Energy Ratio (ER) for the Standard Penetration Test Based on Measured Field Tests

The Standard Penetration Test (SPT) is often used to estimate the soil parameters for geotechnical design projects, using the NSPT index. However, these estimates are performed based on empirical correlations without any scientific basis. Moreover, the test has a large inherent results dispersion due to the use of different types of equipment and execution procedures. Since the NSPT index depends on the amount of energy that is effectively transmitted to the sampler during the hammer fall, it is fundamental to be able to estimate this energy. Given the importance of estimating the energy that reaches the SPT sampler, an instrumented subassembly was developed in order to simultaneously assess the amount of energy transmitted to the drill rods, at sections just above the sampler and just below the anvil. This paper presents a series of SPT experimental results carried out in two different sites located in the State of São Paulo, using two different equipment set-ups (with manual and automatic tripping mechanisms), enabling the assessment of the top and bottom energy ratio under different conditions. Results show that for hand lifting hammer and automatic trip hammer systems the difference in the energy ratio is not significant. However, the dispersion of the results for the hand lifting hammer system is more pronounced due to execution procedures, equipment and operators.


Introduction
Despite criticism, the Standard Penetration Test (SPT) continues to be widely used for geotechnical designs, using the N SPT index as an indicator of soil properties (shear strength, compressibility and undrained shear strength of soils).The criticism is related to results dispersion attributable to the variability inherent in SPT tests.Standard penetration tests are performed using different types of equipment (hammers, drill rod, borehole fluids, sampling tubes, among others), execution procedures and operators.Consequently, the N SPT index, which is often used to estimate geotechnical soil parameters, is broadly variable and its consistency has been questioned.In addition, these estimates are performed based on empirical correlations without any scientific basis.Researchers and practitioners highlight that it is possible to increase the N SPT index accuracy by observing recommended standards and using a more skilled and experienced field crew (Schnaid et al., 2009;Reading et al., 2010).
The estimation of soil properties is performed through empirical correlations using the N SPT index, which corresponds to the number of blows required for the sampler to penetrate 300 mm into to the soil after an initial seating drive of 150 mm.The N SPT index does not represent physical soil resistance but is an indicator of soil resistance, which depends not only on the soil properties but also on the equipment characteristics.The N SPT index is also strongly dependent on the amount of energy delivered to the drill rods (Schmertmann & Palacios, 1979) and to the SPT sampler during hammer impact (Aoki & Cintra, 2000).For each hammer drop there is a corresponding nominal potential energy (PE) that is theoretically equal to 474.5 J (ASTM, 2008).According to the Brazilian standard, the corresponding nominal potential energy for SPT is equal to 478.2 J (ABNT, 2001).
The amount of energy that is initially delivered to the top of the drill rods, and subsequently transmitted to the sampler, can be significantly influenced by many factors, including the type and shape of hammer, drop height, equipment conditions, the length and mass of the drill rods, secondary impacts, soil conditions, verticality of the test, condition of the trip mechanism, among other variables (Schmertmann & Palacios, 1979;Belincanta & Cintra;1998;Aoki & Cintra;2000;Tsai et al., 2004;Odebrecht et al., 2005;Sancio & Bray, 2005;Aoki et al., 2007;Youd et al., 2008;Lee et al., 2010;Reading et al., 2010).
Due to the energy losses in the different mechanical components of the hammer release system and other sources of dissipation, the energy delivered to the rods and sampler is not equal to the nominal potential energy (Schmertmann & Palacios, 1979;Aoki & Cintra, 2000;Odebrecht et al., 2005;Cavalcante et al., 2008;Lukiantchuki, 2012, Santana et al., 2012;Lukiantchuki et al., 2015).Thus, the energy ratio of the SPT setup (E R ) (Eq. 1) is usually defined as the ratio of the amount of energy transferred to the drill rods (Force-Velocity Method, referred to as EFV), to the nominal potential energy (PE) (ASTM, 2010).The EFV is calculated by integrating the force multiplied by the velocity over time.The use of the EFV method in estimating SPT energy is considered to be the most reliable and accurate method for estimating SPT energy during wave propagation (Sy & Campanella, 1991;Howie et al., 2003;Youd et al., 2008).
where F(t) = the normal force, during the wave propagation, at a specific section, and v(t) = the particle velocity.
The energy ratio (E R ) should be evaluated for the system when the N SPT index is used to estimate soil properties for geotechnical designs or for comparing results.However, different types of equipment are used to perform SPT tests, resulting in variable energy ratio values and N SPT indexes.Therefore, researchers and practitioners recommend that the N SPT index should be normalized (Kovacs & Salomone, 1982;Robertson et al., 1983;Seed et al., 1985;Skempton, 1986) to a reference energy ratio of 60% (N 60 ) (ISSMFE, 1989) (Eq.2).
where N SPT = is the blow count; E R = is the energy ratio of the specific SPT set up; E 60 = 60% of the international reference of nominal potential energy (@ 474 J); N 60 = N SPT index corrected to 60% of the international reference for nominal potential energy.
In conventional methods, energy is measured just below the anvil through an instrumented subassembly installed at the top of the drill rods.However, Aoki & Cintra (2000) suggested redefining the SPT energy ratio as the ratio of the maximum amount of energy transferred to the soil sampler system to the nominal potential energy.According to these authors, the energy ratio above the sampler is inversely proportional to the drill rod length (Fig. 1) and the energy ratio would be related to the work done during sampler penetration into the soil and not to the available kinetic energy.
Some researchers (Cavalcante et al., 2008;Odebrecht et al., 2005;Santana et al., 2012;Lukiantchuki et al., 2015) have measured energy in a section just above the sampler.However, little data is available due to the difficulty in placing the instrumentation inside the borehole.The assessment of the amount of energy transmitted to the string of rods, simultaneously at a section just below the anvil and a section just above the sampler, allows for estimating energy losses over the rod (e 4 ) (Eq. 3) (Danziger et al., 2008).
E e e e e PE s = ´´´1 2 3 4 (3) where E s = is the amount of energy that reaches the sampler; e 1 = is the correction factor which relates the energy just before the impact to the free fall energy; e 2 = is the ratio between the energy just below the anvil and the kinetic energy just before the impact; e 3 = is a factor related to the drill rod length and e 4 = is the factor which relates the energy loss over the drill rod length.
Considering the importance of estimating the energy that reaches the SPT sampler, an instrumented subassembly, capable of reading acceleration and force signals just above the sampler, was developed.This paper presents the results of a series of SPT experimental tests performed using two instrumented subassemblies, one placed just below the anvil and the other just above the sampler.This instrumentation allowed the simultaneous assessment of the amount of energy transmitted to the drill rods at sections just above the sampler and just below the anvil.Additionally, the SPT tests were conducted using different hammer types (hand lifting pin weight and automatic trip hammer).Results allow for estimation of the energy ratio at the top (anvil) and at the bottom (sampler) of drill rods, for two different equipment set-ups.

Instrumentation
In this research, two instrumented subassemblies were built, similar to the one developed by Odebrecht et al. (2005).Each instrumented subassembly consists of one segment of rod, to which a pair of accelerometers and one load cell have been installed (Fig. 2a).The load cell is composed of four double strain gauges (350 W each), from a Wheatstone bridge circuit, assembled 90°apart (Fig. 2b).
A pair of PCB Piezotronics piezoeletric accelerometers was rigidly mounted on each instrumented subassembly by means of an aluminum support.A suitable support geometry was defined through dynamic tests with different support geometries (Lukiantchuki, 2012).
Laboratory test results yielded the accelerometer support shown in Fig. 3a as the most suitable for field tests.This support presented the lowest resonance and anti-resonance effects.It was possible to collect data with frequencies up to 14,000 Hz, with low amplitude variations.For recording accelerations at the section just below the anvil, accelerometers capable of measuring accelerations up to 5000 g, in the 0.4-10,000 Hz frequency range (model 350B04) were used.The experimental test results show that accelerations at the section just above the sampler are higher than those at the section just below the anvil (Fig. 3b).For this reason, accelerometers capable of measuring accelerations up to 20,000 g in the 1-15,000 Hz frequency range (model 350M77) were necessary.Lukiantchuki et al. (2011) argue that the tip of the sampler be free (tip resistance is very low) for the first blow in SPT tests, allowing the tip to move downward.This tip movement generates a reflected tensile wave, which doubles the particle velocity at the tip of the sampler (Skov, 1982).When the instrumentation is placed just above the sampler, the time interval between the incoming compressive wave and the reflected tensile wave is very short.This time interval is equal to 2L'/c, where L' is the distance from the accelerometer position to the sampler and c is wave propagation velocity (Fig. 4).Due to the particle velocity superposition, the accelerations are summed.This is the reason for the need to use accelerometers with higher capacity (20,000 g).
To record the signal data, an HBM data acquisition system, model MX410, was used.This four-channel portable data acquisition system is suitable for analyzing high frequency dynamic events.Field tests were conducted at a 96-kHz sampling rate per channel.Additionally, a trigger, a pre-trigger and a low-pass filter (anti-aliasing) corresponding to 15% of the selected sampling rate was used.

Experimental Set-Up
The SPT field tests, performed according to the Brazilian Standard (ABNT, 2001), were carried out at the Experimental Research Site of São Paulo State University (UNESP) in the city of Bauru and at a field site in the city of São Carlos, both cities located in the state of São Paulo, Brazil.The experimental sites, which are geologically similar, are composed of a thick layer of lateritic silty fine sand.This unsaturated, very porous and collapsible layer presents low bearing capacity, with an N SPT index varying from 1 to 30 blows, for depths in the range from 1 m to 30 m.The only difference between the Bauru and São Carlos sites is that the average water level is 30 m and 12 m below the surface, respectively (Figs. 5 and 6).
The field tests were performed using both a hand lifting pin weight hammer and an automatic trip hammer system (Table 1).The hand lifting pin weight hammer system will be referred to hereafter as a hand lifting system.This system uses a pin weight hammer (65 kg) (Fig. 7a) with a manual tripping mechanism.The automatic trip hammer system uses a hammer (61.75 kg) with an automatic tripping mechanism (Fig. 7b).The SPT tests used Brazilian Standard drill rods, which have a cross-section area of around 4.2 ´10 -4 m 2 , a weight of 32 N/m each and the rod joints have a cross-section area of around 8.4 ´10 -4 m 2 .

Calculation and Procedures
The amount of energy (EFV) transmitted to the drill rods can be both theoretically and experimentally estimated.Experimentally, the energy can be calculated using the EFV method, by integrating the product of normal force (F) multiplied by the particle velocity (v) with respect to time, as in Eq. 1.The integration initial instant (t i ) corresponds to the beginning of the event, that is, when the force signal becomes different from zero.The integration final instant (t f ) is when the force and velocity signal become zero and no additional energy transfer occurs.At the final instant, the energy transferred to the drill rods reaches a maximum value.The EFV method provides accurate energy values even when the proportionality between force and velocity is lost (Sy & Campanella, 1991;Howie et al., 2003).The velocity trace was obtained by integrating the acceleration signal measurements.In this procedure, it was assumed that at the initial instant, the acceleration was equal to zero.Likewise, the initial computed velocity was corrected, setting it to zero.In addition, the displacement trace was obtained by integrating the velocity signals.
Then, the maximum displacement value was compared with the SPT sampler penetration measured in the field.The performance of the developed instrumentation can be observed in Fig. 8, in which the maximum top and bottom displacements appear to be very close to the measured sampler penetration.The energy curves are consistent, indicating an energy loss of about 38 J for a depth of 4 m.
The product of velocity and rod impedance (Z) has the same dimension of force (Eq.4): where a = area of the rod cross-section (4.2 ´10 -4 m 2 ); E = modulus of elasticity of the rod (206840 MPa); c = theoretical wave propagation velocity = (E/r R ) 0.5 @ 5120 m/s; r R = mass density of the rod (7880 kg/m 3 ); v = particle velocity; Z = rod impedance (Sancio and Bray, 2005).
In order to verify the suitability of the developed equipment, the force traces (F) and velocity traces (v) multiplied by the rod impedance (Z) were compared.These traces should be proportional in the time interval between the initial instant (t i ) and the instant (t i + 2L'/c), where L' is the distance between the accelerometer and the sampler tip.Due to wave reflections, this trace superposition does not occur after the instant (t i + 2L'/c).
Figure 9 shows a typical record of F and vZ comparisons for the instrumentation placed below the anvil.This test was performed at an initial depth of 10.0 m, the total length of the drill rod (from anvil to sampler) was 10.73 m, so that L' was about 10.43 m.As the N SPT index was 18 blows/0.29 m, each blow by the hammer caused an average sampler penetration of about 0.016 m into the soil.
The F and vZ records begin at point O (t = 4.65 ms) and rise sharply to a maximum value of approximately 60 kN.After the initial peak, the force and velocity magnitudes decrease and tensile wave reflections caused by loose joints appear.At point 6 a compression wave reflection can be observed due to an increase of impedance at the sampler head.At point A the curves are separated due to downward and upward wave effects.

Suitability of the developed instrumentation
In order to ensure the suitability of instrumented blow data, the sampler displacement was measured for each blow and compared to the calculated displacement.As can be observed, independently of the posi-tioning of the instrumentation, the sampler measured and calculated displacements show good agreement (Figs. 10 and 11).This makes sense since the displacement that occurs at the top and bottom of the drill rods is the same.Therefore, the developed instrumentation is adequate for estimating the energy transmitted during wave propagation.

Energy ratio (E R )
The results and conclusions, presented below, are derived from the analysis of 185 instrumented blows.The instrumentation allowed for the simultaneous assessment of the amount of energy transmitted to the drill rods at sections just below the anvil and a section just above the sampler.It was therefore possible to estimate the energy ratio in both sections and analyze the energy loss during the sampler penetration.
For the present analysis, the energy ratio was divided into 8 groups (Gi) considering different ranges (Table 2).
Figures 12 and 13 show the energy ratio (E R ) histogram for the hand lifting and automatic trip hammer systems, which represent the E R values for a section immedi- Table 2 -Groups and energy ratio ranges adopted.

Group
Energy ratio range (%) ately below the anvil.The data indicate that most of the E R values for the hand lifting system are in group G6, which corresponds to energy ratios between 70% and 80%, whereas most of the values for the automatic trip hammer system are in group G7, corresponding to energy ratios between 80% and 90%.In general, results show that the energy ratio corresponding to the automatic trip hammer system is slightly higher than those corresponding to the hand lifting system.This behavior is consistent with the work of Reading et al. (2010), especially because the execution procedures for an automatic trip hammer system with an auto-matic tripping mechanism are less influenced by operators.
In addition, for hand lifting system, buckling of the drill rods occurs during the hammer fall due to the eccentricity of the blow and consequently a higher amount of energy is lost.
Figures 14 and 15 show the energy ratio (E R ) histogram for hand lifting and automatic trip hammer systems, which represent the E R values for a section immediately above the sampler.Results indicate that energy measured above the sampler is more variable than energy measured below the anvil.The data indicate that most of the E R values for the hand lifting system are in group G5, which corresponds to energy ratios between 60% and 70%.For the automatic trip hammer system values are in group G4, for the Bauru site, and in group G6, for the São Carlos site, which corresponds to energy ratios between 50-60% and 70-80%, respectively.
The values of energy ratio as a function of the depth for Bauru and São Carlos sites, considering the different set-ups, are presented in Figs.16 and 17.The behavior of the data shows that the energy transferred to the anvil is not influenced by the rod length (Fig. 16).Santana et al. (2014) observed this same behavior using drill rod lengths varying from 10.80 m to 25.70 m.In the present study, drill rod lengths varying from 2.95 m to 12.95 m were used, due to the limitation of the cable length for the instrumentation placed above the sampler.
Results show that it is possible to use an average value for E R in a section just below the anvil.The coefficient of variation (CV) was about 5% (Table 3), which is a very small value considering geotechnical tests.Phoon & Ching (2012) mentioned that the availability of multiple test data in a typical site investigation can contribute to the coefficient of variation reduction.Additionally, Phoon & Kulhawy (1999) demonstrated that a CV from 25% to 50% can be expected for N SPT index estimation.However, the data presented in this paper, demonstrate that the energy variation can be controlled by following recommended execution procedures, consequently the estimation of N SPT index can be made more reliable.
On the other hand, results also show that E R values in a section just above the sampler are highly variable (Fig. 17).The data indicates that the energy transference to the sampler does not have a defined behavior and it is not possible to use an average value to represent E R in a section just above the sampler.
The energy ratio values measured just below the anvil are less variable than the values measured above the sampler, probably because most of the energy is transferred during the first impact (Fig. 18).On the other hand, the energy before reaching the sampler is dissipated during the wave propagation through the drill rods, connected by somewhat loose rod joints, making it highly variable.Table 3 shows the E R average values, standard deviation (SD) and the coefficient of variation (CV) for the experimental data.Results indicate that the E R for the hand lifting system is about 79% when the instrumentation is placed below the anvil.Santana et al. (2014) estimated values ranging from 67% to 115%.However, the authors observed that the hammer drop height may vary due to operator execution procedures.In the present work, the hammer drop height was strictly controlled and maintained the standard value of 0.75 m.Therefore, E R values higher than 100% can be justified by the additional potential energy of the hammer and rods due to the sampler penetration into the soil (Odebrecht et al., 2005).
Results also show that for the automatic trip hammer system the E R is about 84%, which is 5% higher than the E R value estimated for the hand lifting system.This behavior makes sense because the automatic tripping mechanism is less influenced by operators.Additionally, the blow is more centric and buckling of the drill rods is minimized.
The energy values measured above the sampler were highly variable (Table 3), except for the automatic trip hammer system at the São Carlos site.Generally, results show the difficulty of measuring the energy in this section, which is probably because wave reflections and buckling of the drill rods occur during the wave propagation.The acceleration signals allowed observing the behavior differences between the manual and automatic hammers, following hammer impact (Fig. 19).The acceleration records show that acceleration becomes different from zero at the instant that the hammer hits the anvil (hammer impact).It can be noted that for the automatic hammer, the signals corresponding to both accelerometers installed in the subassembly agree during the impact until the instant of 0.14 ms (Fig. 19a).This indicates that during the hammer impact there was no eccentricity effect.However, for the manual hammer, the acceleration signals are not in agreement during the hammer impact (Fig. 19b).It can be noted that the acceleration signals are not in agreement, most likely due to the blow eccentricity effect.This effect is more frequent for the manual tripping mechanism because the free fall of the hammer is strongly dependent on the operators at the instant they release the hammer.

Energy losses over the rod
The energy losses over the rod were evaluated by comparing energy values calculated in sections just below the anvil and just above the sampler.Figure 20 shows the energy losses over the rod for the hand lifting and automatic trip hammer systems.As it can be observed, results show a significant loss of energy.For the automatic trip hammer system, the data indicates a trend of increasing loss of energy with the length of rods.Results show a loss of energy of about 40% for 12 m of rod length.However, for the hand lifting system the loss of energy shows a dispersion of values and consequently there is no trend to be observed.Odebrecht (2003) and Johnsen and Jagello (2007), identified a trend of increasing energy loss with increasing rod length.Odebrecht et al. (2005) performed instrumented SPT tests in a calibration chamber.However, in field tests, due to the different boundary conditions, different behavior should be expected.Johnsen & Jagello (2007) found a scatter behavior of energy loss using several different SPT systems.However, the increase of energy loss with the increase of rod length is a consensus among researchers.
In the present work, the dispersion of results can be explained by the soil conditions and execution procedures.For shallow depths and soft soil, the energy transfer occurs through multiple impacts (secondary impacts).During the energy transference, the buckling of the drill rod causes significant energy losses.Therefore, the energy loss is highly variable.

The implications of the N SPT index corrections
The energy ratio values have a significant effect on the N SPT index, so that this index should be adjusted to a reference energy ratio of 60% (N 60 ) (Eq. 2).For set-ups with different energy ratios, the N SPT index may vary significantly (Reading et al., 2010), making this correction a necessity.
Figures 21 and 22 show the comparison of the N SPT index for energy ratio values estimated in the present work.For both hand lifting and automatic trip hammer systems, results from the Bauru site (Fig. 21) show that the N SPT index does not vary significantly because the E R values are very similar (Table 3).In this figure, N 80 and N 83 are the N SPT indexes estimated for energy ratios of 80% and 83%, respectively.Consequently, the comparison between the N SPT indexes for different energy ratio indicates that N 60 is about 36% higher than N 80 or N 83 .
Figure 22 shows results from São Carlos.In this figure, N 77 and N 84 are the N SPT indexes estimated for energy ratios of 77% and 84%, respectively.The difference between the energy ratios of the hammer systems is about 7%.The comparison between the N SPT indexes for different energy ratios indicates that N 60 is about 28% and 40% higher than N 77 or N 84 , respectively.However, Reading et al. (2010) demonstrated that for larger differences the N SPT index may vary by about 90%.Therefore, the application of the N SPT correction based on energy ratio, estimated from energy measurements during SPT tests, would give a consistent N SPT index for any kind of equipment set-up.
The European Standard (ISO, 2005) requires that a certificate of calibration including the E R value be available and provides a recommended method for estimating energy ratios and reporting the results.Additionally, the calibrations should be carried out every six months because the hammer may become damaged.

Conclusions
This paper presents the results of SPT tests performed with two instrumented subassemblies, one placed at the top and the other at the bottom of the string of rods.This instrumentation allowed for the assessment of the amount of energy transmitted to the drill rods, at sections just below the anvil and just above the sampler, for two different equipment set-ups, with both a hand lifting and an automatic trip hammer system.The energy ratio was also estimated and compared.The following conclusions can be stated concerning the energy in SPT tests.1.The developed instrumentation allowed for the simultaneous assessment of energy transmitted to the drill rods at sections just above the sampler and just below the anvil.The comparison between measured and calculated sampler displacement showed good agreement.Therefore, the developed instrumentation is adequate for estimating the energy transmitted during wave propagation.2. Experimental in-situ results show that the top energy ratio (E R ) values corresponding to the hand lifting system are between 70% and 80%, whereas the values corresponding to the automatic trip hammer system are between 80% and 90%.Results show that the top energy ratio for the automatic trip hammer system is slightly higher than the hand lifting system.This behavior makes sense because the automatic trip hammer system has an automatic tripping mechanism and conse-  quently the execution procedures are less influenced by operators.Also, for the hand lifting system (manual tripping mechanism), buckling of the drill rods occurs during the hammer fall, due to the eccentricity of the blow and consequently additional energy is consumed.The same effect does not occur for the automatic trip hammer system, because the blow is more centric.3. Experimental in-situ results also show that the bottom energy ratio values for both equipment systems are more variable than the energy values measured below the anvil.4. Results indicate that the energy values assessed just below the anvil are less variable than the values estimated above the sampler, probably because most of the energy is transferred during the first impact.The energy is dissipated during the wave propagation over the drill rods and the amount that reaches the sampler is highly variable.5.The data shows that the energy transferred to the anvil is not influenced by its length.Additionally, results show that it is possible to use an average value for E R in a section just below the anvil.The coefficient of variation (CV) was about 5%, which is considered a very small value for geotechnical problems.On the other hand, results also show that E R values in a section just above the sampler are highly variable.The data indicates that the energy transferred to the sampler does not have a defined behavior and it is not possible to use an average value to represent E R in a section just above the sampler.6. Acceleration records indicate that during the hammer impact, eccentricity effects are more frequent for the manual tripping mechanism.For this tripping mechanism, the free fall of the hammer is strongly dependent on the operators at the instant they release the hammer.7. Experimental results show a significant loss of energy over the drill rod.For the automatic trip hammer system, the behavior of data indicates a trend of increasing energy loss with the length of rods.However, for the hand lifting system the energy loss also shows wide dispersion that cannot be easily explained.A loss of energy of about 40% for a 12-m rod was observed, which can be explained by the soft soil and the buckling of the drill rods during the energy transference, which causes significant energy losses.8.For the Bauru site, results show that the N SPT indexes are very similar for both equipment set-ups.Thus, the comparison between the N SPT indexes for different energy ratios shows that N 60 is about 36% higher than N 80 or N 83 .Results for the São Carlos site also indicated that the N SPT index does not vary for hand lifting and automatic trip hammer systems.The comparison between the N SPT indexes for different energy ratios indicates that N 60 is about 28% and 40% higher than N 77 or N 84 , respectively.The N SPT index correction based on energy ratios estimated from energy measurements during SPT tests would give a consistent N SPT index for any kind of equipment set-up.
The SPT test results of this research indicate that the variability of the amount of the energy that reaches the anvil can be minimized by controlling execution procedures.Additionally, equipment set-up using an automatic tripping mechanism yields lower results dispersion.Results also show that difficulties may arise in measuring and interpreting the amount of energy that reaches the sampler.The variability of results is widely variable in terms of energy and loss of energy, whose explanation is not very clear.
N 60 : N SPT index corrected to 60% of the international reference for nominal potential energy E 60 : 60% of the international reference of nominal potential energy E s : amount of energy that reaches the sampler e 1 : correction factor which relates the energy just before the impact to the free fall energy e 2 : ratio between the energy just below the anvil and the kinetic energy just before the impact e 3 : factor related to the drill rod length q t : tip resistance after correction for pore pressure effects R f : friction ratio t i : the integration initial instant t f : the integration final instant Z: rod impedance a: area of the rod cross-section E: modulus of elasticity of the rod c: theoretical wave propagation velocity r R : mass density of the rod L': distance between the accelerometer and the sampler tip g: gravitational acceleration Figure 2 -Instrumentation developed(Lukiantchuki, 2012).

Figure 3 -
Figure 3 -(a) Accelerometer supports, (b) comparison between acceleration magnitudes measured at sections just below the anvil and just above the sampler.

Figure 8 -
Figure 8 -Energy, velocity and displacement traces (third blow at the depth of 4 m -São Carlos siteautomatic trip hammer system).

Figure 9 -
Figure 9 -Comparison of F and vZ signals for instrumentation placed below the anvil.

Figure 10 -
Figure 10 -Comparison of sampler displacement and calculated displacement for instrumentation placed below the anvil.

Figure 11 -
Figure 11 -Comparison of sampler displacement and calculated displacement for instrumentation placed above the sampler.
Figure 12 -Energy ratio (E R ) histogram for hand lifting system (instrumentation placed below the anvil).

Figure 13 -
Figure 13 -Energy ratio (E R ) histogram for automatic trip hammer system (instrumentation placed below the anvil).

Figure 15 -
Figure 15 -Energy ratio (E R ) histogram for automatic trip hammer system (instrumentation placed above the sampler).

Figure 14 -
Figure 14 -Energy ratio (E R ) histogram for hand lifting system (instrumentation placed above the sampler).
Figure 19 -Comparison between acceleration signals for different hammer systems.

Figure 20 -
Figure 20 -Energy losses over the rod.

Figure 18 -
Figure 18 -Mechanism of energy transfer to the drill rods (below the anvil).

Figure 22 -
Figure 22 -N SPT index for different energy ratios (São Carlos site).