Method statement Sonic Integrity Testing.pdf

Profound .

March 2005

P.Middendorp Msc.

:

Project number : Approved by : P.Middendorp Initials : MPP Number of pages : 25 Number of Appendices : 2

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Method Statement Sonic Integrity Testing (Pile Integrity Testing)

Method statement Sonic Integrity Testing.doc Page 2 of 25 18-03-2005

Contents 1 Principles of Sonic Integrity Testing 3 2 Discontinuities in the pile shaft. 6 3 Signal processing 7 4 The Characteristic Signal 9 5 Interpretation Guidelines 13 6 Interpretation 14 7 Pile preparation 18 8 Placing the Sensor 19 9 Reliable Testing 20 10 References 21 11 Appendix, Influence of hammer blow width 22 12 Appendix, Influence of soil 24


1 Principles of Sonic Integrity Testing

A stress wave (sound wave) is introduced into the pile by means of a hammer blow on the pile head (Fig.1.1). This stress wave travels at the speed of sound ( c ) to the pile toe and reflects back to the pile head (Fig. 1.2). The response of the pile head , as a result of the hammer blow and reflections, is measured with an acceleration transducer. The acceleration is integrated and presented as a velocity signal (v) For each pile at least 3 hammer blows are applied to the pile head and the results are presented as 3 traces in a diagram (Fig. 3). To proof the quality of testing the 3 traces should be similar. The time (T) between the start of the hammer blow and the time of arrival of the reflection from the pile toe is measured. The pile length (L) is calculated with: L = c.T/2 when the stress wave velocity ( c ) is known To present the measuring results the time axis (t) is scaled to a length (depth) (l) axis with L= c.t/2. Due to shaft friction the toe reflection might be of small magnitude. To make the reflection visible, the measured signal is amplified. To remove noise from the signals a filter value can be applied.

L

Depth

Time

T= 2L/c

c

v

Time

Pile toe reflection reflectio

Hammer blow

Figure 1.2

Figure 1.1. Introducing hammer blows


Pile integrity testing analysis is based on the one dimensional stress wave theory. Reflections generated by impedance changes (discontinuities) travel to the pile top and are recorded and analyzed The impedance Z is defined as Z = A √ (E. ρ) In which

A = cross sectional area E = modulus of elasticity ρ = density

Any change in A, E, or ρ or a combination of them will generate a reflection from an impedance change (discontinuity). Potential causes for reflections are:

Pile toe Dimensional changes Soil inclusions Cracks Joints Variations in concrete quality Variations in soil layers Overlap of reinforcement (heavily reinforced piles).

11.5

[mm/s]

12.2

[m]

14.2 12.2

13.9 12.1

0 2 4 6 8 10 12 14 16 [m]1/3SITE 14032-accPILE 76

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 1.3. Result of three hammer blows

Blow Toe reflection

Wave velocity

Length axis

Indication pile length

Peak value blow

Amplification

Filter value

Zero drift correction

type

Site name

Pile ID Program Version Date of

Testing


Limitations of sonic integrity testing are:

Minor impedance changes are not detected Gradually increasing and decreasing pile diameters can not be detected Curved pile shapes can not be detected Small soil inclusions are not detected Local loss of reinforcement cover can not be detected Thickness of debris layer at pile toe can not be detected


2 Discontinuities in the pile shaft. Reflections will occur when discontinuities in the pile cross section or pile material properties are present (Fig.2.1) From the time of occurrence of a reflection the location of the discontinuity can be determined. The sign of the reflection indicates an increase or decrease in the pile cross section or an increase or decrease in pile material quality.

a

Depth

Time t= 2a/c

v

Time

Reflection bulb

a

Depth

Time

t= 2a/c

c

v Time

Reflection necking

Figure 2.1.


3 Signal processing 3.1 Signal amplification Shaft friction (see Appendix) has a strong influence on the pile integrity testing results and has to be taken into account. The signals measured on the pile top are amplified linearly or exponentially to overcome the reduction of the amplitude of the stress wave while traveling to the pile toe and back.

Depth

Time

TimeVelocity at Pile Top

Signal before amplification

Signal after amplification

1x 1-1000x

Time

Depth

Time

TimeVelocity at Pile Top

Signal before amplification


1x 1-1000x

Time

11.8

[mm/s]

18.7

[m]

9.9 18.7

10.2 18.7

0 2 4 6 8 10 12 14 16 18 20 [m]1/3SITE NO_SITEPILE #3_11517

srV2.8

4000[m/s] f:1 Fri Oct 17, 1986

exp:1

Signal before amplification 11.8

[mm/s]

18.7

[m]

9.9 18.7

10.2 18.7

0 2 4 6 8 10 12 14 16 18 20 [m]1/3SITE NO_SITEPILE #3_11517

srV2.8

4000[m/s] f:1 Fri Oct 17, 1986

exp:200


Toe reflection

visible

Figure 3.1, Signal amplification

Toe reflection not visible


3.2 Signal smoothing Discontinuities near the pile top or extending reinforcement will generate high frequency reflections, which mask the global shape of the measured signals. To view the global shape of the signals smoothing is applied. It should be taken into account that by smoothing information about discontinuities is lost. For the final interpretation and presentation of the signals, smoothing should be reduced to a minimum . 10.3

[mm/s]

15.5

[m]

9.4 15.5

11.0 15.5

0 2.5 5 7.5 10 12.5 15 17.5 20 [m]1/3SITE NO_SITEPILE 2K

srV7.0

4000[m/s] f:1 Wed Mar 28, 2001

exp:20

Figure 3.2, Unsmoothed signal

Reflection of serious defect near

pile top

10.3

[mm/s]

15.5

[m]

9.4 15.5

11.0 15.5


srV7.0

4000[m/s] f:8 Wed Mar 28, 2001

exp:20

Figure 3.3, Smoothed signal

Information about defect

got lost


4 The Characteristic Signal To differentiate between a change in soil resistance and a pile discontinuity, the SIT signal from a test pile is compared to a "characteristic signal" deemed to be representative of similar piles in similar soil conditions on site. (The characteristic signal can either be an average of a number of piles on site or the SIT signal of a reference pile chosen prior to testing.) If the test signal is different than the characteristic signal, then any impedance changes are due to the changing pile impedance and not characteristic of the site. Changes not found in the characteristic signal require further analysis to determine the cause.


Figure 4.1 , Example of piles constructed by a similar installation method, in the same ground conditions, of similar age and having the same length.

2.5

[mm/s]

17.4

[m]

3.2 17.4

3.6 17.3

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 4te

V6.14000[m/s] f:7

Thu Sep 16, 1999exp:50

2.8

[mm/s]

18.0

[m]

2.4 18.0

2.6 18.2

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 5te

V6.14000[m/s] f:7


3.5

[mm/s]

18.3

[m]

3.9 18.2

3.2 18.2

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 6te

V6.14000[m/s] f:7


2.3

[mm/s]

18.2

[m]

2.5 18.0

3.7 18.5

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 7te

V6.14000[m/s] f:7


1.7

[mm/s]

18.2

[m]

2.6 17.9

2.5 17.9

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 8te

V6.14000[m/s] f:7


2.4

[mm/s]

17.6

[m]

2.0 17.6

2.6 17.6

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 9te

V6.14000[m/s] f:7


2.5

[mm/s]

18.2

[m]

2.6 18.3

2.2 18.3

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 10te

V6.14000[m/s] f:7


2.1

[mm/s]

18.5

[m]

1.4 18.6

2.8 18.5

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 11te

V6.14000[m/s] f:7



2.5

[mm/s]

17.9

[m]

2.3 18.0

2.5 17.9

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 31te

V6.14000[m/s] f:7


1.6

[mm/s]

18.2

[m]

1.5 18.2

2.2 18.2

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 32te

V6.14000[m/s] f:7


1.3

[mm/s]

18.2

[m]

0.8 18.5

1.3 18.3

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 33te

V6.14000[m/s] f:7


2.2

[mm/s]

17.7

[m]

2.0 17.7

2.1 17.7

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 34te

V6.14000[m/s] f:7


1.6

[mm/s]

18.3

[m]

1.6 18.3

1.5 18.3

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 35te

V6.14000[m/s] f:7


1.9

[mm/s]

18.5

[m]

1.9 18.5

1.5 18.5

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 37te

V6.14000[m/s] f:7


1.4

[mm/s]

17.7

[m]

1.4 17.6

1.3 17.9

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 38te

V6.14000[m/s] f:7


1.7

[mm/s]

17.7

[m]

1.0 17.6

1.1 17.6

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]4/3

PILE 39te

V6.14000[m/s] f:7


Pile deviating from Characteristic Signal

Figure 4.2, Example of pile with signal deviating from the characteristic signal


3.8

[mm/s]

17.9

[m]

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]1/3SITE SITDEMOWINPILE 29

teV6.1

4000[m/s] f:7 Thu Sep 16, 1999

exp:20

Characteristic Signal

Signal of pile 29

Figure 4.3 , Example of a SIT Signal corresponding with the characteristic signal

1.4

[mm/s]

17.7

[m]

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 [m]1/3SITE SITDEMOWINPILE 38

teV6.1

4000[m/s] f:7 Thu Sep 16, 1999

exp:20

Characteristic Signal

Signal of pile 38

Figure 4.4, Example of a SIT Signal deviating from the characteristic signal1 1 1


5 Interpretation Guidelines • Test at least 20 % of the piles on site, with a minimum of 10 at several locations on a site. • Never be satisfied testing only piles which appear suspect. • Determine the characteristic signal for piles of the same type on a site. The characteristic signal

from the site or group average should be compatible with that for other piles of the same type, and should generally correspond with the majority of piles tested on a particular site. The characteristic signal can be established intuitively or made by averaging a number of pile results together, excluding any piles which deviate from the norm.

• Compare the characteristic signal with the available soil data. • Try to understand the causes of deviations—most often a change in pile cross section caused by

soft layers, fill materials, voids in ground, old foundation bases, entry into hard layers, casing lengths, or deliberate pile base enlargements. Note: If a pile enters a rock material, damping will be very high because of greatly increased shaft friction and this will show as an apparent increase in cross section and there will be no reaction from the pile toe.

• If possible, try to determine the pile length from the characteristic signal. • Carry out individual pile interpretation using all individual signals. • Flag as "suspect" signals with important deviations from normal. • Determine the level and type of deviation from normal and physically examine the pile.

Note: Be aware that three dimensional effects will influence the signals for pile diameters larger than 0.4m.


6 Interpretation For interpretation six classes are distinguished Class 1 Pile OK Class 2, Pile head problem Class 3, Discontinuity reducing the nominal impedance of the pile shaft (The nominal impedance represents normally the impedance at the pile top) Class 4, Pile seems too short Class 5, Pile seems too long Class 6, Deviating or no pile toe reflection Examples of results for each class are presented below

11.5

[mm/s]

12.2

[m]

14.2 12.2

13.9 12.1

0 2 4 6 8 10 12 14 16 [m]2/3SITE 14032-accPILE 76

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:10

Figure 6.1. Class 1. Pile good, length OK,


12.2

[mm/s]

12.6

[m]

13.6 12.6

13.1 12.6

0 2 4 6 8 10 12 14 16 [m]2/3SITE 14032-accPILE 36

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 6.2 Pile head problem, Class 2

Possible reason, crack, necking, low material quality.

8.1

[mm/s]

12.1

[m]

8.7 12.1

8.2 12.1

0 2 4 6 8 10 12 14 16 [m]1/3SITE 14032-accPILE 40

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 6.3. Class 3. Reflection from discontinuity at 1.5 m from the pile head.

Possible cause, crack, necking, low material quality.


10.0

[mm/s]

9.9

[m]

9.4 9.9

8.9 9.9

0 2 4 6 8 10 12 14 16 [m]2/3SITE 14032-accPILE 19

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 6.4. Class 4. Pile too short, reflection at 10m, should be 12m

Possible cause: made too short, broken, heavily cracked or serious necking at 10 from the top.

9.9

[mm/s]

13.1

[m]

14.9 13.1

13.2 13.0

0 2 4 6 8 10 12 14 16 [m]2/3SITE 14032-accPILE 81

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 6.5. Class 5. Toe reflection too late.

Possible cause: Pile made too long, lower material quality, pile younger than other piles,


7.0

[mm/s]

12.2

[m]

12.4 12.2

12.3 12.2

0 2 4 6 8 10 12 14 16 [m]2/3SITE 14032-accPILE 93

teV7.2

3800[m/s] f:3 Mon Feb 09, 2004

exp:20

Figure 6.6. Class 6. No clear toe reflection

Possible cause, large friction, deviating material properties at pile toe


7 Pile preparation Driven piles: Trim pile top if cracked Cast-in-situ piles: Remove soft pile top to sound concrete All piles: Cut away overbreak or overspill at ground level. Pile tops must be clean, though not necessarily flat and smooth

Figure 7.1 Example of pile top with sound concrete


8 Placing the Sensor Place the accelerometer with a small amount of Pietoplast to the top of the pile, ensuring that the sensor is vertical. Choose a relatively flat, off-centre spot and press with hand weight. (The Pietoplast helps transfer the signal and damp the resonance from pile to sensor.) Do not position the sensor close to the location of impact. Note: If the pile top is not easily accessible, the accelerometer can also be attached to the pile shaft.

Figure 8.1 Example of placement of transducer

Pietoplast

Acceleration Sensor

Figure 8.2 Example of placement of transducer


9 Reliable Testing The requirements for reliable testing include:

test at least 20 % of piles on site, with a minimum of 10 for the highest confidence, test all piles use proper hammer with a blow length of 1 m or less (see Appendix). record at leas 3 similar signals per pile process all signals in a similar way, with minimal filtering and optimal amplification determine characteristic signal check characteristic signal with soil investigation data (see Appendix) check signals that deviate from the characteristic signal perform qualitative interpretation use all available information Follow the guidelines

If necessary, perform quantitative interpretation with integrity testing signal matching using TNOWAVE (SITWAVE). If there are still doubts, excavate the pile, do coring, or conduct a load test (DLT, STN, SLT) or reject the pile.


10 References

1. Ulrich, G., Stocker, M., (1983), Integritätsuntersuchung an präparierten Betonpfähle, (Integrity Testing Research on Prepared Concrete Piles), Symposium Messtechnik im Erd- und Grundbau, Germany

2. Reiding, F.J., Middendorp, P., P.J. van Brederode, 1984, A digital approach to sonic pile

testing. 2nd International Conference on Stress Waves, The Hague, Netherlands, Balkema

3. Fleming, K, Reidng, F.J., Middendorp, P, 1985, Faults in cast in place piles and their detection. Proceedings of the Second International Conference on Structural Faults and Repair, The Institution of Civil Engineers, Westminster London.

4. Middendorp, P., Reiding, F.J., 1988, Determination of Discontinuities in Piles by TNO Integrity

and Signal Matching Techniques, 3rd International Conference on The Application of Stress Wave Theory on Piles, Ottawa, Canada

5. Starke, W.F., Janes, M.C., 1988, Accuracy and Reliability of Low Strain Testing, 3rd

International Conference on The Application of Stress Wave Theory on Piles, Ottawa, Canada

6. England, M., 1991, A guide to Low-Strain Integrity Testing , Report Cementation Piling and

Foundations Ltd, Field Data Acquisition Department, United Kingdom.

7. ASTM D5882-96 (1996) Standard Test Method for Low Strain Integrity Testing of pile, American Society for Testing and Materials

8. Turner M.J., (1997) Integrity Testing In Piling Practice, CIRIA Report 144 9. German Society for Geotechniques (DGGT) (1998), Recommendations for Static and Dynamic

Pile Tests ,

10. Chow, Y.K., Phoon, K.K., Chow, W.F., (2004) Three-Dimensional Stress Wave Analysis of Pile Integrity Tests, 7th International Conference on The Application of Stress Wave Theory on Piles, Kuala Lumpur, Malaysia

11. Seidel, J.P., Tan, S.K.,(2004), Elimination of the Rayleigh Wave Effect on Low Strain

Integrity Test Results. 7th International Conference on The Application of Stress Wave Theory on Piles, Kuala Lumpur, Malaysia


11 Appendix, Influence of hammer blow width Reference:: Profound report 01-PROF-B0566, SIT User’s day. 21 March 2001 (In Dutch)

SIT Results Reference Pile 9, Hammer blow width 0.75m

15.2

[mm/s]

16.3

[m]

18.6 16.3

16.7 16.3

0 2 4 6 8 10 12 14 16 18 [m]1/3SITE NO_SITEPILE 9

teV6.1

4200[m/s] f:3 Wed Mar 28, 2001

exp:20

Hammer blow width

0.75 m

SIT Result, Pile 2 , Hammer blow width 0.75m. Defect visible

10.3

[mm/s]

15.5

[m]

9.4 15.5

11.0 15.5


srV7.0

4000[m/s] f:1 Wed Mar 28, 2001

exp:20

Reflection defect

Pile 9, Reference pile, no defects Pile 2, with 49% cross section reduction cut


9.9

[mm/s]

16.2

[m]

10.2 16.3

10.4 16.2


srV6.1

4000[m/s] f:1 Wed Mar 28, 2001

exp:5

SIT Result, Pile 2 , Hammer blow width 2 m, Defect not visible!!!!

5.2

[mm/s]

15.9

[m]

5.2 15.8

5.8 15.8


teV6.1

4000[m/s] f:1 Wed Mar 28, 2001

exp:5

SIT Result, Reference Pile 9 , Hammer blow width 2 m,

Blow length 2m


12 Appendix, Influence of soil The shaft friction reduces the amplitude of the stress wave. Stiff soil layers cause a higher damping than soft layers. This means that the detection of the pile toe reflection (pile length) has a limit. For very stiff clays only the first 5 m from the pile head yield information about the pile shaft. Pile toe reflections from 80 m have been detected in soft soils. In the Netherlands the pile toe reflection can be measured up to 35m.

Soil layers changes act as discontinuities along the pile shaft and resulting reflections are visible in the signals. For this reason it is essential that soil investigation results are included in the interpretation. Reflections from a greater depth are weaker and to make them visible the signals are amplified exponentially with time.

7.0

[mm/s]

18.3

[m]

8.9 18.4

9.2 18.4

0 2 4 6 8 10 12 14 16 18 20 [m]1/3SITE PILE_1PILE #1_11114

noV2.8

4000[m/s]av:0

f:1 Tue Oct 14, 1986

exp:1

Precast pile tested before driving

11.8

[mm/s]

18.7

[m]

9.9 18.7

10.2 18.7

0 2 4 6 8 10 12 14 16 18 20 [m]1/3SITE PILE_1PILE #3_11517

srV2.8

4000[m/s]av:0

f:5 Fri Oct 17, 1986

exp:100

Precast pile tested afer driving

Influence from soil layers

Pile toe reflection, no amplification

Pile toe reflection, amplification 100x


Figure Test result cast in situ pile

22.6

[mm/s]

12.8

[m]

25.0 12.8

25.1 12.8

0 2 4 6 8 10 12 14 [m]1/3SITE PREFAB ZOETERMEERPILE 78

teV6.1

4200[m/s]av:0

f:3 Mon Sep 27, 1999

exp:50

. Integrity testing results of cast in situ pile

. Integrity testing results and relation with soil investigation result. (CPT)

Documents

Method statement Sonic Integrity Testing.pdf