Energy Measurement in the Brazilian SPT System · zilian standard NBR-6484/2001. In the first...

Energy Measurement in the Brazilian SPT System

C.M. Santana, F.A.B. Danziger, B.R. Danziger

Abstract. This paper presents results of the instrumentation of 373 blows from two SPT deployments performed in theSarapuí II Test Site, located in Duque de Caxias, Rio de Janeiro. In these blows the hammer drop height, its velocity atimpact, the rate of blows and also the energy transferred to the rod stem have been measured. It is therefore possible toknow the loss of energy for the SPT process (and the corresponding efficiency factors), since the hammer is delivered atzero velocity up to the time the transmitted energy reaches the rod stem.Keywords: SPT, energy, efficiency.

1. IntroductionDespite the existing problems associated with the re-

liability and repeatability of the Standard Penetration Test,Campanella & Sy (1994) emphasize that the SPT continuesto be the most used in situ test for foundation design, evalu-ation of liquefaction potential and compaction control ofsands and sandy silts. Many authors associate the wide-spread use of the test to the simplicity of the test procedure,robustness of the equipment and low operational cost (e.g.,Broms & Flodin, 1988; Décourt, 1989).

Some factors influencing the N value obtained fromSPT have been discussed in several papers (e.g., Fletcher,1965; Ireland et al., 1970; De Mello, 1971; Serota & Low-ther, 1973; Kovacs et al., 1977, 1978; Palacios, 1977;Schmertmann & Palacios, 1979; Kovacs, 1979, 1980,1994; Kovacs & Salomone, 1982; Riggs et al., 1983; Belin-canta, 1985, 1998; Skempton, 1986; Belincanta & Cintra,1998; Décourt et al., 1988; Tokimatsu, 1988; Décourt,1989; Clayton, 1990; Matsumoto et al., 1992; Morgano &Liang, 1992; Teixeira, 1993; Abou-matar & Goble, 1997;Aoki & Cintra, 2000; Fujita & Ohno, 2000; Cavalcante,2002; Odebrecht, 2003; Daniel et al., 2005; Youd et al.,2008; Santana et al., 2012).

One of these papers, by Schmertmann & Palacios(1979), has shown that the number of blows N varies in-versely with the energy delivered to the rod stem, to N equalat least 50. After some discussions concerning the need tostandardize and the choice of the proper energy to be usedas a reference for the N value (e.g., Kovacs & Salomone,1982; Robertson et al., 1983; Seed et al., 1985; Skempton,1986), ISSMFE (1989) has established 60% of the theoreti-cal free fall energy (or nominal potential energy) as the in-ternational reference. Therefore the corresponding N60 isobtained as

N NE

E60

60

� (1)

where N = measured number of blows, E = energy corre-sponding to N and E60 = 60% of the international referenceenergy E*, E* = 474 J.

Décourt (1989) and Kulhawy & Mayne (1990) havesummarized the factors affecting the energy transmissionfrom the hammer to the rods. According to Décourt (1989),the energy entering the rod stem (or enthru energy, Ei) canbe obtained as

E e e e Ei � 1 2 3 * (2)

where e1, e2 and e3 are efficiency (or correction) factors. Theefficiency factor e1 relates the kinetic energy just before theimpact to the free fall energy and is mainly dependent onthe way the hammer is lifted and released. A number ofstudies have been carried out on this subject (e.g., Kovacs etal., 1977, 1978; Kovacs, 1979, 1980; Kovacs & Salomone,1982; Skempton, 1986; Tokimatsu, 1988; Décourt, 1989).The factor e2 is associated to the loss of energy due to thepresence of the anvil (e.g., Skempton, 1986; Décourt,1989). The efficiency factor e3 is related to the rod lengthand e3 values smaller than 1 have been proposed (e.g.,Schmertmann & Palacios, 1979; Skempton, 1986) to takeinto account the separation between hammer and anvil forrod lengths smaller than 10 m, due to the upcoming stresswave. However, recent research (Cavalcante, 2002; Ode-brecht, 2003; Daniel et al., 2005; Odebrecht et al., 2005;Danziger et al., 2006) has shown that a number of impactsmay occur in a single blow, each impact being responsiblefor part of the energy delivered to the rod stem. Thus, e3

should be taken as 1. The e1, e2 and e3 values are discussedbelow together with the corresponding values obtainedherein.

The efficiency factors are related to the theoretical (ornominal) free fall energy, thus they are not the real ones. In-stead, the efficiency factors are influenced by the errors as-sociated with the non-use of the real free fall energy during

Soils and Rocks, São Paulo, 37(3): 243-255, September-December, 2014. 243

Christian Matos de Santana, D.Sc., Civil Engineer, Departamento Nacional de Infraestrutura de Transportes, Aracaju, SE, Brazil. e-mail: [email protected] Artur Brasil Danziger, D.Sc., Full Professor, COPPE e Escola Politécnica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil. e-mail:[email protected] Ragoni Danziger, D.Sc., Associate Professor, Escola de Engenharia, Universidade Estadual do Rio de Janeiro, Rio de Janeiro, RJ, Brazil. e-mail:[email protected] on June 27, 2014; Final Acceptance on December 15, 2014; Discussion open until April 30, 2015.

the test. To the authors’ knowledge, very few studies havebeen conducted regarding the potential energy actuallyused in the test (e.g., Riggs et al., 1983; Cavalcante et al.,2011), the latter only relating to the hand lifted pinweighthammer system regularly used in Brazil. However, a veryexperienced crew performed the SPTs in the study carriedout by Cavalcante et al. (2011), and the obtained resultscannot be considered typical but rather a benchmark for thebest results possible to be obtained with this system.

This paper presents research to measure the potentialenergy of the regular Brazilian system in regular opera-tional conditions, i.e. with a crew with regular experience.Also, the impact velocity of the hammer has been evalu-ated. The blow count rate was also measured, provided thatthere are recommendations for the rate to be used in lique-faction analysis (Seed et al., 1985). The energy reaching therod stem has been measured and used to evaluate the effi-ciency factors, which have been therefore evaluated basedboth on the nominal free fall energy and on the measuredenergy.

2. Equipment

2.1. SPT analyzer

The SPT Analyzer measures the energy transmitted tothe rod stem, besides other quantities. It is composed of adata acquisition unit, instrumented rods and connection ca-bles, as can be seen in Fig. 1.

The acquisition data unit has two channels for theforce signal and other two for the acceleration signal. Itsmaximum sample frequency is 20 kHz. The maximumreading interval is 102.4 ms.

Rods 1 m in length have been instrumented, each witha pair of force measuring devices and a pair of accelerome-ters. Electric strain-gauges have been used for monitoringthe force in the instrumented rods, forming a Wheatstonebridge directly fixed to the rods. Piezoelectric accelerome-ters, with 0.02 g resolution and capacity of 5000 g, have

been used to record the rod acceleration. The accelerome-ters can be fixed to the rods in diametrically opposite posi-tions and between the force sensors. The data acquisitionsystem transforms the acceleration records into velocityupon integration with time.

2.2. High speed camera

A Casio EX-FH20 high speed camera, capable of re-cording up to 1000 pictures per second, was used to recordthe hammer drop height and impact velocity.

The images recorded by the camera were digitizedand analyzed picture by picture in order to enable identifi-cation of the maximum height drop during hammer raiseand the moment the hammer hit the anvil. To help deter-mine the hammer position, an Invar ruler is positioned be-side the SPT set. Figure 2 shows the system employed inthe instrumentation of the SPT.

3. Tests Performed

3.1. Test characteristics

Two SPT deployments have been monitored in Sara-puí II Test Site, situated at the margin of the WashingtonLuiz Highway, in the area of the Navy Radio Station in themunicipality of Duque de Caxias/RJ. Geotechnical charac-teristics of the test site have been provided by Jannuzzi(2009, 2013).

According to Jannuzzi (2009) the soil profile in theregion is formed by a very soft clay layer with a typicalthickness of 7.5 m to 8.0 m, followed by minor layers ofclay, sands and silts and clays once more. The water table isat ground level.

The same crew including a chief-operator and threeauxiliary-operators were in charge of the two SPT borings.

An anvil with a mass of 977 g was used (see Fig. 3). Itshould be pointed out that although the Brazilian standardNBR-6484/2001 states that the anvil should have a mass

244 Soils and Rocks, São Paulo, 37(3): 243-255, September-December, 2014.

Santana et al.

Figure 1 - SPT analyzer. Figure 2 - System employed in SPT energy monitoring.

ranging from 3.5 kg to 4.5 kg, anvils with a mass of around1 kg are very often used all over Brazil. Reference must bemade to, for example, Skempton 1986, Décourt 1989,Belincanta 1998 and Belincanta & Cintra 1998 for the in-fluence of the anvil mass on the energy transmitted to therod stem.

A sisal rope was used for lifting and releasing thehammer. The pinweight hammer with a wood cushion isshown in Fig. 4. No measurement was made of the hammermass in the present study. However the SPT company incharge of the tests has informed that the hammer mass isverified periodically and is equal to 65 kg. Measurementsmade in previous research (Cavalcante, 2002) indicate thatthis information may be considered reliable, and errors inthe hammer mass may be generally considered negligible.The hammer drop height has been visually controlled, as in

the usual procedure, with the aid of a mark at the pinweighthammer.

The rods employed in the tests had an external diame-ter of 33 mm, 3.2 kg per meter, as recommended in the Bra-zilian standard NBR-6484/2001.

In the first boring, named Boring 1, 141 blows havebeen monitored. The rod stem length (including the sam-pler) varied from 10.80 to 22.80 m (nominal test depthsvarying from 9 to 21 m).

In the second boring, named Boring 2, 232 blowshave been monitored. The rod stem length (including thesampler) varied from 11.70 to 25.70 m (nominal test depthsvarying from 10 to 23 m).

3.2. Instrumentation results

In order to avoid significant loss of image quality, arate of 210 pictures per second was used, corresponding to amaximum resolution of 480 x 360 pixels. The records ob-tained by the high speed camera have been transferred to acomputer, separated picture by picture, and analyzed byAutoCAD software in a way that it would be possible to de-fine the hammer height during drop by the action of eachblow. An Invar ruler acted as a reference.

The height measured in the picture just before ham-mer release is defined as the hammer drop height. The ham-mer impact velocity has been obtained by the analysis ofthe hammer height picture by picture, since the instant of itsrelease up to the imminence of impact (last picture beforehammer contact with the anvil). Thus it has been possible toadjust a function that describes the relation between theheight drop of the hammer and the time, according to Fig. 5.

The derivation of the hammer drop height in relationto time, when the height drop tends to zero is the impact ve-locity.

Different polynomial functions of two and three de-grees have been tested in various blow counts that producedgood agreement. The difference observed in the velocityduring impact selecting one or other polynomial functionhas been of very low significance. The option has been thento try to adjust a second-degree polynomial function in or-der to simplify the numerical estimation.



Figure 3 - Anvil employed in Sarapuí II SPTs.

Figure 4 - Equipment employed in the tests in Sarapuí II Experi-mental Test Site.

Figure 5 - Hammer drop height vs. time (obtained during film-ing).

Cavalcante et al. (2011) used polynomial functions offourth degree to describe hammer drop height function withtime.

The hammer acceleration in time is the second deriva-tive of the hammer drop height in time. In this way, the useof a second-degree polynomial function implies the consid-eration of constant hammer acceleration during the hammerrelease.

The hammer acceleration during its release is influ-enced by the gravitational force (which is approximatelyconstant) and by friction forces. In this way, the resultantfrom the friction forces is considered constant during thehammer release.

Considering that the second or third degree polyno-mial functions did not produce significant change in the ad-justments, it is reasonable to consider that the friction vs.time function, in the analyzed cases, is approximately con-stant.

The average values of hammer drop height (hd), ham-mer velocity at impact (vi), potential energy of hammer atrelease (Ep) and kinetic energy at impact (Ek) in each blowsequence of borings 1 and 2 are presented in Tables 1 and 2,respectively. The potential energy and kinetic energy at im-pact have been calculated as:

Ep = m.g.hd (3)

Ek = 0.5.m.vi

2 (4)

where m = hammer mass, considered as 65 kg and g = grav-ity acceleration, considered as 9.81 m/s2.

In the three first blow sequences of Boring 2 no film-ing has been carried out. Furthermore, in a significant num-ber of blows from deployments 1 and 2 (152), it has notbeen possible to determine the impact velocity due to prob-lems with the video. In a smaller number of blows (102)video problems prevented the determination of hammerdrop height in both deployments.

Due to errors in the SPT Analyzer operation, the rodenergy in six blows from sequences 6 and 9 from Boring 2has not been monitored. However, the hammer drop heightand impact velocity of these blows have been measured.

Figures 6 and 7 show the frequency distribution of theSPT hammer drop height in borings 1 and 2, respectively.Figures 8 and 9 illustrate the percentage of blows applied indifferent ranges of hammer drop height in borings 1 and 2,respectively.

Figures 10 and 11 show the hammer drop height blowby blow in each sequence from borings 1 and 2, in all avail-able cases.

Tables 1 and 2 present, for each blow sequence, thefollowing measurements: the number of blow counts for45 cm sampler penetration (N45), the average frequency ofblow count application, the working shift when the blowshave been applied (see definition below), the nominal depthof the test and the length of the rod stem, the hammer dropheight, the impact velocity and the energies measured, aswell as energy ratios.

The average frequency of the blows has been calcu-lated considering the interval from the initial lifting of thehammer, in the first blow of each sequence, up to the finalof the hammer impact for the last blow. Figure 12 shows thehammer drop height vs. the frequency of blows.

In order to evaluate the variation in hammer dropheight during the day, the working period of the boringcrew was divided in four shifts, namely: first shift, from 8and 10 AM; second shift, from 10 to 12 AM; third shift,from 14 to 16 PM and fourth shift, from 16 to 18 PM. Figure13 shows the corresponding variation.

The energy reaching the rod stem (enthru energy) (Ei)has been calculated from Eq. 5. The values of force (F) andvelocity (v) have been obtained through the measurementsfrom the strain-gauges and accelerometers installed on therods. Tables 1 and 2 present the measured values.

E F v dti � � �� (5)


Santana et al.

Figure 6 - Frequency distribution of the hammer drop heightfrom Boring 1.

Figure 7 - Frequency distribution of the hammer drop height fromBoring 2.



Tab

le1

-M

easu

red

valu

esfo

rB

orin

g1.

Sequ

ence

N45

1Fr

eque

ncy

(Blo

ws/

min

)Sh

ift

Dep

th2

(m)

L3

(m)

h d4(m

)v i7

(m/s

)E

p8(J

)E

k9(J

)E

i10(J

)E

p/E*11

Ek/E

*(e

1)E

i/E*

Ek/E

p

(e1*

)E

i/Ep

Ei/E

k

(e2)

A5

SD6

ASD

ASD

ASD

ASD

15

32.1

49.

0010

.80

0.87

0.07

3.83

0.16

554.

047

.547

8.2

40.5

429.

663

.41.

161.

000.

900.

860.

780.

90

22

31.3

210

.00

11.7

90.

710.

073.

53-

451.

941

.540

5.7

-36

6.7

65.2

0.94

0.85

0.77

0.90

0.81

0.90

38

22.8

211

.00

12.8

00.

770.

063.

610.

0849

2.8

35.9

423.

918

.638

2.1

27.1

1.03

0.89

0.80

0.86

0.78

0.90

415

29.0

312

.00

13.8

00.

870.

064.

020.

1555

3.4

37.8

525.

840

.049

3.5

34.3

1.16

1.10

1.03

0.95

0.89

0.94

53

-3

13.0

014

.81

0.69

0.05

3.41

0.27

437.

432

.838

0.2

60.7

328.

641

.80.

910.

800.

690.

870.

750.

86

64

39.7

315

.00

16.8

30.

670.

033.

380.

0342

9.1

20.0

371.

65.

931

8.1

22.7

0.90

0.78

0.67

0.87

0.74

0.86

718

27.2

219

.00

20.6

80.

690.

033.

670.

0944

1.1

18.4

436.

921

.741

1.4

18.4

0.92

0.91

0.86

0.99

0.93

0.94

819

24.0

320

.00

21.7

60.

830.

063.

750.

1452

9.4

39.0

458.

033

.543

1.5

18.1

1.11

0.96

0.90

0.87

0.82

0.94

926

26.1

421

.00

22.7

70.

860.

083.

930.

1854

7.3

47.9

502.

946

.144

7.1

35.4

1.14

1.05

0.93

0.92

0.82

0.89

1041

19.4

422

.00

23.8

00.

780.

053.

680.

1149

7.8

29.1

441.

325

.540

9.6

22.6

1.04

0.92

0.86

0.89

0.82

0.93

1 N45

=nu

mbe

rof

blow

sfo

r45

cmsa

mpl

erpe

netr

atio

n;2 D

epth

=no

min

alde

pth

ofth

eSP

T(m

);3 L

=le

ngth

ofth

ero

dst

em,i

nclu

ding

the

sam

pler

leng

th(m

);4 h d

=SP

Tha

mm

erdr

ophe

ight

;5 A

=av

erag

eva

lue;

6 SD=

stan

dard

devi

atio

n;7 v i

=SP

Tha

mm

erim

pact

velo

city

;8 E

p=

actu

alpo

tent

iale

nerg

yof

the

SPT

ham

mer

atth

ere

leas

em

omen

t;9 E

k=

kine

ticen

ergy

ofth

eSP

Tha

mm

erat

the

imm

inen

ceof

impa

ct;

10E

i=

ener

gym

easu

red

just

belo

wth

ean

vil;

11E

*=

theo

retic

alpo

tent

iale

nerg

yof

SPT

ham

mer

from

Bra

zilia

nsy

stem

(478

.2J)

.

Figure 14 shows typical force and velocity signalsmeasured just below the anvil. Figure 15 shows typical val-ues of energy vs. time.

Figure 16 illustrates values of Ei normalized by theactual potential energy (Ep), as a function of the rod length,for borings 1 and 2.

3.3. Analysis of the results

The average hammer drop height of sequences fromBoring 1 varied from 67 to 87 cm, with actual potential en-ergy in the range 429.1 - 554.0 J, reaching a difference of29%. The hammer has been lifted higher than 80 cm, orlower than 70 cm (difference higher than 5 cm from thestandard value) in 64% of the blows. The average hammerdrop height of the whole data from Boring 1 is 80 cm, with astandard deviation of 9 cm.

The scatter in hammer drop height values was smallerin Boring 2. The average hammer drop height varied from69 to 87 cm, with potential energies in the range 440.8-553.0 J, reaching a difference of 25%. The hammer has


Santana et al.

Tab

le2

-M

easu

red

valu

esfo

rB

orin

g2.

Sequ

ence

N45

Freq

uenc

y(B

low

s/m

in)

Shif

tD

epth

(m)

L (m)

h d(m

)v i

(m/s

)E

p(J

)E

k(J

)E

i(J

)E

p/E*

Ek/E

*(e

1)E

i/E*

Ek/E

p

(e1*

)E

i/Ep

Ei/E

k

(e2)

ASD

ASD

ASD

ASD

ASD

17

--

10.0

011

.70

--

--

--

--

451.

729

.7-

-0.

94-

--

28

--

11.0

012

.70

--

--

--

--

449.

334

.2-

-0.

94-

--

35

--

12.0

014

.70

--

--

--

--

472.

242

.7-

-0.

99-

--

45

27.1

313

.00

15.7

00.

750.

043.

690.

1247

7.4

28.2

442.

529

.540

8.1

15.1

1.00

0.93

0.85

0.93

0.85

0.92

511

26.6

314

.00

16.7

00.

780.

063.

780.

1649

7.9

41.1

466.

239

.246

3.9

31.2

1.04

0.97

0.97

0.94

0.93

0.99

69

28.6

415

.00

17.7

00.

850.

133.

920.

2754

1.2

80.8

501.

868

.055

1.4

15.0

1.13

1.05

1.15

0.93

--

77

33.3

416

.00

18.7

00.

740.

053.

710.

1247

1.2

32.3

448.

228

.142

7.2

19.6

0.99

0.94

0.89

0.95

0.91

0.95

815

29.8

117

.00

19.7

00.

780.

063.

780.

1649

6.5

39.3

465.

138

.844

5.5

41.7

1.04

0.97

0.93

0.94

0.90

0.96

95

25.7

218

.00

20.7

00.

740.

013.

640.

0847

0.4

8.8

430.

318

.248

1.8

44.9

0.98

0.90

1.01

0.91

--

1016

32.9

219

.00

21.7

00.

800.

063.

860.

1451

2.4

39.3

484.

935

.546

5.9

28.4

1.07

1.01

0.97

0.95

0.91

0.96

1125

28.3

220

.00

22.7

00.

870.

063.

980.

1255

3.0

36.5

516.

232

.148

2.2

35.6

1.16

1.08

1.01

0.93

0.87

0.93

1246

27.6

221

.00

23.7

00.

750.

073.

700.

1847

5.8

41.7

446.

543

.344

5.6

42.2

1.00

0.93

0.93

0.94

0.94

1.00

1351

24.0

322

.00

24.7

00.

720.

043.

670.

1145

7.8

22.7

438.

926

.744

1.0

18.1

0.96

0.92

0.92

0.96

0.96

1.00

1428

20.5

423

.00

25.7

00.

690.

043.

570.

0944

0.8

22.8

414.

321

.440

3.5

26.8

0.92

0.87

0.84

0.94

0.92

0.97

Figure 8 - Percentage of blow counts applied in different rangesof hammer drop height from Boring 1.

Figure 9 - Percentage of blow counts applied in different rangesof hammer drop height from Boring 2.



Figure 10 - Hammer drop height measured in Boring 1.


Santana et al.

Figure 11a - Hammer drop height measured in Boring 2 (sequences from 4 to 13).

been lifted to heights greater than 80 cm or lower than70 cm in 46% of the blows. The average hammer dropheight is 76 cm, with a standard deviation of 8 cm.

Therefore, in spite of both borings had been per-formed by the same crew, using the same equipment and onthe same site, under the same conditions, a difference on theaverage hammer drop height in Borings 1 and 2 was veri-

fied (80 and 76 cm, respectively). Two other aspects ob-served in the tests: i) a variation in the hammer drop heightin the same blow sequence; ii) a variation in the averagehammer drop height from different sequences.

A tendency to increase the hammer drop height withthe advance of the sequence was observed in both borings.Only in two out of 21 sequences the opposite behavior wasverified. It was hypothesized that the increase in hammerdrop height is caused by the fatigue of the crew, resulting inless care in the procedure, although the opposite shouldseem more probable. The average frequency of the blowswas 19.4 and 39.7 blows per minute, in Borings 1 and 2, re-spectively, which is a significant difference. The smallerfrequencies were observed in the longer sequences, withmore than 25 blows, probably also caused by fatigue of thecrew. The shorter sequences, with five or even fewer blows,presented the higher frequencies. The average frequency ofblows for all sequences (Borings 1 and 2) was27.8 blows/min with a standard deviation of 4.7 blows/min.

Many authors (e.g., Kovacs, 1979; Seed et al., 1985;Skempton, 1986; Décourt, 1989) discuss the influence ofthe frequency of blows on SPT results.



Figure 12 - Hammer drop height vs. frequency of blows per min-ute.

Figure 11b - Hammer drop height measured in Boring 2 (se-quence 14).

Figure 13 - Hammer drop height vs. shift (first shift, from 8 and10 AM; second shift, from 10 to 12 AM; third shift, from 14 to 16PM and fourth shift, from 16 to 18 PM).

Figure 14 - Typical signals of force and velocity measured justbelow the anvil (Blow 21 of Sequence 9, Boring 1).

Figure 15 - Typical values of energy (just below the anvil) vs.time (Blow 21 of Sequence 9, Boring 1).

In fact, the dynamic condition of SPT may generateexcess pore water pressures that can influence the soil resis-tance to penetration, even in sands. These excess pore waterpressures can be influenced by the driving frequencies.Tests submitted to different frequencies can result in differ-ent N values in the same soil. Seed et al. (1985) showed thatthe N values might be affected by the blow application fre-quency, depending on soil characteristics.

Danziger et al. (2009) and Souza et al. (2012a,2012b) listed different values of the ratio qc/N (where qc iscone resistance) for loose and dense sands. Unlike the CPT,where the test is performed in drained conditions in the caseof sands, SPT may generate positive excess pore pressuresin loose sands and negative excess pore pressures in densesands. This results in lower qc/N values when tests are car-ried out in dense sands, and higher qc/N values when testsare performed in loose sands.

Although not very pronounced, Fig. 12 shows a ten-dency of more scatter on the hammer drop height with theincrease of the frequency of blows, not only above, but alsobelow the standard value.

The work shift does not seem to have influenced thehammer drop height, as indicated by the data presented in

Fig. 13, where a similar scatter was observed for all workshifts.

The average hammer velocity at impact varied from3.38 to 4.02 m/s (kinetic energy of 371.6 and 525.8 J, re-spectively) in the sequences from Boring 1, whereas in thesequences from Boring 2 the average hammer velocity atimpact varied from 3.57 to 3.98 m/s (kinetic energy of414.3 and 516.2 J, respectively), see Tables 1 and 2. Thisvariation is a consequence of the inadequate control in thehammer drop height.

The efficiency factor e1 (Ek/E*), defined by Décourt(1989), varied in Boring 1 from 0.78 to 1.10 and in Boring 2from 0.87 to 1.08. These values are greater than those foundby Cavalcante et al. (2011) and varied in a broader rangethan those presented by Décourt (1989), see Fig. 17. Valuesof e1 greater than 1.00 are explained by the hammer dropheight above the standard value in various sequences.

In order to avoid the influence of the hammer dropheight on the efficiency factors, Santana et al. (2012) pro-posed the use of an efficiency factor e1*, given by Ek/Ep. Thevalues of e1* varied from 0.86 to 0.99 in Boring 1 and from0.91 to 0.96 in Boring 2. These values are greater than thosepresented by Décourt (1989) for the manual system and byCavalcante et al. (2011), see Fig. 18. As expected, e1* val-ues have less scatter than e1 values.

The average energy measured just below the anvil, Ei,varied from 318.1 J (efficiency of 67% in relation to thetheoretical potential energy or nominal energy) to 493.5 J(efficiency of 103%) in Boring 1, whereas in Boring 2 var-ied from 403.5 J (efficiency of 84%) to 551.4 J (efficiencyof 115%). This significant scatter in Ei values is mainly aconsequence of the variation on the hammer drop height,see Tables 1 and 2.

These results indicate that even SPT performed by thesame boring crew, in similar conditions, can result in N val-ues with distinct significance. When the efficiency of theenergy measured just below the anvil is calculated in rela-tion to the actual potential energy, the range in efficiency issignificantly lower, varying from 74% to 93% in Boring 1and from 85% to 96% in Boring 2, see Tables 1 and 2.


Santana et al.

Figure 16 - Rod length vs. energy just below the anvil normalizedby the actual potential energy (Ei/Ep).

Figure 17 - Values of the efficiency factor e1 (adapted fromSkempton, 1986, Décourt, 1989 and Cavalcante et al., 2011).

The efficiency factor e2 (Ei/Ek) varied from 0.86 to0.94 in Boring 1 and from 0.92 to 1.00 in Boring 2. Thesevalues are in the range - average line of Décourt (1989) dataand Cavalcante et al. (2011) data -, considering the anvil of977 g, see Tables 1 and 2 and Fig. 19.

It is possible that high values of factor e2 are associ-ated to the downward movement of the rod stem duringhammer blow, generating an increase in potential energythat is transferred to the rods in subsequent hammer im-pacts of the same blow. This occurrence, described byOdebrecht (2003), is more relevant in low resistance soils.The SPT Analyzer is capable of measuring the whole en-ergy transferred to the rod stem, only if the process occursbefore 102 ms. However, the kinetic energy is calculated inrelation to the first hammer impact with the anvil, so the e2

value can be overestimated should other impacts occur.

The results of Ei/Ep as a function of the rod lengthmeasured in Borings 1 and 2 are presented in Fig. 16. Thisfigure illustrates that the energy transferred to the rod stemis not significantly affected by its length, at least in therange of lengths analyzed, from 10.80 to 25.70 m. This is

corroborated by previous studies (e.g., Cavalcante, 2002;Odebrecht, 2003; Daniel et al., 2005; Danziger et al., 2008)indicating that the energy transmitted to the rod stem doesnot depend on its length and the e3 factor should be consid-ered equal to 1.00.

4. Conclusions

The paper presented the instrumentation results oftwo SPT deployments performed by the same crew, usingthe same procedures and equipment in the Sarapui II Exper-imental Test Site. The main conclusions are summarized asfollows:i) Although both borings had been performed by the same

crew, using the same equipment and on the same site,under the same conditions, a difference was found inthe average hammer drop height in Borings 1 and 2 (80and 76 cm, respectively). Two other aspects observedin the tests: a variation in the hammer drop height in thesame blow sequence; a variation in the average hammerdrop height from different sequences.

ii) A tendency to increase the hammer drop height as the se-quence advances was observed in both borings. Theopposite behavior was verified in only two out of 21sequences. It was hypothesized that the increase inhammer drop height is caused by the fatigue of thecrew, resulting in the careless of the procedure, al-though the opposite should seem more probable.

iii) The average frequency of blows was 19.4 and 39.7blows per minute in Borings 1 and 2, respectively. Thesmaller frequencies were observed in the longer se-quences, with more than 25 blows, probably caused bythe fatigue of the crew. The shorter sequences, withfive or even less blows, presented the higher frequen-cies.

iv) A slight trend of higher scatter on the hammer dropheight was observed when the rate of blows increased.



Figure 18 - Values of the efficiency factor e1* (adapted fromSkempton, 1986, Décourt, 1989 and Cavalcante et al., 2011).

Figure 19 - Efficiency factor e2 as a function of the anvil mass (adapted from Décourt, 1989 and Cavalcante et al., 2011).

v) The work shift does not seem to have influenced thehammer drop height, since a similar scatter was ob-served for all work shifts.

vi) The efficiency factor e1 (Ek/E*) varied in Boring 1 from0.78 to 1.10 and in Boring 2 from 0.87 to 1.08. Thesevalues are higher than those found by Cavalcante et al.(2011) and varied in a broader range than those pre-sented by Décourt (1989). Values of e1 greater than1.00 are explained by the hammer drop height abovethe standard value in various sequences.

vii) The values of e1* (Ek/Ep) varied in Boring 1 from 0.86 to0.99 and in Boring 2 from 0.91 to 0.96, in a narrowerrange than the e1 values.

viii) The average energy measured just below the anvil, Ei,varied from 318.1 J (efficiency of 67% in relation tothe theoretical potential energy or nominal energy) to493.5 J (efficiency of 103%) in Boring 1, whereas inBoring 2 varied from 403.5 J (efficiency of 84%) to551.4 J (efficiency of 115%). The scatter in Ei values ismainly due to the variation in the hammer drop height.The obtained results indicate that even SPT performedby the same boring crew, in similar conditions, can re-sult in N values with distinct significance.

ix) When the efficiency of the energy measured just belowthe anvil is calculated in relation to the actual potentialenergy, the range in efficiency is significantly lower,varying from 74% to 93% in Boring 1 and from 85%to 96% in Boring 2.

x) The efficiency factor e2 (Ei/Ek) varied in Boring 1 from0.86 to 0.94 and in Boring 2 from 0.92 to 1.00.

xi) The values of Ei/Ep vs. the rod length indicate that the en-ergy transferred to the rod stem is not significantly af-fected by its length, and the efficiency factor e3 shouldbe considered as 1.00.

Acknowledgments

Roberto Marinho and Max Gomes de Souza, for theirhelp in performing instrumented SPTs. Elvyn Marshall,who proofread the paper.

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