Chapter 4. Dynamic and Static Pile Load Test Data - Design and Construction of Driven Pile Foundations – Lessons Learned on The Central Artery_Tunnel Project.pdf

8/10/2019 Chapter 4. Dynamic and Static Pile Load Test Data - Design and Construction of Driven Pile Foundations Lessons

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Static resistance is formulated from an idealized elastoplastic soil model, where the quake parameterdefines the displacement at which the soil changes from elastic to plastic behavior. The dynamicresistance is formulated using a viscous damping model that is a function of a damping parameter and thevelocity.

First, the forces and accelerations acting on the actual pile during initial impact are recorded with a straingauge and accelerometer mounted at the pile head. The measured acceleration is used as input to thepile model along with reasonable estimates of soil resistance, quake, and damping parameters. Theforce-time signal at the pile head is calculated using the model and is compared to the measured

force-t ime signal. The soil-resistance distribution, quake, and damping parameters are subsequentlymodified until agreement is reached between the measured and calculated signals. An example of acomparison between a measured and calculated force signal from one of the test piles is shown in figure20. Once an acceptable match is achieved, the solution yields an estimate of ultimate static capacity, thedistribution of soil resistance along the pile, and the quake and damping parameters.

Figure 20. Example of CAPWAP signal matching, test pile 16A1-1.(33)

Static Load Test Methods

Static load tests were performed during the test phase of each contract to verify the design assumptions

and load-carrying capacity of the piles. Telltale rods INSTALLED at various depths within the pileswere used to evaluate the load transfer behavior of the piles with regard to the surrounding soil andbearing stratum. The static tests were carried out in general accordance with project specifications

section 940.62.B.4,(14)Short Duration Test, and the ASTM's D1143-81, which is entitled "Standard Test

Method for Piles Under Static Axial Compression Load."(36)

The static load test data presented in thisreport were obtained from the project files. (See references 37 through 50.)

Static loads wereAPPLIED and maintained using a hydraulic jack and were measured with a load cell.A typical load test arrangement is shown in figure 21. Reaction to the jack load is provided by a steelframe that is attached to an array of steel H-piles located at least 3 m away from the test pile. Pile headdeflections were measured relative to a fixed reference beam using dial gauges. Telltale measurementswere made in reference to the pile head or the reference beam using dial gauges. Pile head and telltaledeflection data were recorded for each loading increment.

pter 4. Dynamic and Static Pile Load Test Data - Design and Constru... http://www.fhwa.dot.gov/publications/research/infrastructure/geotechni...

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Figure 21. Typical stat ic load test arrangement

showing instrumentation.(51)

An excerpt from the loading procedures for short-duration load test section 940.62 is given below(14)

:

Apply 25 percent of the allowable design load every one-half hour up to the greater of the following:

[two alternatives are described; the most general is 200 percent of the design load]. Longer time

increments may be used, but each time increment should be the same. At 100 percent of the design

load, unload to zero and hold for one-half hour; then reload to 100 percent and continue 25 percent

incremental loads. At 150 percent, unload to zero and hold for one-half hour; then reload to 150percent and continue 25 percent incremental loads. In no case shall the load be changed if the rate of

settlement is not decreasing with time.

a.

At the maximum applied load, maintain the load for a minimum of one hour and until the settlement

(measured at the lowest point of the pile at which measurements are made) over a one-hour period is

not greater than 0.254 mm (0.01 inch).

b.

Remove 25 percent of the load every 15 minutes until zero load is reached. Longer time increments

may be used, but each shall be the same.

c.

Measure rebound at zero load for a minimum of one hour.d.

After 200 percent of the load has been applied and removed, and the test has shown that the pile hasadditional capacity, i.e., it has not reached ultimate capacity, continue testing as follows. Reload the

test pile to the 200 percent design load level in increments of 50 percent of the allowable design load,

allowing 20 minutes between increments. Then increase the load in increments of 10 percent until

either the pile or the frame reach their allowable structural capacity, or the pile can no longer support

the added load. If failure at maximum load does not occur, hold load for one hour. At maximum

achieved load, remove the load in four equal decrements, allowing 15 minutes between decrements.

e.

The capacity of the test piles was selected as the greater capacity defined by two failure criteria. Thefirst criteria establishes the allowable design capacity as "50 percent of the applied test load whichresults in a net settlement of the top of the pile of up to 1.3 cm, after rebound, for a minimum of one hourat zero load." The second criterion uses Davisson's criteria as described below.

The Davisson offset limit load criterion was used on the project to define the ultimate capacity, or failure,

of the test piles.(52)

The ultimate load is interpreted as the point at which the displacement of the pilehead meets a limit that is offset to the elastic compression line of the pile. For piles less than 61 cm in


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diameter, the limit is defined by the following linear relationship:

(1)

where,

Sf = Movement of pile top (cm).

D = Pile diameter or width (cm).

Se = Elastic compression of total pile length (cm).

The elastic compression in this case refers to the pile deflection that would occur if 100 percent of theapplied load was transferred to the toe of the pile (i.e., zero shaft friction), and is given by the followingequation:

(2)

where,

Q = Applied load.

L = Total length of pile.

A = Cross-sectional area of the pile.

E = Modulus of elasticity of the pile.

The average load in the pile at the midpoint between two telltale locations was estimated from the elasticshortening of the pile using the following equation:

(3)

where,

A = Area of pile.

E = Modulus of elasticity of the pile.

D1 = Deflection at upper telltale location.

D2 = Deflection at lower telltale location.

L = Distance between the upper and lower telltale locations.

Both equations 2 and 3 require the modulus of elasticity of the pile. The specifications require that theelastic modulus be determined via compression tests performed on representative concrete samples(ASTM C 469-87a). However, this method is not really applicable to the concrete-filled steel pipe piles. It

was common practice on the CA/T project to use the upper telltale and pile head deflections to calculatethe modulus of the pile using equation 3. This approach was justified by assuming that any preaugeringthat was performed prior to pile installation would reduce the shaft friction, especially near the pile head.In some cases, the elastic modulus of the PPC piles was determined from a combination of telltale andcompression test data using engineering judgment.

LOAD TEST RESULTS

More than 160 dynamic tests were performed on the selected contracts to evaluate pile capacity duringboth the testing and production phases. Of these 160 tests, the results of 28 tests are presented in thisreport because they correspond to static load tests on 15 piles. Information about each pile tested isshown in table 7, and pile driving information is presented in table 8.

Table 7. Summary of pile and preauger information.

Test Pile Name Contract Pile Type Preauger Depth (m) Preauger Diameter (cm)


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Test Pile Name Contract Pile Type Preauger Depth (m) Preauger Diameter (cm)

ET2-C2 C07D1 41-cm PPC 0 NA1

ET4-3B C07D1 41-cm PPC 0 NA

375 C07D2 41-cm PPC 9.1 45.7

923 C07D2 41-cm PPC 24.1 45.7

I90 EB SA C08A1 41-cm PPC NI2 40.6

14 C08A1 41-cm PPC 27.4 40.6

12A1-1 C09A4 31-cm PPC 30.5 45.7

12A2-1 C09A4 31-cm PPC 32.0 45.7

16A1-1 C09A4 41-cm PPC 30.5 45.7

I2 C09A4 41-cm PPC 30.5 40.6

3 C09A4 41-cm pipe 24.4 40.6

7 C09A4 41-cm pipe 24.4 40.6

IPE C19B1 32-cm pipe 7.6 30.5

IPW C19B1 32-cm pipe 12.2 30.5NS-SN C19B1 41-cm PPC 8.2 40.6

Notes:

1. NA = Not applicable.

2. NI = Data not identified.

Table 8. Summary of p ile driving information.

Test Pile

Name

Test

Type1

Hammer

Type2

Embedment

Depth (m)

Minimum

Transferred

Energy (kN-m)

Recorded

Penetration

Resistance

(blows/2.5 cm)

Permanent

Set (cm)

ET2-C2 EOD I 47.5 NI3 7,7,7 0.36

34DR 58.0 11 0.23

ET4-3B EOD II 41.1 NI 8,7,10 0.25

NI 50.8 14 0.18

375 EOD II 16.8 50.2 12,13,39 0.08 7DR 54.2 > 12 < 0.20

923 EOD II 32.9 46.1 7,7,7 0.36

7DR 51.5 > 8 0.33

I90 EBSA

EOD III 46.6 25.8 12,10,10 0.25

1DR 25.8 13 0.20

14 EOD III 45.4 25.8 10,10,16 0.15

1DR 23.1 21 0.13

12A1-1 EOD III 41.8 20.7 4,4,5 0.51 1DR 28.6 > 7 > 0.36

12A2-1 EOD III 38.7 15.3 3,4,4 0.64

1DR 18.6 8 0.33


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Test Pile

Name

Test

Type1

Recorded Penetration Resistance (blows/2.5

cm)

Ultimate Capacity2(kN)

Shaft Toe Total

7DR > 8 (1,664) (1,708) (3,372)

I90 EB SA EOD 12,10,10 934 712 1,646

1DR 13 (1,156) (1,112) (2,268)

14 EOD 10,10,16 (449) (2,237) (2,687)1DR 21 (894) (1,926) (2,820)

12A1-1 EOD 4,4,5 685 979 1,664

1DR > 7 (1,103) (743) (1,846)

12A2-1 EOD 3,4,4 316 845 1,161

1DR 8 1,023 431 1,454

16A1-1 EOD 6,7,7 956 1,063 2,015

3DR 11 (983) (876) (1,859)

I2 EOD 4,4,4 400 1,130 1,530

1DR 5 1,526 489 2,015

3 EOD 11,12,14 (983) (2,086) (3,069)

1DR 30 (1,228) (1,690) (2,918)

7 EOD 11,11,11 (80) (2,740) (2,820)

3DR > 16 (983) (1,984) (2,962)

IPE EOD 5,5,5 489 1,334 1,824

1DR 7 645 1,535 2,180

IPW EOD 5,5,5 778 1,223 2,002

1DR 8 1,290 1,468 2,758

NS-SN EOD 8,15,16 (583) (1,806) (2,389)

7DR 26 (858) (1,935) (2,793)

Notes:

1. EOD = End of initial driving, #DR = # days before restrike.

2. Values shown in parentheses denote conservative values.


Many of the capacities are listed in parentheses, which indicates that the values are most likely

conservative (i.e., the true ultimate capacity is larger). It is recognized in the literature that dynamiccapacities can be underestimated if the hammer energy is insufficient to completely mobilize the soil

resistance.(53)

Specifically, research has shown that blow counts in excess of 10 blows per 2.5 cm may

not cause enough displacement to fully mobilize the soil resistance.(53,54)As shown in table 8, the majorityof the piles during restrike exceeded 10 blows per 2.5 cm and are thus likely to be lower than the trueultimate capacity of the piles.

The conservativeness of the CAPWAP capacities in certain piles can be illustrated by comparing the loadversus displacement curve at the toe evaluated with CAPWAP to that obtained in a static load test. Thetoe load-displacement curves from test pile 16A1-1 are shown in figure 22. Blow counts of seven blowsper 2.5 cm were recorded for this pile during initial driving. The static load test data shown in figure 22were extrapolated from the telltale data. As shown in figure 22, the maximum resistance mobilized by thepile toe from CAPWAP is approximately 1060 kN. At least 1670 kN were mobilized in the static load test;however, the ultimate value is actually higher since failure was not reached.


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Figure 22. Load-displacement curves for pile toe, test pile 16A1-1.

Soil quake and damping parameters obtained from the CAPWAP analyses are summarized in table 10. Itis often assumed that the quake values are approximately 0.25 cm in typical wave equation analyses. Thetoe quake values in this study range from 0.25 to 1.19, with an average of 1.6 cm. Large toe quake

values on the order of up to 2.5 cm have been observed in the literature.(55,56)

However, the quake values

in this study appear to be within typical values.(57)

Table 10. Summary of CAPWAP soil parameters.

Test Pile Name Test Type1

Quake (cm) Damping (s/m)

Shaft Toe Shaft Toe

ET2-C2 EOD

34DR 0.43 0.84 0.72 0.23

ET4-3B EOD

- 0.56 0.36 0.89 0.82

375 EOD 0.64 1.19 0.33 0.07

7DR 0.51 0.86 0.23 0.20

923 EOD 0.38 1.14 0.72 0.43

7DR 0.23 0.81 0.46 0.43

I90 EB SA EOD 0.13 0.89 0.16 0.561DR 0.38 0.56 0.69 0.69

14 EOD 0.25 0.76 0.39 0.43

1DR 0.25 0.41 0.59 0.43

12A1-1 EOD

1DR 0.38 0.56 0.75 0.16

12A2-1 EOD

1DR 0.25 0.51 0.49 0.33

16A1-1 EOD

3DR 0.25 0.10 1.41 1.15

I2 EOD 0.25 0.51 0.75 0.26

1DR 0.13 0.25 0.46 0.10


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Test Pile Name Test Type1

Quake (cm) Damping (s/m)

3 EOD 0.48 0.64 0.13 0.10

1DR 0.15 0.56 0.33 0.10

7 EOD 0.23 0.64 0.46 0.10

3DR 0.25 0.36 0.52 0.10

IPE EOD 0.25 0.69 0.62 0.23

1DR 0.38 0.89 0.59 0.23

IPW EOD 0.38 0.64 0.43 0.23

1DR 0.25 0.36 0.59 0.20

NS-SN EOD 0.30 0.91 0.52 0.33

7DR 0.13 0.46 0.72 0.49

Notes:

1. EOD = End of initial driving, #DR = # days before restrike.

2. s/m = seconds/meter.

Comparison of CAPWAP Data

A comparison between the EOD and BOR CAPWAP capacities is shown in figure 23. The line on thefigure indicates where the EOD and BOR capacities are equal. Data points that are plotted to the left ofthe line show an increase in the capacity over time, whereas data that fall below the line show adecrease in capacity. In the four piles (12A2-1, I2, IPE, and IPW) where the soil resistance was believedto be fully mobilized for both the EOD and BOR, the data show an increase of 20 to 38 percent occurringover 1 day. The overall increase in capacity is attributed to an increase in the shaft resistance.

Figure 23. CAPWAP capacities at end of initial driving (EOD) and beginning of restrike (BOR).

Static Load Test Data

Static load tests were performed on 15 piles approximately 1 to 12 weeks after their installation. The testresults are summarized in table 11. In general, two types of load deflection behavior were observed in thestatic load tests (figures 24 through 27).

Table 11. Summary of static load test data.

Test Pile

Name

Time After Pile Installation

(days)

Maximum Applied

Load (kN)

Maximum Pile Head

Displacement (cm)


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Test Pile

Name

Time After Pile Installation

(days)

Maximum Applied Load

(kN)

Maximum Pile Head

Displacement (cm)

ET2-C2 13 3,122 1.7

ET4-3B 20 3,558 2.4

375 15 3,447 1.6

923 33 3,447 2.4

I90 EB SA 23 3,781 1.6

14 6 3,105 2.2

12A1-1 30 1,512 1.4

12A2-1 24 1,014 0.5

16A1-1 17 3,612 2.6

I2 6 3,558 1.7

3 9 3,959 2.4

7 10 3,167 2.0

IPE 84 2,384 1.3IPW 10 2,891 4.1

NS-SN 30 2,535 1.3

Test pile 12A1-1 (figure 24) represents a condition where the axial deflection of the pile is less than thetheoretical elastic compression (assuming zero shaft friction). This pile was loaded to 1,557 kN in fivesteps and at no point during the loading did the deflection exceed the estimated elastic compression ofthe pile. This behavior is attributed to shaft friction, which reduces the compressive forces in the pile andlimits the settlement. The significant contribution of shaft friction is also apparent in the load distributioncurve shown in figure 25, which shows the load in the pile decreasing with depth. This behavior is typicalof test piles ET2-C2, ET4-3B, I90-EB-SA, 12A1-1, 12A2-1, I2, and 3.

Figure 24. Deflect ion of pile head during static

load testing of pile 12A1-1.

Figure 25. Distr ibution of load in pile 12A1-1.

Test pile 14 (figure 26) represents a condition where the axial deflection is approximately equal to thetheoretical elastic compression. This suggests that more of the applied loads are being distributed to thetoe of the pile with less relative contribution of shaft friction. This is apparent in figure 27, which showsnegligible changes in the load within the pile with depth. This behavior is typical of test piles 375, 923, 14,16A1-1, 7, IPE, and IPW.


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Figure 26. Deflection of pile head dur ing

static load testing of pile 14.

Figure 27. Distr ibution of load in pile 14.

Of the 15 static load tests, only one test pile (IPW) was loaded to failure according to Davisson's criteria.These data are shown in figures 28 and 29. This pile showed a significant increase in the deflection at

approximately 2,580 kN, subsequently crossing the Davisson's line at approximately 2,670 kN at adisplacement of around 2.5 cm. The telltale data obtained near the toe of the pile indicated that the pilefailed in plunging.

Figure 28. Deflection of pile head dur ing

stat ic load testing of pile IPW.

Figure 29. Distribution of load in pi le IPW.

All test piles achieved the required ultimate capacities in the static load tests. The required ultimatecapacities were determined by multiplying the allowable design capacity by a factor of safety of at least2.0, as specified in the project specifications. A slightly higher factor of safety of 2.25 was used incontract C19B1. Three of the 15 static tests did not demonstrate that 100 percent of the design load wastransferred to the bearing soils. Two of the piles (12A1-1 and 12A2-1) could not transfer the load to thebearing soils because of the high skin friction (figures 24 and 25). Test pile I2 could not demonstrate loadtransfer because the bottom telltale was not functioning.

Comparison of Dynamic and Stati c Load Test Data

The capacities determined by CAPWAP and from the static load tests are summarized in table 12, alongwith the required ultimate capacities. Of the 15 test piles, only one pile (IPW) was loaded to failure in astatic load test. Likewise, only four BOR CAPWAP analyses and eight EOD CAPWAP analyses mobilizedthe full soil resistance. This means that the true ultimate capacity of the majority of the piles tested wasnot reached, and this makes a comparison of static load test and CAPWAP results difficult.

Test pile IPW was brought to failure in the static load test. Coincidentally, it is anticipated that theCAPWAP capacities for this pile also represent the fully mobilized soil resistance because of the relatively


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low blow counts (i.e., < 10) observed during driving. Based on a comparison of all data for pile IPW, itscapacity increased by approximately 35 percent soon after installation, yielding a factor of safety ofapproximately 3.0. Note that this pile was preaugered to a depth of approximately half of the embedmentdepth. The capacity of 2,669 kN determined in the static load test is slightly less than the restrikecapacity of 2,758 kN. However, this difference is partly attributed to modifications that were made to thepile after the dynamic testing, but prior to static testing. These modifications included removal of 0.6 m ofoverburden at the pile location and filling of the steel pipe pile with concrete, both of which woulddecrease the capacity of the pile measured in the static load test.

Table 12. Summary of dynamic and static load test data.

Test Pile

Name

Required

Allowable

Capacity (kN)

Required

Minimum

Factor of

Safety

Required

Ultimate

Capacity (kN)

CAPWAP Ultimate

Capacity1(kN)

Ultimate Capacity

From Stat ic Load

Test (kN)

EOD BOR

ET2-C2 1,379 2.00 2,758 NI2 (3,247) (3,122)

ET4-3B 1,379 2.00 2,758 NI (3,719) (3,558)

375 1,379 2.00 2,758 (4,226) (4,759) (3,447)

923 1,379 2.00 2,758 2,571 (3,372) (3,447)

I90 EBSA

1,379 2.00 2,758 1,646 (2,268) (3,781)

14 1,379 2.00 2,758 (2,687) (2,820) (3,105)

12A1-1 756 2.00 1,512 1,664 (1,846) (1,512)

12A2-1 507 2.00 1,014 1,161 1,454 (1,014)

16A1-1 1,245 2.00 2,491 2,015 (1,859) (3,612)

I2 1,245 2.00 2,491 1,530 2,015 (3,558)

3 1,583 2.00 3,167 (3,069) (2,918) (3,959)

7 1,583 2.00 3,167 (2,820) (2,962) (3,167)

IPE 890 2.25 2,002 1,824 2,180 (2,384)

IPW 890 2.25 2,002 2,002 2,758 2,669

NS-SN 1,112 2.25 2,504 (2,389) (2,793) (2,535)

Notes:

1. Capacities shown in parenthesis denote values that are conservative (dynamic load tests) or wherefailure was not achieved (static load tests).


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Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDTScheduled Update: Archive - No Update

Technical Issues: [email protected]

Topics: research, infrastructure, geotechnical

Keywords: research, infrastructure, geotechnical, Driven piles, heave, research, infrastructure, geotechnical, CAPWAP, static load test,Boston tunnel

TRT Terms: research, infrastructure, geotechnical, Pile foundations, research, soil engineering, structural engineering, Tunneling

Updated: 04/23/2012


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Documents

Chapter 4. Dynamic and Static Pile Load Test Data - Design and Construction of Driven Pile Foundations – Lessons Learned on The Central Artery_Tunnel Project.pdf