STATNAMIC LOAD TESTING OF HIGH CAPACITY MARINE FOUNDATIONS

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1 STATNAMIC LOAD TESTING OF HIGH CAPACITY MARINE FOUNDATIONS Mike Muchard, P.E., Applied Foundation Testing, Inc., Tampa, FL, USA STATNAMIC load testing augmented with embedded instrumentation has been successfully used to obtain structural and geotechnical design information on large diameter spun cast concrete cylinder piles on many large marine construction projects. The paper focuses on one particularly interesting large marine project involving a new bridge to St. George Island, Florida where instrumentation and STATNAMIC load testing was used in conjunction with static load testing on high capacity piles. The information presented includes embedded instrumentation measurements taken during pile fabrication (posttensioning), during pile driving, during static load testing, and STATNAMIC load testing. As much of this data is the first of its kind on concrete cylinder piles, it is exciting to present the foundation industry with this additional insight as to the behavior of this pile type and the accuracy of the methods used to test them. INTRODUCTION Construction projects in marine applications require demanding foundation designs. Large diameter spun cast concrete cylinder piles have many advantages in this application, most notable is their ability to resist large axial and lateral loads. As designers continue to push the capacity envelope, STATNAMIC load testing augmented with embedded instrumentation has become the preferred method of testing these high capacity foundations. Mainly because STATNAMIC load testing is a very efficient, cost effective, and reliable means of applying the large test loads needed to verify these demanding designs. Test loads up to 4,500 tons are available with current STATNAMIC equipment. The largest STATNAMIC load test to date on a concrete cylinder pile was 2,500 tons. The St. George Island Bridge project was the first in a long line of high capacity cylinder pile projects where STATNAMIC load testing was successfully used. The St. George Island Bridge project included a pile load test program consisting of 4-static load tests and 6-STATNAMIC load tests, all of which included multiple levels of embedded strain gages. Dynamic pile testing was also performed on the test piles and upwards of 50 production piles. All dynamic pile testing and static load testing was performed independently by the project geotechnical consultant. The data obtained from embedded instrumentation, dynamic, STATNAMIC and static load testing provided engineers a better understanding of cylinder pile structural and geotechnical performance as well as a perspective on the methods used to test them. Monitoring the embedded strain gages during post tensioning showed pre-stress magnitudes ranging from 760 psi to 1,010 psi and that some eccentricity was locked-in the pile during the process. Using these measurements, a composite modulus of elasticity of 5,725 ksi was calculated. The eccentricity is believed to be partly from non-uniform contact points between the segments and arching of the pile during tensioning. Strain measurements made during the pile driving show the propagation of the stress wave and actual driving stresses within the pile. This information was useful in demonstrating the differences between dynamic testing and STATNAMIC testing. Capacity predictions from dynamic testing showed as much as 41-percent under-prediction between the CAPWAP restrike and the static load tests. The CAPWAP derived skin friction and end bearing compared to the measured load distribution during the static tests showed a much greater disparity to the point of being totally unreliable. During subsequent STATNAMIC and static load testing, the instrumentation was used to determine load distribution e.g. side shear and end bearing resistance. The data also shows the high reliability of STATNAMIC load testing of cylinder piles when compared to static testing. In three of four piles tested, the STATNAMIC

2 method was within 2-percent of the maximum static capacity. In the fourth case, STATNAMIC was 9-percent below the maximum static capacity because with only 2-days between tests the pile did not re-gain its full side shear capacity. CYLINDER PILE DESCRIPTION Cylinder piles are constructed by spin casting multiple cylindrical segments and posttensioning them together to make up the desired length. The most popular pile diameters vary from 36 inches to 66 inches with wall thicknesses of 5 to 8 inches. Individual segment lengths which make up a pile typically range from 4 feet to 16 feet. The piles discussed in this paper were all 54 inches O.D. by 8 inch wall thickness and comprised of five 16-foot sections for a total length of 80 feet. The pile segments were constructed with longitudinal ducts in which post-tensioning strands were threaded after castings. High strength, low slump concrete was placed in the rotating forms. The centrifugal forces impel the concrete to the walls of the forms which results in a very strong and dense structural element. The 54 inch piles on this project had ultimate capacities ranging from 750 tons to 900 tons. But ultimate capacities as much as 1,630 tons have been used on 54 inch cylinder piles in the Chesapeake Bay Area. INSTRUMENTATION Due to the manufacturing process of cylinder piles, successful instrumentation presents many challenges. Figures 1 and 2 show some of the instrumentation processes. To the author s knowledge, this was the first time cylinder piles were successfully instrumented with embedded strain gages. Embedded strain gages were installed in the test piles for the purpose of measuring load distribution during STATNAMIC and static load testing. However, to gain additional knowledge of this pile type, the gages were monitored during the post tensioning process and during pile driving as well. As shown in Figure 6 following the Reference section. The bottom 4-sections of the piles were instrumented with 3 gages on a plane for a total of 12 gages per pile. Figures 1 and 2: Instrumentation of cylinder piles. Courtesy of Applied Foundation Testing, Inc. PILE INSTALLATION Installation of cylinder piles are most commonly done by means of large air, steam or hydraulic powered hammers. Heavy ram - short stroke hammers seem to be the preferred combination in the industry. Typical air and steam hammers driving cylinder piles are shown in Figures 3, 4 and 5. The 54 inch diameter piles presented in this paper were driven with a 40,000 lb ram air-hammer capable of a maximum stroke of 3-feet. Therefore providing a maximum rated energy of 120,000 ft-lbs. As is typical of air-steam hammers, this hammer was equipped with half-stroke capabilities. Three of the test piles (LT-1, LT-2 and LT-3) were driven to practical refusal of 20 blows per inch. Test Pile LT-5 was driven to a final blow count of 10 blows per inch. This was the only pile of the four stopped prior to practical refusal.

3 Figures 3, 4 and 5: Driving of cylinder piles with 40,000 lb air hammer (left) and 60,000 lb ram steam hammer (center and right). Courtesy of Boh Brothers and Weeks Marine Construction, Inc. SOIL CONDITIONS The subsurface conditions at the project generally consisted of four major strata. Water depths ranged from 10 to 20 feet. The upper layer was soft or loose silty bay bottom soils with N-values from 0 to 2. Underlying this was a more dense silty fine sand layer with N-values from 20 to 50. This strata was underlain by a weathered limestone formation ranging in thickness from 7 to over 15 feet. Below the limestone, dense silty fine sands and stiff sandy silts were present with N-values of 70 decreasing to 30 with depth. Soil conditions at each test pile location are shown in Figure 6 attached following the References. DYNAMIC TESTING METHOD Dynamic pile testing was performed on all of the test piles at the St. George Island Bridge project using a Pile Driving Analyzer (PDA). Dynamic testing techniques are well documented in many references. It was accomplished by externally attaching accelerometers and strain transducers near the pile top and taking measurements during driving. Pile capacity during driving was estimated by the Maximum CASE Method. Other data obtained include maximum compression stress, maximum tension stress and hammer energy transferred to the pile at the gage locations. It should be common practice to use 4-sets of external gages due to the large diameter of typical cylinder piles. A signal matching approach was used to back calculate various dynamic soil parameters and estimate pile load distribution. The computer program CAPWAP uses the data measured during a single blow as a boundary condition and the user performs many iterations on soil parameters to make a calculated force wave match the measured one. This approach is highly user dependent and does not provide a unique answer. STATNAMIC LOAD TEST METHOD The STATNAMIC load test is based on Newton's second and third laws which state that force is equal to mass times acceleration and that for every action there is an equal and opposite reaction. Loads up to 4,500 tons can be generated (axially or laterally) by propelling a reaction mass off the foundation. Since the mass is in contact with the foundation prior to the test, the force associated with propelling of this mass acts equally and oppositely onto the foundation. STATNAMIC load testing requires no reaction piles, no reaction beam, and no

4 hydraulic jack. The STATNAMIC device (Figure 7) mounts on the pile top and includes a load cell and displacement measuring system. Although not necessary, embedded strain instrumentation is always recommended in the foundation. A special solid fuel is burned to generate gas pressure inside a cylinder and ram (analogous to a gas actuated jack). As the pressure builds, it reacts against a heavy mass above the pile. The pressure eventually builds high enough to propel the reaction mass upward; in turn a downward load is simultaneously applied to the pile top which is many times greater than the weight of the reaction mass. The stages of a STATNAMIC test are shown in Figure 8. The STATNAMIC device typically produces a time dependent load on the order of 1/2 second or less. The fuel burn rate produces a smooth increasing force and controlled venting of the pressure produces a soft unloading. The load produced is not an impact which makes the analysis more simplified and reliable than dynamic testing. Load is measured with a calibrated load cell and displacement is measured with a photo-voltaic sensor triggered by a stationary laser reference. Three to four additional motion sensors also provide measurement of displacement at the pile top. Figure 7: 2,200 Ton STATNAMIC Load Test device used on cylinder piles at St. George Island Bridge. Courtesy of Applied Foundation Testing, Inc.

5 A - Foundation Member B - Calibrated Load Cell C - Laser Displacement System D - Piston & Cylinder E - Silencer F - Reaction Mass G - Catch Mechanism Stage 1 - Before The Test - reaction mass is in contact with pile. Stage 2 - The Test - Burning fuel creates high pressures, simultaneously propelling the reaction mass up and loading the pile in downward compression. The applied load and pile displacement are measured using high precision instrumentation and a data acquisition systems. Stage 3 - After The Load Test - The reaction mass is safely caught using hydraulic systems or by mechanical means. It is easily lowered for cyclic loading on the same pile. Figure 8: Stages of a STATNAMIC Load Test STATIC LOAD TESTING METHOD The load test set-up shown in Figures 9 and 10 was capable of applying almost 1,600 tons. As one can imagine from these photos, this test was very time consuming and costly. The axial load was applied to the pile using three 600 ton hydraulic jacks coupled in parallel. Multiple load cells (one above each jack) within this system provided a direct within this system provided a direct measure of the applied force. Multiple LVDT s were used to measure displacement. All instrumentation including the strain gages were used to measure displacement. All instrumentation including the strain gages were monitored with a data acquisition system. Reaction load was provided through frame supported water filled barges. Four pipe piles supported his weighted box apparatus. Loading rate was generally governed by filling the barges with water. The barge filling rate was about 7 tons of water per minute. The total duration of the test including loading and unloading was in most cases over 5 hours. In the interest of safely, the load on the test pile was always kept a minimum of 200 tons below the dead weight of the reaction mass. Thus, the four reaction piles were constantly under an approximate load around 50 tons each. The same is true for the unloading, where the pressure on the jack was slowly released as the water was drained from the tanks. Since the pipe pile supports were nearly 25-feet clear distance from the test pile and under minimal compression load, side effects on the test results from reaction piles were effectively negated. POST TENSION MONITORING RESULTS Sixteen tendons were stressed two at a time, 180 degrees from each other to a force of 63 kip each. Strand loading was monitored via gage pressure on the two calibrated jacks. Figure 11 contains the strain measurements from 11 individual gages (one gage did not survive the casting process). The data shows significant eccentricity to the point of going into tension in one line of gages. However, when gages at each level are averaged the eccentric stress appear more uniform. The eccentricity is believed to be partly from non-uniform contact points between the segments and arching of the pile during tensioning. It is also interesting to note that the point of jacking was only 2.5 feet from the yellow gage level which shows the

6 highest average strain but also the most uniform strain. Since we know the jacking force, strain in the pile and the cross section area, the overall pile modulus was measured. The modulus based on the average of all strain gages was 5,725 ksi and corresponds to a concrete compressive strength of over 10,000 psi. The stress in the pile can then be determined based on the calculated modulus which is shown in Figure 12. This information can be used to validate structural design assumptions and also be used to more accurately determine allowable driving stresses for spun cast concrete cylinder piles. Figure 9: Static load test set-up. Courtesy of Boh Brothers Constructions Co. Figure 10: Detail pile top of static load test set-up. Courtesy of Boh Brothers Construction Co, Inc.

7 Post Tensioning Strain (All Gages) Strain (ue) Blue1 ue Blue2 ue Blue3 ue Green1 ue Green2 ue Red1 ue Red2 ue Red3 ue Yellow1 ue -300 Yellow2 ue yellow3 ue Time (seconds) Figure 11: Strain measurements during post tensioning Post Tensioning Stress Stress (ksi) Blue Green Red Yellow Time (seconds) Figure 12: Average stress at each gage level during post tensioning

8 MEASUREMENTS DURING DRIVING Measurements of the embedded strain gages were made during pile driving on selected test piles. Conventional dynamic monitoring was also performed on all test piles during initial drive and restrike. An example of the calculated force datfrom the strain measurements within the pile for a single hammer blow is shown in Figure 13. The force measured with the PDA is also superimposed on this figure. The internal measurements clearly indicate the effects of stress wave propagation. Due to this wave propagation, load distribution can not be simply interpreted from these data. The data shows the presence of compression and tension waves in the upper gages but the toe gages remain in compression. It is interesting to note that the highest tension was recorded near the head of the pile after several wave periods. It is also noted that the maximum compression and tension forces corresponded very well to the calculated values from the PDA. An interesting comparison of the mechanics of a hammer impact and the STATNAMIC test may be made from this information. The hammer blow impact duration is approximately 1 millisecond as shown in Figure 13. The STATNAMIC load test produced a compressive load lasting 100 milliseconds as shown in Figure 15. The STATNAMIC load is applied in a linearly increasing manner with a gradually decreasing unloading. The load is not an impact because the reaction mass is in contact with the pile prior to the test. Because the STATNAMIC load duration is sufficiently longer than the natural period of the foundation, a much simpler and more accurate analysis can be made. The natural period of the pile is its length divided by its overall wave speed, L/c or 6 milliseconds for this pile. The STATNAMIC loading duration was approximately 16 times longer than the natural period of the pile. Where, a hammer impact is many times shorter than the natural period of the pile. This longer loading duration has several interesting characteristics. The most important is tensile stress waves are not present. This is shown in Figures 13 and 15 where the hammer blow produces over 800 kips of tension and the STATNAMIC test none. The figures also show that the pile is straining in-phase during the STATNAMIC test compared to those from an impact load. Therefore, the pile may be analyzed as a unit with all parts mobilizing capacity along its length simultaneously, like in a conventional static load test. But the static capacity is not simply the measured load and deflection. Because the pile, as a whole, undergoes rapid translation during the compressive loading, inertia and damping forces are easily subtracted. Force vs. Time During Pile Driving Level 1 Level 2 Level 3 Level 4 PDA Force Force (kips) Time (Seconds) Figure 13: Measured Driving Forces in a Cylinder Piles

9 In contrast to the STATNAMIC loading, a hammer blow produces a wave down and wave back up the pile several times which mobilizes resistance at different locations at different points in time. This is especially noticeable in the upper strain gages. Thus, a more complex analysis technique involving a signal match approach is required to determine the resistance distribution. The signal match approach introduces approximately 18 unknowns and has no unique solution. CAPWAP analyses were performed on the dynamic test data at end of initial drive and restikes with elapsed times ranging from 7 to 17 days. Only the CAPWAP restrike results are presented in Table 1. Table 1: CAPWAP results from restrike tests CAPWAP CAPWAP CAPWAP Skin End Pile Total Friction Bearing No. (kips) (kips) (kips) LT LT LT LT STATNAMIC AND STATIC LOAD TEST RESULTS The load applied during a STATNAMIC test is of sufficient duration to maintain the pile in compression during the entire load event (like a static test). But because the pile is pushed rapidly, the loading force is also resisted by damping and inertia forces from the soil. Therefore, these forces must be subtracted to obtain the static resistance component. The derived static capacity from the STATNAMIC tests was determined using the Segmental Unloading Point Method (SUP) developed at the University of South Florida (Mullins et al., 2002). The Segmental Unloading Point Method (SUP) discretizes a foundation into segments defined by the locations of the embedded strain gages. This allows the standard Unloading Point Method (UPM) to be applied to each segment using measured data. Then the total derived static response is calculated as the sum of the derived static response from the individual segments. The applied STATNAMIC load and derived static load (via SUP method) for test pile 3 at the St. George Island project is shown in Figure 14. The difference between these two curves in Figure 14 is the inertia and damping components. The load calculated at each strain gage versus time from the same STATNAMIC test is shown in Figure 15. The loads in this figure are not corrected for inertia and damping. Note that the upper three levels of strain gages showed almost no load transfer with all the capacity being contributed by the bottom 16 feet of pile. The physical set-up of the STATNAMIC equipment devised by the Contractor was quite ingenious in that it was self supported solely by the test pile. Therefore, falsework piles were not necessary to support the test equipment. This set up produced an atypical bouncing effect of the pile caused when the reaction mass was caught. This can be seen at the end of the data set in the load-displacement data as well as the strain data. Data reduction of the static load test is very straight forward. A typical plot of the measured load versus measured displacement is shown in Figure 16 for test pile 3. The load distribution calculated from the strain gages as a function of time during the static test are shown in Figure 17. As was typical for most of the piles, the strain data for LT-3 show that all the load was transferred to the bottom 16 feet of pile. Using these data reduction approaches, the static and STATNAMIC results were plotted together in Figures 18 through 21. As shown in these figures the ultimate capacity was achieved in two of the piles (LT-1 and LT-5). In pile LT-1, Figure 18, the STATNAMIC testing was done in three load cycles to a maximum of 2,166 kips following the static test which went to 2,136 kips. The STATNAMIC test showed both the stiffness and ultimate capacity that would be expected during a reload. It is also interesting to note that the pile developed more end bearing and less skin friction in the subsequent STATNAMIC test. The test order was reversed for LT-2, which is shown in Figure 19. STATNAMIC testing of LT-2 included three load cycles to a maximum of 3,372 kips. The following static test was taken to the limits of the equipment at 3,109 kips. Since this pile demonstrated an elastic response in each test, the comparison should be based on loads from similar deflections. Using this logic, the results were nearly identical from each test

10 method. The STATNAMIC and Static load test results for Pile LT-3 were plotted on top of each other in Figure 20 to show the nearly perfect agreement in every way. It is believed that this pile was on the verge of achieving plunging failure. The skin friction and end bearing distribution on pile LT-2 and LT-3 also had very good agreement. Test pile LT-5 shown in Figure 21 had a different response than the other test piles in that it had the most skin friction capacity of the four piles. The pile had a maximum static capacity of 2,889 kips of which 1,280 kips was end bearing. The STATNAMIC test was performed only two days later and showed a lower capacity of 2,631 kips. What is interesting is the STATNAMIC test showed a similar end bearing of 1,300 kips. So it is reasonable to assume the difference in capacity was due to a loss in skin friction between tests. A summary of results from the static and STATNAMIC tests is presented in Table 2.

11 Load vs. Displacement from STATNAMIC Test - LT-3 Displacement (inches) Statnamic Applied Load Derived Static Load Load (kips) Figure 14: Applied STATNAMIC load and derived static load based on SUP method Load vs. Time from STATNAMIC Test - LT Load (kips) Load Cell L 1 L 2 L3 L 4 (Bottom) Time (sec) Figure 15: Load distribution within the pile during the STATNAMIC load test (uncorrected for inertia and damping)

12 Load vs. Displacement from Static Test - LT Displacement (inches) Load (kips) Figure 16: Applied load vs displacement during static load test of LT-3 0 Load vs Time from Static Load Test - Test Pile LT Load (kips) Top L 1 L 2 L 3 L 4 (Bottom) Time (hours) Figure 17: Load distribution based on strain measurements during the Static load test of LT-3

13 Load vs. Displacement from STATNAMIC and STATIC Tests 0.00 Displacement (inches) Test Pile 1 Static Load Test Statnamic Load Cycle 1 Statnamic Load Cycle 2 Statnamic Load Cycle Load (kips) Figure 18: Static and STATNAMIC load test results from LT-1 Load vs. Displacement from Statnamic and Static Tests Test Pile 2 Statnamic Load Tests Static Load Test Displacement (inches) Load (kips) Figure 19: Static and STATNAMIC load test results from LT-2

14 Load vs. Displacement from STATNAMIC and STATIC Tests Test Pile 3 Statnamic Load Test Static Load Test Displacement (inches) Load (kips) Figure 20: Static and STATNAMIC load test results from LT Load vs. Displacement from Statnamic Test and Static Test Test Pile 5 Displacement (inches) Static Load Test Statnamic Load Test Load (kips) Figure 21: Static and STATNAMIC load test results from LT-5

15 Table 2: Summary of static and STATNAMIC load test results Static Total (kips) Static Skin Friction (kips) Static End Bearing (kips) STATNAMIC Total (kips) STATNAMIC Skin Friction (kips) STATNAMIC End Bearing (kips) Pile No. LT LT LT LT CONCLUSIONS AND COMPARISON OF THREE TEST METHODS Three types of load tests were performed on the St. George Island Bridge project consisting of dynamic testing, STATNAMIC load testing, and static load testing. Four of the test piles utilized all three methods on the same pile which are the focus of this paper. In these comparisons, the static load test was selected as the bench mark reference test. Of the four piles tested, only two (LT-1 and LT-5) achieved the ultimate pile capacity. In all cases, the CAPWAP restrike underpredicted the total pile static capacity. These under-predictions ranged from 9 percent to 41 percent as shown in Figure 22. The predicted load distribution from CAPWAP had much larger disparities though. The best CAPWAP comparison was at LT-1, where the restrike was only 9 percent below the ultimate static capacity. However, it did not match so favorably to the skin friction and end bearing distribution where it under-predicted the skin friction by 222% and over-predicted the end bearing by 79%. At LT-5, the CAPWAP restrike under-predicted the ultimate static capacity by 30 percent. It also under-predicted the skin friction by 245% and over-predicted the end bearing by 21%. In LT-2 and LT-3, the CAPWAP restrike again showed significant under-prediction of 35% and 41%, respectively. At LT-2, the skin friction was over-predicted by 127% and the end bearing was under-predicted by over 117%. Similar magnitude discrepancies were shown in LT-3 with an under-prediction on skin friction of 194% and 54% under-prediction of end bearing. In three of the four piles, STATNAMIC derived static capacities were within 2 percent of the static load tests. In the fourth pile, the STATNAMIC derived capacity was 9 percent below the static load test primarily due to the short time interval (only 2 days) between the static test and the STATNAMIC test. Figure 23 graphically depicts this comparison. In LT-1 the end bearing during the reload was 71 % greater than the initial static test. Also accompanied by a reduction in side shear capacity of 35%. In LT-2 the STATNAMIC showed 107 kips lower side shear capacity and 50 kips higher end bearing capacity than in the static test. The side shear and end bearing capacity from STATNAMIC was within 1% to 2% of the static test for LT-3. In LT-5 the STATNAMIC end bearing was within 2 % of the static test, but was 17% lower on side shear as discussed above. Although CAPWAP significantly underpredicted the pile capacity as illustrated in Figure 22, dynamic testing of cylinder piles still adds benefit in gaining driving stress and hammer energy data. The high reliability of determining pile capacity with STATNAMIC load testing augmented with strain instrumentation was demonstrated on this project and illustrated in Figure 23. In this application, STATNAMIC has demonstrated it can provide engineers, owners and contractors a more cost effective means to test high capacity marine foundations.

16 CAPWAP / Static Comparison REFERENCES Mullins, G., Lewis, C., and Justason, M.D., Advancements in Statnamic data Reduction Techniques. ACSE Geotechnical Special Publication 116, pp CAWAP (kips) Under-Prediction Williams Earth Sciences, Inc. Report of Load Test Results (Static, STATNAMIC, and PDA), St. George Island Bridge SR 300 over Apalachicola Bay, January, 31, Static Load (kips) Figure 22: CAPWAP restrike to static test 3500 STATNAMIC / Static Comparison 3000 Statnamic Load (kips) Static Load (kips) Figure 23: STATNAMIC to static test

17 Figure 6: Soil and pile profiles St. George Island Bridge