Energy Efficiency and Rod Length Effect in SPT …docs.trb.org/prp/10-0317.pdf · 1 Energy...

Energy Efficiency and Rod Length Effect in SPT Hammers 1 Submitted: August 1, 2009 2 Revised: November 15, 2009 3 Word Count: 7495 (4495 + 9 Figures + 3 Tables) 4 5 Michael Valiquette, P.E., North Carolina Department of Transportation Geotechnical 6 Engineering Unit, 3301 Jones Sausage Road, Raleigh, NC, 27529, phone: (919) 662-4710, fax: 7 (919) 662-4710. e-mail: [email protected] 8 9 Brent Robinson, P.E., Department of Civil, Construction and Environmental Engineering, North 10 Carolina State University, Campus Box 7908, Raleigh, North Carolina, 27695. phone (919) 515-11 1229. fax (919) 515-7908. e-mail: [email protected] 12 13 Roy H. Borden, Ph.D., P.E., Department of Civil, Construction and Environmental Engineering, 14 North Carolina State University, Campus Box 7908, Raleigh, North Carolina, 27695. phone 15 (919) 515-7630. fax (919) 515-7908. e-mail: [email protected] 16 17

TRB 2010 Annual Meeting CD-ROM Paper revised from original submittal.

ABSTRACT: Twenty-eight SPT hammers owned by NCDOT and private consultants were 1 used to investigate the average energy efficiency and variability of manual vs. automatic 2 hammers, as well as the effect of SPT rod length on hammer efficiency. The results agree 3 with published data in several regards. Automatic hammers in the study were found to 4 have an average transferred efficiency of 80.9%. This agrees very well with the 80% 5 efficiency assumed in geotechnical engineering practice for automatic hammers. Manual 6 hammers in the study averaged 63.9%, also close to the 60% efficiency assumed for manual 7 hammers. Manual hammers were found to be twice as variable in transferred energy from 8 blow to blow within an SPT blow count than automatic hammers. The study demonstrated 9 that the measured transferred energy appeared to be affected by rod length. Lengths 10 shorter than approximately 40 feet caused reduced energy to be transferred into the rod. 11 An empirical formula is presented for correcting short rod length energy losses. The data 12 did not demonstrate a strong dependence on SPT N-Value, although the data set lacked 13 observations where N-value was less than 6 blows per foot.14


Valiquette, Robinson, Borden 1

INTRODUCTION 1 The Standard Penetration Test (SPT) consists of counting the number of blows from a 2

standard hammer needed to advance a drive rod with a split spoon sampler the final 12 inches of 3 an 18 inch drive. The total number of blows applied over the final twelve-inches is referred to 4 as the N-value. Each standard hammer blow consists of dropping a 140 pound ram a distance of 5 30 inches, yielding a theoretical energy for each hammer blow of 350 foot-pounds. SPT tests are 6 standardized according to ASTM D1586. 7

It is well accepted that SPT hammers do not deliver the full theoretical energy with each 8 blow due to friction losses and inefficient ram tripping devices. Differing hammer designs and 9 even different hammers of the same design will deliver varying percentages of the full theoretical 10 energy. Previous researchers have also noted that manual cathead hammers are strongly 11 influenced by operator factors, and short rod lengths and rod stiffness may reduce the energy 12 transferred. This study summarizes the results of field calibrations of 28 SPT rigs used for soil 13 borings performed by NCDOT, particularly focusing on the effects of hammer type and drive rod 14 length on the measured transferred energy. Other factors affecting energy transfer, such as loose 15 threaded rod connections and automatic hammer lift mechanism speed, are likely present in the 16 hammers studied but have not been addressed in this paper due to a lack of applicable data. 17 18 PREVIOUS STUDIES 19 Efficiency 20

Schmertmann and Palacios (1) recognized that energy transferred by the SPT hammer to 21 the drill rod had a pronounced effect on the measured N-value and that the drop system used to 22 allow the hammer to freefall also had a significant effect on the transferred energy. Transferred 23 energy measurements on SPT rods are made by instrumenting the driven rod with strain 24 transducers (or load cells) and accelerometers, such that the force and velocity generated at a 25 point along the rod’s length by the impact can be measured with time. The transferred energy is 26 calculated by integrating the product of force and velocity over time. The reported transferred 27 energy is the maximum value of this integration. A typical set of force, F(t), velocity, V(t), and 28 energy, E(t) curves is shown in FIGURE 1. 29 30



1 FIGURE 1 Force (F), Velocity (V), Energy (E) and Displacement (D) curves from an SPT 2 calibration. 3 4

The EFV formula is specified in the ASTM D 4633-05 standard. This equation is 5 integrated from the beginning of hammer impact until the end of the record and the maximum 6 transferred energy at any time in the record is determined. The EFV equation was used for the 7 basis of energy measurements included in these calibrations. 8 9

dttVtFtEEFV )()(max)(max (Eq. 1, From ASTM D 4633-05) 10

11 The baseline energy transfer ratio has been set to the average transferred energy for 12

manual safety SPT hammers, or 60%, which yields the corrected blow count N60, as proposed by 13 Skempton (2). Most N-value based soil property correlations have been developed (or assumed 14 to have been developed) using manual donut and safety hammers delivering only 45 to 60% of 15 the theoretical energy, or 158 to 210 foot-pounds. Higher transferred energy leads to lower field 16 blow counts and lower transferred energy creates higher field blow counts. Equation 2 is used 17 to correct raw N values to N60 if energy measurements are not made, with correction factors for 18 the energy delivered by a hammer type (CE), the borehole diameter (CB), the rod length (CR), and 19 the sampling method (CS). Typical values for the corrections factor can be found in the FHWA’s 20 Geotechnical Engineering Circular No. 5, “Evaluation of Soil and Rock Properties”, GEC 5 (3). 21 22

SRBEraw CCCCNN 60 (Eq. 2, GEC No. 5) 23 24 25 26



Theoretical Energy Reduction with Reducing Rod Length 1 Schmertmann and Palacios (1) originally proposed rod length correction factors based on 2

theoretical wave mechanics and showed the returning tension stress wave arriving at time 2L/c, 3 where L is the length of rod and c the stress wave speed in steel, abruptly cuts off the incoming 4 compressive stress wave and separates the anvil from the hammer. Schmertmann and Palacios 5 (1) presented an equation that calculates the energy transfer efficiency factor for differing 6 hammer and rod lengths and cross-sectional areas. This equation is also the basis for the 7 recommendations by Skempton (2), for the ASTM K2 correction factor and appears to be the 8 basis of the N-value rod length correction factor, CR, presented in the GEC 5 (3). These 9 relationships are plotted, with others, in FIGURE 2. These correction factors were developed for 10 an energy calculation method using the square of the force curve, which was the standard until 11 the mid-1990s when reliable accelerometers became available to measure velocity near the top of 12 the rod. This method, now replaced almost 15 years ago, is described in depth in Schmertmann 13 and Palacios (1), Sancio and Bray (4), Daniel et al. (5) and elsewhere. 14

Morgano and Liang (6) published a review of rod length effect of energy transfer in SPT. 15 The EFV method was used to calculate the transferred energy of different rod lengths from 16 actual field data and radar measurements of ram velocity. It was found that actual transferred 17 energy was higher for short rod lengths than that predicted by the Schmertmann and Palacios 18 theoretical equation. The higher energy measurements were very likely due to the EFV method 19 including energy from subsequent hammer impacts within a single hammer blow. Rod 20 correction factors as recommended by Morgano and Liang (6) based on their field data are 21 plotted in FIGURE 2 for comparison with other rod correction factors. 22

0.80

0.85

0.90

0.95

1.00

1.05

0 20 40 60 80

Rod Length (ft)

Rod L

ength

Corr

ection F

acto

r (%

)

.

Inverse of Morgano EFV Rod Correction

Schmertmann EF2 Theoretical Energy Reduction

GEC5 Rod Length Corrections

23 FIGURE 2 Previously proposed rod length correction curves. 24



The last decade has shown significant renewed interest in the question of whether rod 1 length corrections are correct for energies derived from both force and velocity records. Youd 2 and Idriss (7) recommend the use of such factors for methods using N-value to determine 3 liquefaction resistance, as does the GEC 5 (3) and ASTM. Sancio and Bray (4) studied the effect 4 of rod length on energy transfer in SPT. Their study, consisting of energy measurements with 5 increasing rod lengths from a single manual hammer from many borings in a silty sand in 6 Turkey, indicated that the EFV method was measuring higher transferred energies than when 7 energy was measured only over 2L/c (E2E). The E2E values measured were consistent with N-8 value, but gradually increased with increasing rod length, while EFV values tended to increase 9 with both rod length and N-value. 10

Sancio and Bray (4) also investigated the effect penetration resistance, N-values, had on 11 energies measured with the EFV method and observed that EFV measured energy increased with 12 increasing N values from 2 blows per foot (bpf) up to approximately 10 bpf for rods less than 34 13 feet in length. The influence of N-value was no longer observed for rods longer than 33 feet in 14 length. They noted that their dataset did not contain significant amounts of data for N greater 15 than 10 bpf at short rod lengths. Inspection of FIGURE 3, recreated from Sancio and Bray (4) 16 and normalized for later comparison to the 65% average EFV ETR for rod lengths greater than 17 33 ft, indicate that the upper limit of N-value influence on EFV measurements may actually be 18 lower, possible around N values of 6 or 7 blows per foot. Reduced energy with shorter rod 19 length effect was not seen in their data for N-values above 10 bpf. Presented data for short rod 20 lengths tends to show EFV data points at N-values of no more than 10 to 14 bpf. 21

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

N-value (bpf)

EF

V T

ran

sfe

r R

ati

o /

Ave

rag

e E

FV

Tra

nsf

er

Rat

io f

or

L >

33

ft (

%)

Rod Length = 14 ft; Manual19 ft> 33 ft

22 FIGURE 3 Sancio and Bray (2004)'s variation of average EFV energy transfer with rod 23 length at each measured N-value (combined and replotted). 24

25



Daniel et al. (5) performed laboratory and numerical studies to investigate the validity of 1 short rod correction, considering multiple impacts of the hammer on the rod during each blow. 2 They took issue with Morgano and Liang (6)’s EFV based data, and with the short rod 3 corrections currently in place from the F2 methods. The results of their laboratory and 4 computational modeling based investigation indicated that EFV measured energies were 5 independent of rod length for the 11.5 and 21.3 ft lengths used. The study was conducted with a 6 safety hammer and string-cut drop mechanism. Daniel et al. (5) concluded with the supposition 7 “…that the ’correct’ short rod correction will be closer to ’no correction’ than to the short rod 8 correction factors that have been published to date.” 9

Howie et al. (8) included the laboratory data above as well as data from two field test 10 sites. The EFV energy results from the two tests indicated mixed results—one showed no 11 reduction in energy as rod lengths decreased, while the other showed an approximately 10% 12 reduction as rod length decreased from 40 to 15 ft. They reported Davidson (9) observed a 13 similar drop, and posited that the reduction could be a result of data acquisition rates. 14 15 CURRENT STUDY 16

The NCDOT Geotechnical Engineering Unit conducted a survey of Standard Penetration 17 Test (SPT) energy measurements on twenty eight subsurface investigation drill rigs commonly 18 used on NCDOT projects. Twenty drill rigs were equipped with automatic SPT hammers, and 19 the remaining eight drill rigs were equipped with rope and cathead operated SPT safety 20 hammers. Both private and public sector drill rigs were included in the survey. Each drill rig 21 was equipped with only one hammer. The objective of the NCDOT survey was to produce an 22 energy efficiency rating specific to each hammer for use when calculating equivalent 60% 23 energy efficient blow counts (N60). The resulting SPT energy measurement database was used as 24 the basis of this paper. 25

Field data was collected by GRL Engineers, Inc. between November 28, 2005 and 26 January 9, 2006. Standard SPT blow counts were measured on 5 foot increments at rod lengths 27 starting between 14 to 24 feet and terminating between 54 and 74 feet, depending on the test 28 location. Actual SPT sample depths are generally 3 to 5 feet less than the rod length used due to 29 the rod extending above the ground surface. All SPT drives were driven for the standard three 30 six-inch increments. SPT blow counts shown in this report are the sum of the number of blows 31 applied over last two six-inch increments. Total number of blows measured per hammer ranged 32 between 71 and 498 with an average of 271 total blows. Only one boring was performed with 33 each hammer through the course of the survey. Transferred energies were recorded for every 34 hammer blow for each SPT drive. 35 36 SPT Energy Measurements 37

Energy measurements were taken with a PAK model Pile Driving Analyzer (PDA). The 38 PDA records strain and acceleration readings from strain gages and accelerometers mounted to a 39 short SPT drill rod inserted into the drill steel string directly below the hammer anvil. As 40 discussed previously, the strain and acceleration readings were integrated to produce force and 41 velocity traces with respect to time. 42

Considering all energy measurements (FIGURE 4), regardless of rod length or N-value, 43 the measurements made on the rigs used on NCDOT projects showed an overall average energy 44 transfer ratio of 78.6% and 62.3% for automatic and manual drop rigs, respectively. Neglecting 45 measurements with rod lengths of less than 38 feet, the average energy transfer ratio rises 46



slightly to 80.8% and 64.4%. These overall averages are very similar to the observations 1 reported by Skempton (2), and in GEC 5. The importance of calibrating individual rigs is shown 2 as some hammers are consistently below or consistently above the values recommended for each 3 hammer type in the aggregate. With the exception of hammer A-5, the automatic hammers have 4 less variability over the length of the boring than the manual hammers. In general, the lower 5 bound measurements for each drill rig occurred when the SPT sampler was at a penetration of 6 less than 38 ft. 7

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 91011121314151617181920212223242526272829Drill Rig

Ave

rage

EFV (%

)

All Rod Lengths

Rod Lengths > 38 ft

GEC 5

Manual HammersAutomatic Hammers

A-1 A-2 A-3 A-4 A-5 A-11

A-10

A-9A-8A-7A-6 A-12

A-14

A-13

A-16

A-15

A-17

A-20

A-19

A-18

M-1 M-8M-7M-6M-5M-4M-3M-2

8 FIGURE 4 Average energy transfer ratios over all rod lengths and rod lengths over 38 ft. 9 Range bars show range of maximum and minimum average EFV for an N-value. Drill rigs 10 ordered from lowest to highest transferred energy. 11 12

FIGURE 5 and FIGURE 6 show EFV measured energy from every recorded hammer 13 blow for Manual Hammer 4 and Automatic Hammer 6. Corresponding field N-values for each 14 rod length are shown. As can be anticipated, the manual hammers generally exhibited 15 significantly more scatter in hammer energy from blow to blow within each rod length drive. 16 With some exceptions, the standard deviation of transferred energy within an N-value for 17 automatic hammers averaged 1 to 2%, while for manual hammers the same standard deviation 18 ranged from 3.5 to 6%. Similarly, shorter rod lengths tended to yield greater standard deviations 19 than longer rod lengths. While this lends some credence to Daniel et al. (5)’s observation that 20 the rod correction may lie within the range of the accuracy of the energy measurement, 21 differences from short to longer rod lengths often approach approximately 10%. 22

Automatic Hammer 6 exhibited a typical transferred energy reduction with shorter rods, 23 as seen in other automatic hammers within the study. Manual Hammer 4 did not exhibit the 24 same short rod behavior. It is felt that this lack of short rod behavior in Manual Hammer 4 is due 25 to inconsistent safety hammer cathead operation at differing rod lengths. Both hammers were 26 selected for presentation as N-values are relatively consistent along the boring depths. 27



20

30

40

50

60

70

80

90

100

0 20 40 60 80

Rod Length (ft)

Energy Transfer Ratio (%)

N=13

N=10

N=13

N=10

N=12N=10

N=13N=15

N=10

N=9 N=7

1 FIGURE 5 Energy Transfer Ratio for Manual Hammer No. 4. 2

20

30

40

50

60

70

80

90

100

0 20 40 60 80

Rod Length (ft)

Energy Transfer Ratio (%)

N=11

N=11N=9

N=8N=10

N=9N=12

N=11

N=9

3 FIGURE 6 Energy Transfer Ratio for Automatic Hammer No. 6. 4 5



Observed Effect of Rod Length on Measured Energy Transfer 1 Field data from the NCDOT SPT energy survey were used to investigate the rod length 2

effect. First, an average energy was calculated over an N-value at each rod length for every 3 hammer. Secondly, a baseline energy was developed for each individual hammer by averaging 4 the energy measurements of rod lengths longer than 38 feet. Rods longer than 38 feet were 5 selected for calculating a hammer’s baseline energy as the measured energy reached a relative 6 consistent maximum, as can be seen in Figure 7. Then, also on a per hammer basis, the average 7 energy at a given rod length was divided by the hammer’s baseline energy, thus normalizing the 8 shorter rod energies to the hammer’s baseline efficiency. This allows the energy reduction vs. 9 rod length to be compared among different hammers even though individual hammers may have 10 differing baseline efficiencies. The normalized efficiency vs. rod length effect was then 11 averaged within the automatic and manual hammer groups for each rod length. The results are 12 plotted in Figure 7. A best fit curve equation was plotted for the automatic hammer group as 13 these were more consistent from blow to blow than the manual hammer group. Equation 3 is the 14 empirical best fit curve corresponding to the 20 automatic hammers included in the NCDOT 15 survey. Results of this equation for various rod lengths are presented in TABLE 1 and plotted in 16 FIGURE 7. Equation 3 is applicable only for rod length, L, shorter than 45 feet. 17 18

7922.00083.000008.0 2 LLCr (L ≤ 45 ft) (Eq. 3) 19 20 TABLE 1 Energy Reduction with Reducing Rod Length 21

Rod

Length (ft)

Field Auto. Average Reduced

EFV Energy (%)

Cr, Field Auto

Best Fit Reduced

EFV Energy (%)

Schmertmann and Palacios Theoretical EF2 Energy

Reduction (%)

GEC 5 N-Value

Rod \Length

Correction

Inverse of ASTM

D4633-86 K2 Factor

Inverse of Morgano and Liang

EFV Energy

Correction 14 0.90 0.89 0.83 0.85 0.82 0.92 19 0.91 0.92 0.91 0.85 0.90 0.95 24 0.95 0.94 0.95 0.95 0.94 0.97 29 0.96 0.96 0.97 0.95 0.97 0.98 34 0.98 0.98 0.99 1.00 0.98 0.99 39 0.99 0.99 0.99 1.00 0.99 1.00 44 1.00 1.00 1.00 1.00 1.00 1.00 49 1.00 1.00 1.00 1.00 1.00 1.00 54 1.00 1.00 1.00 1.00 1.00 1.00

22 As can be seen in FIGURE 7, the EFV calculated transferred energy clearly drops off 23

with shorter rod lengths. The Schmertmann and Palacios (1) theoretical energy reduction and 24 subsequent ASTM D4633-86 K2 factor over predict the energy loss at shorter rod lengths that 25 was measured in this study. The Morgano and Liang (6) EFV correction factors offer better 26 agreement with the NCDOT field data for the shortest rod length where the tension cutoff 27 becomes more significant. This result is logical since the NCDOT and Morgano and Liang (6) 28 energy measurements are based on the EFV method. These results also indicate that the 29 published GEC 5 rod length correction factor for N-values underestimates the transferred energy 30 at short rod lengths, 19 ft and less, and may over correct N-values if used. 31



Automatic EFV Field Data Best Fit Curve

y = -8E-05x2 + 0.0083x + 0.7922

0.80

0.85

0.90

0.95

1.00

1.05

0 20 40 60 80

Rod Length (ft)

Tra

nsfe

red E

nerg

y a

t Length

, L /

Avg. E

nerg

y >

38 ft (%

)

Avg. Automatic Hammer EFV Field Data

Inverse of Morgano EFV Rod Correction

Schmertmann EF2 Theoretical Energy Reduction

GEC5 Rod Length Corrections

Automatic Hammer EFV Field Data Best Fit Field Curve

1 FIGURE 7 Energy transfer normalized to average energy (Plus and minus one standard 2 deviation bars are shown for Automatic Hammer EFV Field Data). 3 4 TABLE 2 Manual Hammers – Average Efficiency and Variability 5

Average Uncorrected

Efficiency Shorter Than

38'

Average Uncorrected

Efficiency Longer Than

38'

Average Uncorrected

Efficiency, All Lengths

Average Rod

Corrected Efficiency, All Lengths

Average Standard Deviation from Blow

to Blow M-1 64.8 72.6 69.0 70.9 3.92 M-2 65.2 66.3 65.8 67.7 3.75 M-3 52.7 60.5 57.0 58.6 3.16 M-4 52.8 54.4 53.6 55.3 5.13 M-5 73.8 83.6 79.1 81.4 3.67 M-6 65.8 70.3 68.2 70.2 5.61 M-7 54.4 52.3 52.6 53.2 4.26 M-8 49.5 55.2 52.6 54.1 4.91 Average 59.9 64.4 62.3 63.9 4.3 St. Deviation 8.6 10.8 9.8 10.2 0.8 Lower Limit 42.6 42.8 42.7 43.6 2.6 Upper Limit 77.1 86.0 81.8 84.2 6.0 Avg w/in 2 St. Deviation 59.9 64.4 62.3 63.9 4.3



TABLE 3 Automatic Hammers—Average Efficiency and Variability 1

Average Uncorrected

Efficiency Shorter Than

38'

Average Uncorrected

Efficiency Longer Than

38'

Average Uncorrected Efficiency, All Lengths

Average Rod

Corrected Efficiency, All Lengths

Average Standard Deviation

from Blow to

Blow A-1 78.8 83.8 81.5 83.8 2.12 A-2 77.8 83.0 80.6 82.9 1.71 A-3 72.2 76.9 74.7 76.9 2.29 A-4 73.4 75.6 74.3 77.0 1.83 A-5 66.3 78.3 71.6 73.8 3.41 A-6 72.9 78.9 75.6 78.2 2.04 A-7 73.3 78.0 75.4 78.1 1.68 A-8 61.9 64.7 63.1 65.4 1.75 A-9 77.6 79.8 78.6 81.4 2.01 A-10 71.7 74.8 73.5 75.6 1.42 A-11 81.0 81.1 81.1 82.0 1.78 A-12 78.6 84.0 82.7 83.5 1.53 A-13 81.4 82.5 82.3 83.1 1.77 A-14 77.7 82.8 80.0 82.8 3.00 A-15 79.7 83.6 81.5 84.4 1.53 A-16 79.2 84.7 81.7 84.5 1.15 A-17 78.4 82.8 80.8 83.1 1.06 A-18 76.0 81.5 78.4 81.2 1.56 A-19 83.2 87.1 85.2 87.9 5.54 A-20 85.8 92.4 89.4 91.9 1.50 Average 76.3 80.8 78.6 80.9 2.0 St. Deviation 5.5 5.4 5.5 5.5 1.0 Lower Limit 65.4 69.9 67.7 70.0 0.1 Upper Limit 87.3 91.7 89.5 91.8 4.0 Avg Within 2 St. Deviation 77.1 81.7 79.4 81.7 1.85

2 Based on the data above, Equation 2 can be revisited. If the energy is measured on the 3 drill rig for each recorded N-value, the raw N-values can be adjusted linearly to the assumed 4 60% efficient hammer blow count (N60) by Equation 2, with ETR the energy transferred to the 5 rod divided by rated hammer energy (%). 6 7

%60

(%)ETRCC BE (Eq. 4) 8

Finally, a common method for applying energy corrections to a drill rig after it has been 9 ‘calibrated’ is to calculate an average transferred energy over an entire boring or series of 10 boreholes. In this case, ETRavg/60% is simply CE, and no rod length correction factor is used for 11 any of the data. 12



The possible effects of determine a hammer’s efficiency using only short or long rods can 1 be seen in TABLE 2 and TABLE 3. Basing a hammer efficiency on short rod EFV readings 2 only can underestimate the hammer’s performance. Such a scenario is possible if a hammer is 3 calibrated at a shallow bedrock site. Average automatic hammer efficiency for rods shorter than 4 38 feet is 76.8% vs. 80.8% for rods longer than 38 feet. The average automatic hammer 5 efficiency over all rods lengths is 78.6%. 6

An extreme example of using only short rods in a hammer calibration can be seen by 7 using the average efficiency from only 14 ft rods. The average automatic hammer energy 8 transfer with 14 ft rods was 10% less than the average for lengths longer than 38 ft. This would 9 reduce the hammer calibrated efficiency by 10%. For a typical 80% efficient automatic hammer, 10 the hammer’s energy efficacy would be incorrectly calibrated at 72%. The real world impact of 11 incorrect hammer calibration depends on the sensitivity individual engineering analysis 12 techniques to N60. For example, an incorrect low N60 value would affect a liquefaction analysis 13 more than axial pile bearing capacity analysis used to estimate pile order lengths. 14

As an illustration, consider a hypothetical deposit of sand, with a unit weight of 120 15 lb/ft3, the groundwater table at the surface and a constant N1 of 8 bpf obtained with the 80% 16 efficient automatic hammer described above. Were the hammer calibrated with short rod 17 lengths, the 72% efficiency would yield an (N1)60 value of 10 bpf, while the calibration using 18 longer rod lengths with 80% efficiency would yield an (N1)60 of 11 bpf. Using the Driven 19 program (Blue-Six Software, 10), the required length of an HP12x53 section with an ultimate 20 load of 200 kips would be 73.6 and 72.2 ft for the 72 and 80% correction factors, respectively. 21 Thus, the improper energy would lead to an increase in estimated pile length of 1.9%, or well 22 within the variation of static pile capacity estimation methods. 23

Now consider the same profile for a liquefaction study over a depth of 75 feet. Using the 24 SPT correlations described by Youd and Idriss (7), under a magnitude 7.5 earthquake with a 25 peak ground acceleration of 0.1 g, a factor of safety against liquefaction can be estimated. For 26 the given profile, a factor of safety against liquefaction of less than 1.0 is predicted from the 27 ground surface to 56 feet for the 72% hammer efficiency, while the 80% efficiency predicts 28 liquefaction to depths of 36 feet. Using the shorter rod for calibration of the SPT hammer would 29 require design accounting for liquefaction effects over an additional 20 feet of the profile. 30

31 Normalized Efficiency vs. N-Values 32

Sancio and Bray (4) have previously shown data indicating that EFV based energy 33 measurements are affected by soil resistance when N is below 10 bpf. Inspection of their data 34 (FIGURE 3) indicates that this upper limit of 10bpf may be actually lower, possibly around N-35 values of 6 or 7 bpf. The NCDOT SPT dataset has the opposite problem as the Sancio and Bray 36 dataset. The Sancio and Bray dataset did not contain a significant amount of energy data for N-37 values above 10 bpf at shorter rod lengths. Conversely, the NCDOT energy data does not 38 contain significant energy data with N-values below 8 bpf at shorter rods; only one or two tests 39 for each rod length had N-values of 5, 6, 7 or 8 bpf. Additionally, the Sancio and Bray dataset 40 consisted of only one hammer tested over many borings. The NCDOT automatic hammer 41 dataset consists of 20 automatic hammers, one boring each, each with a different baseline energy 42 efficiency. Creating graphs similar to FIGURE 3 from the NCDOT database requires that the 43 individual hammers be normalized to their baseline energy to allow comparisons between 44 individual hammers. FIGURE 8 and FIGURE 9 are the normalized energy plots for manual and 45 automatic hammers, respectively. Data from 34 ft long rods was included in the baseline energy 46



used to produce FIGURE 8 and FIGURE 9 to allow a more direct comparison with the Sancio 1 and Bray (4) data in FIGURE 3. 2

Inspecting the NCDOT dataset, N-value vs. EFV efficiency graphs for rod lengths of 14 3 ft and 19 ft, it appears likely that empirical rod length correction, Eq 3, was not heavily 4 influenced by EFV measurements at N-values above 8 blows per foot. Sancio and Bray’s (4) 5 data set showed some reduction in transferred energy as N-values dropped below approximately 6 7 bpf, and their data set included more measurements at N-values of less than 5 bpf than were 7 included in the NCDOT data set. Nevertheless, FIGURE 8 and FIGURE 9would not appear to 8 indicate a strong effect of N-value on transferred energy. 9 10

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

N-value (bpf)

EF

V T

ran

sfer

Rat

io / A

vera

ge

EF

V T

ran

sfer

Rat

io f

or

L >

33

ft (

%) NCDOT Manual, L=14 ft

L=19 ft

L > 33 ft

11 FIGURE 8 Soil effects on energy transfer ratio for manual hammers in the study, 12 normalized to the average ETR for rod lengths greater than 33 ft. 13



0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80 90 100

N-value (bpf)

EFV

Tra

nsf

er R

atio / A

vera

ge E

FV

Tra

nsf

er R

atio f

or

L >

33

ft (%

)

NCDOT Auto, L=14 ft

L=19 ft

L > 33 ft

1 FIGURE 9 Soil effects on energy transfer ratio for automatic hammers in the study, 2 normalized to the average ETR for rod lengths greater than 33 ft. 3 4 CONCLUSIONS 5 The following conclusions can be drawn from this study: 6 7

1. Commonly assumed manual and automatic hammer efficiencies of 60% and 80%, 8 respectively, are reasonable as the NCDOT fleet averaged 63.9% and 80.9% for the two 9 types of hammers. However, the importance of calibrating individual rigs is shown as 10 some hammers are consistently below or consistently above the values recommended for 11 each hammer type in the aggregate. 12

13 2. The NCDOT hammer database demonstrated that manual hammers are twice as variable 14

as automatic hammers from blow to blow for a given hammer. This is probably not as 15 significant as it is self canceling provided that baseline hammer efficiency is known. It is 16 an indication that manual hammers are inherently more variable in day to day operation. 17 Unfortunately, this study did not include data from different operators on the same 18 manual hammer. Additionally, it is likely that manual hammer’s inherent variability 19 masks the rod length energy reduction effect. 20

21 3. The NCDOT and Morgano and Liang (6) studies both demonstrate Schmertmann and 22

Palacios’s (1) theoretical wave mechanics based rod correction factors and the ASTM 86 23 K2 factors over correct EFV energy measurements for short rod lengths. This conclusion 24 is in agreement with the ASTM D-4633-05 critique. 25

26



4. Hammer efficiency calibrations should be based on rod lengths 35 feet and longer if at all 1 possible. If not feasible due to shallow bedrock, measured transferred energies could be 2 corrected for the rod length effect using Equation 3 or Morgano and Liang’s (6) rod 3 length correction factors. The ASTM D-4633-05 comment that measurements from 4 lengths longer than 30 to 40 feet are more accurate is not sufficiently strong enough to 5 prevent a hammer calibration from being based entirely on shorter rod lengths. The 6 effect of this may be a 5% lower rated hammer efficiency as shown in Tables 2 and 3. 7 The magnitude of this effect will increase for average efficiencies based only on very 8 short rods, 19 ft or less. 9

10 5. The GEC 5 rod length N-value adjustment factors, CR in Eq. 2, would have overcorrected 11

the field N-values measured in this study. It appears that the GEC 5 rod length correction 12 factors are based on the Schmertmann and Palacios theoretical correction factors. Further 13 studies would need to be performed to determine if field N-values need to be corrected 14 for rod length effects. 15

16 6. The NCDOT data currently do not, unfortunately, allow for investigation of Sancio and 17

Bray’s (4) observations using the E2E quantity as an energy measurement that does not 18 vary with penetration resistance. However, the NCDOT data also do not show the 19 dramatically lower energies at low N-values shown in Sancio and Bray’s data set. 20 Similarly, while Howie (8) suggests the lower energy measured may be a function of 21 digitizing frequency, the data presented here was collected at a uniform sampling rate. 22 Daniel et al. (5) proposes doing away with rod length corrections all together, but the data 23 obtained in this study does not yet seem to fully support that conclusion, only that the 24 current corrections are perhaps adjust too much. As has been concluded before, a more 25 extensive field and laboratory study with automatic or manual hammers might help 26 answer these nagging questions. 27

28 29

DISCLAIMER 30 The contents of this paper reflect the views of the authors, who are responsible for the fact and 31 accuracy of the data presented herein. The contents do not necessarily reflect the official views 32 or policies of the North Carolina Department of Transportation. This report does not constitute a 33 standard, specification or regulation. 34 35 ACKNOWLEDGEMENTS 36 This data was collected under a contract with the North Carolina Department of Transportation 37 by GRL Engineers, Inc. The USDOT also provided funding via an Eisenhower Transportation 38 Fellowship to the second author. 39 40 REFERENCES 41 42

1. Schmertmann, J. and Palacios (1979). “Energy Dynamics of the SPT.” Journal of the 43 Geotechnical Engineering Division, vol. 105, no. 8, p. 909-928. 44



2. Skempton, A. (1986). “Standard Penetration Test Procedures and the Effects in Sands of 1 OVereburden Pressure, Relative Density, Particle Size, Ageing and Over Consolidation.” 2 Geotechnique, 36:3 September. 3

3. Geotechnical Engineering Circular No. 5 (2002). Sabatini, P., Bachus, R., Mayne, P. 4 Schneider, J., and Zettler, T. “Evaluation of Soil and Rock Properties. 5 http://isddc.dot.gov/OLPFiles/FHWA/010549.pdf (Accessed November, 2009) 6

4. Sancio, R. and Bray, J. (2005). “An assessment of the effect of rod length on SPT energy 7 calculations based on measured field data.” Geotechnical Testing Journal, vol. 28, no. 1, 8 p 22-30. 9

5. Daniel, C., Howie, J., Jackson, R. and Walker, B. (2005). “Review of standard 10 penetration test short rod corrections.” Journal of Geotechnical and Geoenvironmental 11 Engineering, v 131, n 4, p 489-497. 12

6. Morgano, C.M., and Liang, R., “Energy transfer in SPT – Rod length effect” Proceeds of 13 the 4th International Conference of the Application of Stress-Wave Theory to Piles, pp. 14 121-127 15

7. Youd and Idriss (2001). “Liquefaction resistance of soils: Summary report from the 16 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance 17 of soils.” Journal of Geotechnical and Geoenvironmental Engineering, v 127, n 4, p 297-18 313. 19

8. Howie, J., Daniel, C., Jackson, R. and Walker, R. (2003). “Comparison of Energy 20 Measurement Methods in the Standard Penetration Test” Geotechnical Research Group, 21 Department of Civil Engineering, The University of British Columbia. 419 pages. 22 http://www.civil.ubc.ca/people/faculty/JHowie/SPT_Report/SPT_Report.pdf (Accessed 23 November 2009) 24

9. Davidson, J., Maultsby, J., and Spoor, K. (1999). “Standard Penetration Test Energy 25 Calibrations.” Florida Department of Transportation, Contract No. BB261. 26

10. Blue-Six Software, Inc. (1998). “Driven 1.2” User’s Manual. FHWA Report No. 27 FHWA-SA-98-074. http://www.fhwa.dot.gov/engineering/geotech/software/sa98074.pdf 28 (Accessed November 2009). 29


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