period is consistent with the 2006 International Building Code (International Code Council, Inc., 2006).
Table 2. Summary of USGS Deaggregation Data for Selected Cities LOCATION MEAN EQ
Sacramento, California 6.16 0.24 Salt Lake City, Utah 6.78 0.76
San Diego, California 6.12 0.41 San Francisco, California 7.39 0.73
Seattle, Washington 6.70 0.63 Washington, D.C. 4.80 0.07
AVERAGE 6.23 0.32
An earthquake magnitude of 6.5 and a peak horizontal ground acceleration of 0.25g were selected for both Models 1 and 2, respectively, based on the available data. These values are generally low for the higher seismicity areas along the west coast of the U.S. and are somewhat higher than typical for low-seismicity areas of the country. After considering the hammer efficiencies outlined in Table 1 above, the upper and lower bounds were selected for the automatic, safety and winch hammers. The upper bound of the winch hammer was modified from 0.50 to 0.60 to represent a theoretical worst-case correction for hammer energy efficiency (CE) of 1. The hammer efficiency values used in our LiquefyPro calculations are shown in Table 3 below. The donut hammer was not modeled separately because the upper and lower bounds were essentially the same as the winch hammer as demonstrated in Table 1.
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Table 3. SPT Hammer Energy Efficiency Ranges Modeled in LiquefyPro SPT HAMMER TYPE SPT HAMMER ENERGY EFFICIENCY
RATIO (ER) RANGE Automatic trip 0.46 and 0.99
Safety rope and pulley 0.36 and 0.7 Donut rope and pulley 0.27 and 0.6
Winch wire line 0.25 and 0.6 Once the two soil profiles, the earthquake loading, and SPT hammer energy efficiency values were selected, the FOS against liquefaction was calculated using the LiquefyPro software. Table 4 presents a summary of the input variables used in the LiquefyPro analyses.
Table 4. Summary of LiquefyPro Input
EARTHQUAKE PGA (g):
0.25 SETTLEMENT CALCULATION:
Liquefiable zone only
EARTHQUAKE MAGNITUDE:
6.5 GROUND IMPROVEMENT
OF FILL ON GROUND
SURFACE:
not applicable (“0” feet of fill
height)
HOLE DEPTH: Model 1: 15 feet Model 2: 25 feet
IN-SITU TEST DATA:
Refer to Fig. 1 above
WATER TABLE (DURING
EARTHQUAKE AND IN-SITU
TESTING):
10 feet HAMMER ENERGY RATIO:
Refer to Table 3 above
SETTLEMENT ANALYSIS (WET):
Tokimatsu/Seed BOREHOLE DIAMETER:
2.5 to 4.5 inches
FINES CORRECTION
(LIQUEFACTION):
Idriss/Seed (SPT only)
SAMPLING METHOD:
Standard sampler
FINES CORRECTION
(SETTLEMENT):
During liquefaction
FACTOR OF SAFETY:
1.3
Note: The LiquefyPro computer program reference to Idriss/Seed for fines correction should actually reference NCEER (1997), page 7. The results of the analyses are presented below for Model 1 and Model 2 in graphical and tabular form in Fig. 2, Fig. 3, Table 5 and Table 6, respectively. The results demonstrate that the hammer energy efficiency ratio significantly impacts whether a site is determined to be potentially liquefiable—when based on the SPT liquefaction evaluation. For example, if a drill rig equipped with a safety hammer were used to evaluate liquefaction on a site similar to Model 1, the site could be classified as “not liquefiable,” liquefiable from 10 to 15 feet below grade, or
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somewhere in between depending upon the energy efficiency ratio assumed by the geotechnical engineer. For Model 2, the discrepancy in identifying the liquefiable layer was much worse, especially for the automatic and safety hammers. The automatic hammer results range from “not liquefiable” to about 12 feet of liquefiable soil. The safety hammer ranged from 1 to 15 feet of liquefiable soil.
FIG. 2. Liquefaction FOS Calculations for Model 1 (bold vertical line represents FOS of 1.3).
FIG. 3. Liquefaction FOS Calculations for Model 2 (bold vertical line represents FOS of 1.3).
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Table 5. Summary of LiquefyPro Output for Model 1 HAMMER CALCULATED LIQUEFIABLE LAYER THICKNESS
Automatic
At the lower bound hammer energy efficiency ratio of 0.46, 2.2 feet of the soil was liquefiable (i.e. from 12.8 to 15 foot depth). At the upper bound hammer energy efficiency ratio of 0.99, none of the soil was determined to be liquefiable.
Safety
At the lower bound hammer energy efficiency ratio of 0.36, all 5 feet of the soil was liquefiable (i.e. from 10 to 15 foot depth). At the upper bound hammer energy efficiency ratio of 0.70, none of the soil was liquefiable.
Winch/Donut
At the lower bound hammer energy efficiency ratio of 0.25, all 5 feet of the soil was liquefiable (i.e. from 10 to 15 foot depth). At the upper bound hammer energy efficiency ratio of 0.60, none of the soil was liquefiable.
Table 6. Summary of LiquefyPro Output for Model 2
HAMMER CALCULATED LIQUEFIABLE LAYER THICKNESS Automatic At the lower bound hammer energy efficiency ratio of 0.46,
12.2 feet of the soil was liquefiable (i.e. from 12.8 to 25 foot depth). At the upper bound hammer energy efficiency ratio of 0.99, none of the soil was determined to be liquefiable.
Safety
At the lower bound hammer energy efficiency ratio of 0.36, all 15 feet of the soil was liquefiable (i.e. from 10 to 25 foot depth). At the upper bound hammer energy efficiency ratio of 0.70, 1 foot of the soil was liquefiable (i.e. from 24 to 25 foot depth).
Winch/Donut
At the lower bound hammer energy efficiency ratio of 0.25, all 15 feet of the soil was liquefiable (i.e. from 10 to 25 foot depth). At the upper bound hammer energy efficiency ratio of 0.60, 5.6 feet of the soil was liquefiable (i.e. from 19.4 to 25 foot depth).
CONCLUSIONS This limited study demonstrates that for the same SPT N value, each hammer type (automatic, safety, donut and winch) can potentially over or under predict the occurrence of liquefaction. Under-predicting the occurrence of liquefaction could result in severe consequences (i.e. damage to property and loss of life) should a site experience liquefaction during an earthquake. This study demonstrates that hammer calibration could help to reduce that risk.
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It is our opinion that public agencies should require calibrated SPT hammer test results when evaluating liquefaction, strength loss, dynamic settlement, and lateral spread hazards. It is our hope that this practice will be adopted by the engineering community as the Standard of Practice and compulsory by local and state building codes. It should be noted that while this study included analyses for a winch hammer, this hammer type is not recommended for SPT-based liquefaction studies due to its inability to produce repeatable test results. Additionally, ASTM does not sanction the use of the winched wireline SPT hammer. REFERENCES ASTM D 1586-99, Revised 1999. “Standard Test Method for Penetration Test and
Split-Barrel Sampling of Soils.” ASTM International, West Conshohocken, Pennsylvania, Volume 4.08.
Batchelor, C., Goble, G., Berger, J., and Miner, R. (1995). “Standard Penetration Test Energy Measurements on the Seattle ASCE Field Testing Program.” Report for the Seattle Section of the American Society of Civil Engineers.
California Division of Mines and Geology and others (1997). “Guidelines for evaluating and mitigating seismic hazards in California.” California Division of Mines and Geology Special Publication 117.
Davidson, J.L., Maultsby, J.P., and Spoor, K.B. (1999). “Standard Penetration Test Energy Calibrations. Final Report and Appendices.” Florida University, Gainesville; Florida Department of Transportation; and Federal Highway Administration.
International Code Council, Inc. (2006). “2006 International Building Code.” Kovacs, W.D., Salomone, L.A., and Yokel, F.Y. (1983). “Comparison of Energy
Measurements in the Standard Penetration Test Using the Cathead and Rope Method.” National Bureau of Standards Report to the US Nuclear Regulatory Commission, November, 1983.
NCEER (1997). “Proceedings of the NCEER Workshop on Evaluation of Liquefaction Resistance of Soils.” Edited by Youd, T.L. Idriss, I.M., Technical Report No. NCEER-97-0022, December 31, 1997.
Seed, H.B., Tokimatsu, K., Harder, L.F., and Chung, R.M. (1985). “Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations.” ASCE Journal of Geotechnical Engineering, Volume 11, No. 12, December: pp. 1425-1445.
Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M., Riemer, M.F., Sancio, R.B., Bray, J.D., Kayen, R.E., and Faris, A. (2003). “Recent advances in soil liquefaction engineering: a unified and consistent framework.” Report No. EERC 2003-06, Earthquake Engineering Research Center.
Sjoblom, D., Bischoff, J., Cox, K. (2002). “SPT Energy Measurements with the PDA,” presented at the 2nd International Conference on the Application of Geophysical and NDT Methodologies to Transportation Facilities and Infrastructure, Los Angeles, California, May 15, 19, 2002.
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