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CHAPTER 4. DYNAMIC AND STATIC PILE LOAD TEST DATA
This chapter presents the methodology and results of dynamic and static pile load test data for the selected contracts. At least two static load tests were performed per contract, and the results of 15 tests are presented herein. The Pile Driving Analyzer® (PDA) was also used on these piles for comparison, and analyses were performed periodically during production pile installation. Issues related to design loads and load test criteria are discussed, including factors of safety and load transfer requirements. A comparison is made between the results of the static load tests and the CAse Pile Wave Analysis Program (CAPWAP®) analyses. The CAPWAP data suggest that the quake values generally exceed the values typically recommended in wave equation analyses. A review of the literature is presented to evaluate the significance of this finding. High blow counts recorded during the end of driving also suggest that the majority of the estimated pile capacities from CAPWAP are conservative.
LOAD TEST METHODS
Dynamic Load Test Methods
Approximately 160 dynamic pile load tests were performed to evaluate pile capacity, driving stresses, and hammer performance during the installation of test piles and production piles. The data presented in this report were obtained from project files. (See references 25, 26, 27, 28, 29, 30, 31, 32, 33, 34.)
The PDA was used to record, digitize, and processes the force and acceleration signals measured at the pile head. These signals were used to estimate static capacity using the Case Method, a simplified field procedure for estimating pile capacity, as well as the more rigorous CAPWAP. The dynamic load test results discussed in this report are primarily from the CAPWAP analyses. A description of the fundamentals of dynamic testing, including CAPWAP, is presented in Design and Construction of Driven Pile Foundations (Federal Highway Administration (FHWA) report no. FHWA-HI-97-013).(17) The dynamic testing was carried out in general accordance with project specifications section 940.62.C,(14) Dynamic Load Tests, and D4945-89 of the American Society for Testing and Materials (ASTM). D4945-89 is entitled "Standard Method for High Strain Testing of Piles."(35)
CAPWAP is an iterative curve-fitting technique where the pile response determined in a wave equation model is matched to the measured response of the actual pile for a single hammer blow. The pile model consists of a series of continuous segments and the total resistance of the embedded portion of the pile is represented by a series of springs (static resistance) and dashpots (dynamic resistance). Static resistance is formulated from an idealized elastoplastic soil model, where the quake parameter defines the displacement at which the soil changes from elastic to plastic behavior. The dynamic resistance is formulated using a viscous damping model that is a function of a damping parameter and the velocity.
First, the forces and accelerations acting on the actual pile during initial impact are recorded with a strain gauge and accelerometer mounted at the pile head. The measured acceleration is used as input to the pile model along with reasonable estimates of soil resistance, quake, and damping parameters. The force-time signal at the pile head is calculated using the model and is compared to the measured force-time signal. The soil-resistance distribution, quake, and damping parameters are subsequently modified until agreement is reached between the measured and calculated signals. An example of a comparison between a measured and calculated force signal from one of the test piles is shown in figure 20. Once an acceptable match is achieved, the solution yields an estimate of ultimate static capacity, the distribution of soil resistance along the pile, and the quake and damping parameters.
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 piles were used to evaluate the load transfer behavior of the piles with regard to the surrounding soil and bearing 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 this report were obtained from the project files. (See references 37 through 50.)
Static loads were applied 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 steel frame that is attached to an array of steel H-piles located at least 3 m away from the test pile. Pile head deflections were measured relative to a fixed reference beam using dial gauges. Telltale measurements were made in reference to the pile head or the reference beam using dial gauges. Pile head and telltale deflection data were recorded for each loading increment.
An excerpt from the loading procedures for short-duration load test section 940.62 is given below(14):
The capacity of the test piles was selected as the greater capacity defined by two failure criteria. The first criteria establishes the allowable design capacity as "50 percent of the applied test load which results in a net settlement of the top of the pile of up to 1.3 cm, after rebound, for a minimum of one hour at 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 pile head meets a limit that is offset to the elastic compression line of the pile. For piles less than 61 cm in diameter, the limit is defined by the following linear relationship:
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 the applied load was transferred to the toe of the pile (i.e., zero shaft friction), and is given by the following equation:
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 elastic shortening of the pile using the following equation:
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 the elastic 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 calculate the modulus of the pile using equation 3. This approach was justified by assuming that any preaugering that 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 and compression test data using engineering judgment.
LOAD TEST RESULTS
More than 160 dynamic tests were performed on the selected contracts to evaluate pile capacity during both the testing and production phases. Of these 160 tests, the results of 28 tests are presented in this report because they correspond to static load tests on 15 piles. Information about each pile tested is shown in table 7, and pile driving information is presented in table 8.
1. NA = Not applicable.
2. NI = Data not identified.
1. EOD = End of initial driving, #DR = # daysbefore restrike.
2. Hammer types: I = Delmag D 46-32, II = HPSI 2000, III = ICE 1070, IV = HPSI 1000, V = Delmag D 19-42, VI = Delamag D 30-32.
3. NI = Data not identified.
Dynamic Results and Interpretation
Dynamic tests were performed both at the end of initial driving of the pile (EOD) and at the beginning of restrike (BOR), typically 1 to 7 days (1DR, 7DR, etc.) after installation. In most cases, the dynamic tests were performed before the static load tests. Test piles ET2-C2 and ET4-3B, however, were dynamically tested during a restrike after a static load test was performed. The ultimate capacities of the 15 test piles as determined by CAPWAP analysis are summarized in table 9. The table lists when the test was performed, as well as the predicted shaft and toe resistance.
1. EOD = End of initial driving, #DR = # days before restrike.
2. Values shown in parentheses denote conservative values.
3. NI = Data not identified.
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 dynamic capacities 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 majority of the piles during restrike exceeded 10 blows per 2.5 cm and are thus likely to be lower than the true ultimate capacity of the piles.
The conservativeness of the CAPWAP capacities in certain piles can be illustrated by comparing the load versus displacement curve at the toe evaluated with CAPWAP to that obtained in a static load test. The toe load-displacement curves from test pile 16A1-1 are shown in figure 22. Blow counts of seven blows per 2.5 cm were recorded for this pile during initial driving. The static load test data shown in figure 22 were extrapolated from the telltale data. As shown in figure 22, the maximum resistance mobilized by the pile 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.
Soil quake and damping parameters obtained from the CAPWAP analyses are summarized in table 10. It is often assumed that the quake values are approximately 0.25 cm in typical wave equation analyses. The toe 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)
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 the figure indicates where the EOD and BOR capacities are equal. Data points that are plotted to the left of the line show an increase in the capacity over time, whereas data that fall below the line show a decrease in capacity. In the four piles (12A2-1, I2, IPE, and IPW) where the soil resistance was believed to be fully mobilized for both the EOD and BOR, the data show an increase of 20 to 38 percent occurring over 1 day. The overall increase in capacity is attributed to an increase in the shaft resistance.
Static Load Test Data
Static load tests were performed on 15 piles approximately 1 to 12 weeks after their installation. The test results are summarized in table 11. In general, two types of load deflection behavior were observed in the static load tests (figures 24 through 27).
Test pile 12A1-1 (figure 24) represents a condition where the axial deflection of the pile is less than the theoretical elastic compression (assuming zero shaft friction). This pile was loaded to 1,557 kN in five steps and at no point during the loading did the deflection exceed the estimated elastic compression of the pile. This behavior is attributed to shaft friction, which reduces the compressive forces in the pile and limits the settlement. The significant contribution of shaft friction is also apparent in the load distribution curve shown in figure 25, which shows the load in the pile decreasing with depth. This behavior is typical of test piles ET2-C2, ET4-3B, I90-EB-SA, 12A1-1, 12A2-1, I2, and 3.
Test pile 14 (figure 26) represents a condition where the axial deflection is approximately equal to the theoretical elastic compression. This suggests that more of the applied loads are being distributed to the toe of the pile with less relative contribution of shaft friction. This is apparent in figure 27, which shows negligible 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.
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 a displacement of around 2.5 cm. The telltale data obtained near the toe of the pile indicated that the pile failed in plunging.
All test piles achieved the required ultimate capacities in the static load tests. The required ultimate capacities were determined by multiplying the allowable design capacity by a factor of safety of at least 2.0, as specified in the project specifications. A slightly higher factor of safety of 2.25 was used in contract C19B1. Three of the 15 static tests did not demonstrate that 100 percent of the design load was transferred to the bearing soils. Two of the piles (12A1-1 and 12A2-1) could not transfer the load to the bearing soils because of the high skin friction (figures 24 and 25). Test pile I2 could not demonstrate load transfer because the bottom telltale was not functioning.
Comparison of Dynamic and Static Load Test Data
The capacities determined by CAPWAP and from the static load tests are summarized in table 12, along with the required ultimate capacities. Of the 15 test piles, only one pile (IPW) was loaded to failure in a static load test. Likewise, only four BOR CAPWAP analyses and eight EOD CAPWAP analyses mobilized the full soil resistance. This means that the true ultimate capacity of the majority of the piles tested was not 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 the CAPWAP capacities for this pile also represent the fully mobilized soil resistance because of the relatively low blow counts (i.e., < 10) observed during driving. Based on a comparison of all data for pile IPW, its capacity increased by approximately 35 percent soon after installation, yielding a factor of safety of approximately 3.0. Note that this pile was preaugered to a depth of approximately half of the embedment depth. The capacity of 2,669 kN determined in the static load test is slightly less than the restrike capacity of 2,758 kN. However, this difference is partly attributed to modifications that were made to the pile after the dynamic testing, but prior to static testing. These modifications included removal of 0.6 m of overburden at the pile location and filling of the steel pipe pile with concrete, both of which would decrease the capacity of the pile measured in the static load test.
1. Capacities shown in parenthesis denote values that are conservative (dynamic load tests) or where failure was not achieved (static load tests).
2. NI = Data not identified.