COMPARISON BETWEEN STATNAMIC AND STATIC OF DRILLED SHAFTS IN VARVED

Carl Ealy, Highway Research Engineer, Federal Highway Administration, McLean, Virginia, USA Magued Iskander, Ph.D., P.E., Associate Professor, Polytechnic University, Brooklyn, New York, USA Mike Justason, P.Eng., Berminghammer Equipment, Hamilton, Ontario, Canada Danny Winters, Graduate Research Assistant, University of South Florida, Tampa Florida, USA Gray Mullins, Ph.D., P.E., Assistant Professor, University of South Florida, Tampa, Florida, USA

Static and STATNAMIC (STN) rapid load test (RLT) was performed on 3-drilled shafts. The purpose of the RLT part of the study was to obtain data in support of FHWA research on innovative load testing methods. The research site is located at the National Experimentation Research Site (NGES) in Amherst Massachusetts and is underlain by Connecticut valley varved clay. Two of the tested shafts were constructed without planned defects and the third with manufactured defects to evaluate current non-destructive integrity testing (NDE) methods. Quick Load Test (QLT) and Constant Rate of Penetration (CRP) static axial load tests were performed on two shafts to plunging failure. Good agreement between static and RLT derived static failure loads was attained for a 5% failure criterion (load at a movement equal to 5% of shaft diameter).

IINTRODUCTION Figure 1 is typical of the varved clay material underlying A joint research effort on drilled shafts involving the FHWA, the site. ADSC and Polytechnic University was implemented as part of the Y2K conference held April 9th through April 14th at the National Geotechnical Experimental Site (NGES) located at the University of Massachusetts in Amherst. The primary objectives of the research were to evaluate the effectiveness of various non destructive test (NDT) methods in detecting planned defects of varying size and locations in drilled shafts installed in a varved clay and the effects of those defects on the axial load-movement and load-transfer behavior. A secondary objective was to obtain data in support of FHWA research on innovative load testing methods. This report describes the statnamic portion of the research and presents the results of a comparison between the derived static and conventional static load-movement behavior for two shafts.

Six 0.9 m diameter, 15 m long shafts were built for this Figure 1 - Connecticut Valley Varved Clay research. Load tests were performed on Shafts 2, 4 and 6. Details of the defect detection program and its effects on capacity are reported in Iskander et al (2001). Site specific testing was performed with the University of South Florida’s minicone at the locations of test Shafts 2 and 4 and reaction shaft 5. Results for test Shafts 2 and 4 are SITE DESCRIPTION shown in Figure 2 and Figure 3. The in the The test site is located at the National Geotechnical vicinity of Shafts 2 and 4 varied from 35 to 193 kPa over the Experimentation Site on the campus of the University of first 1.5 m except for a few high readings up to 290 kPa for Massachusetts in Amherst. Details of the , Shaft 2. From 1.5 to 4.6 m the shear strength decreases stratigraphy and mechanical properties of the site are from 193 to 35 kPa and then decreases to approximately 18 described in detail by Lutenagger (2000). Briefly, the kPa from this depth to the end of the soundings. The above stratigraphy consists of a crust of miscellaneous clay/ fill tests were performed during the instrumentation and extending from the surface to about 1.5 m. This is underlain construction phase of the project. At the time of the by approximately 2 m of sandy silt that in turn overlies statnamic tests the water table as measured in varved clay that varies in thickness throughout the test area approximately 15 to 30 m from center of test site ranged but extends to least to 19 m below the sandy silt layer. from 0.05 to 0.75 m below the surface

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Planned defects were incorporated in test Shaft 4 and all fs (kPa) three-reaction shafts. Typical defects installed in Shaft 4 are 0 50 100 150 200 250 300 shown in Figure 5. Details of the location and construction 0 of the defects are discussed by Iskander et al (2001). 2

4 6

Depth (m) Depth 8 Shaft 4 Shaft 2 10

12 = Fig. 2 Variation of shear stress with depth. qc (kPa) 0 1000 2000 3000 4000 5000 -2 0 2 Figure 5 - Instrumented rebar cage for Shaft 4. 4 6 Shaft 4

Depth (m) Depth Reaction shafts 3 and 5 were constructed via free fall; all Shaft 2 8 others were tremied. The tremie process for constructing

10 the instrumented shafts is shown in Figure 6. Considerable

12 difficulty was encountered in feeding the tremie pipe past Fig. 3 Bearing stress with depth and through the defects in Shaft 4 and in general it was difficult keeping the tremie pipe from slamming up against the sides of the rebar gage during descent and during DRILLED SHAFT CONSTRUCTION concreting of all shafts. The holes for the test and reaction shafts were drilled without water or slurry with a -Mec model R515. For most shafts, very little time elapsed between excavation and concreting and were constructed without discernible incident. Shaft 2 was intended to be constructed defect free, however approximately 4 hours elapsed between excavating and concreting Shaft 2 during which time intermittent sloughing could be heard. Depth soundings after excavation and again before concreting suggested a soft bottom possibly from bottom heave and/or sloughed material accumulating in bottom of shaft. The measured depth prior to concreting was 13.4 m. These events resulted in a soft bottom as as unknown sidewall condition to due to sloughing.

Figure 6 - Shaft construction by tremie method

INSTRUMENTATION Test Shafts 2, 4 and 6 were instrumented with 5 levels of bondable resistance strain gages encapsulated on “sister bars”, 2 at each level 180 degrees apart. Shafts 2 and 4 also had 5 telltales located at the level of the strain gages. Reaction shaft 5 was instrumented with vibrating wire strain gages for future lateral load testing as well as with telltales and sister bars at selected depths. Additionally, Shafts 2 and 6 were instrumented with accelerometers at the tip. An

automated electronic data acquisition system was used to

monitor and record instrument readings during static and Figure 4 – Test and Reaction Shaft Locations statnamic testing. STATIC TESTING

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Static tests were performed on Shafts 2 and 4 in accordance Prior to testing, the top surface of the shafts were filled and with ASTM D1143-87 Quick Load Test Method (QLM). leveled as necessary to provide an even contact surface for the load cell. The testing sequence is summarized in table 1.

Table 1 - Testing history

SHAFT DATE TIME TEST REM

7/20/00 21:58 4 QLT 45 KN/

7/21/00 10:00 4 QLT 90 KN/ 7/22/00 9:24 2 QLT 45 KN/ 7/22/00 17:47 2 CRP 7/24/00 14:30 6 STN 275 pell

7/24/00 15:07 6 STN 300 pell 7/25/00 10:00 2 STN 300 pell Figure 7- 7/25/00 16:00. 4 STN No acel 7/25/00 16:35 4 STN 314 pell Schematic of Statnamic test.

Shaft 4 Two QLM tests were performed on Shaft 4. Loads were applied in 45 KN increments for the first test and 90 KN increments for the second test.

Shaft 2 Two static tests were performed on Shaft 2. The first was a QLM test as above in 45 KN increments followed approximately 8 hours later by a constant rate of penetration (CRP) test.

STATNAMIC TESTING The Statnamic (STN) rapid load test method was conceived by Patrick Bermingham, of Berminghammer Foundation Equipment in the late 1980’s and jointly developed by Berminghammer Foundation Equipment and TNO, Figure 8 - 4 mN Statnamic device with Catch Mechanism. Netherlands. The impetus for developing the method was to find an economical solution for load testing high capacity Statnamic load tests were performed on shafts 6, 2 and 4. deep foundations. The method is described in detail by Data reduction was accomplished using the Statnamic Bermingham and Janes (1995), Horvath (1995), and Analysis Workbook (SAW) and an alpha version of numerous others. Briefly the test is performed by placing a SUPERSAW analysis programs written by the University of mass equivalent to approximately 5% of the desired ultimate South Florida for FHWA. Both programs use Middendorp’s load over a pressure chamber that rests on the test Unloading Point Method (UPM) (Middendorp et. al., 1992). foundation. The controlled burning of fuel in the pressure Briefly, the UPM models the pile-soil system as a single chamber accelerates the mass. The reaction of the mass lumped mass with soil resistance characterized by a spring applies a force to the tested foundation equal and opposite and dashpot to represent the static and damping resistance to the statnamic (STATtic-dynAMIC) force exerted on the respectively. The assumptions used in the method and their reaction mass. Measurements of load are taken with a load validity are discussed by Middendorp (1992), Middendorp cell and movement of the test foundation is normally and Bielefeld (1995), Seidel (1996), Hajduk et al (2000) and measured via a laser sensor. Acceleration of the top of the numerous others. In simplest form the foundation is shaft is measured with an accelerometer that also serves as assumed to behave as a rigid body and the static soil a backup for determining top movement. This was the capacity remains constant after shearing at max force to method used for determining top movement for the Amherst max displacement. Using these assumptions, a damping statnamic tests. A schematic of a typical statnamic load test constant can be derived from the measured data that is used setup is shown in Figure 7. to correct the statnamic data. This correction along with correction for inertial forces yields the derived static load- movement curve. The SAW and SUPERSAW are Testing at the Amherst site was done using the 4-MN spreadsheet-based programs that automate the statnamic device and catch mechanism shown in Figure 8.

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Time (msec) 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 application of the UPM to statnamic data. SUPERSAW 6 divides the pile into segments and applies the UPM to each 5 segment. The derived results for each segment are 4 summed to provide the top derived static load-movement 3 top accelerometer toe acelerometer relationship. The program also generates t-z curves 2

(average skin over a segment plotted against 1 movement of segment) and derived static load distribution 0 with depth. The results obtained for the above methods are Acceleration (g) Acceleration -1 summarized in Figures 9-18 and discussed below. -2

-3 Statnamic Force (kN) 5 -4 Fig. 11 Top and toe accelerations vs time 0 for shaft 2. -5 -10 The statnamic and derived static load distribution with time is shown in Figure 12. Several gages were lost due to -15 electrical noise or damage during construction. The strain -20 readings for the surviving gages were converted to loads by -25 multiplying by the cross sectional area and composite -30 modulus at the gage level. Prior to performing the analysis,

-35 all gage levels were zeroed by subtracting the to readings. Shaft Head Movement (mm) Therefore, a preload must be reintroduced at each level to -40 0 -500 -1000 -1500 -2000 -2500 -3000 -3500 -4000 account for loads caused by the initial static load of the statnamic reaction mass. Fig. 9 Statnamic Force-Movement Shaft 2 Time (sec) Shaft 2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 1000 The statnamic force-movement relationship for Shaft 2 is shown in Figure 9. The shape of the curve is somewhat 0 atypical with the point of maximum movement occurring nearly at zero load with sharp rebound to a final set of -1000 approximately 22 mm. The magnitude of the final set indicates full mobilization of both dynamic and static soil -2000 resistances. The maximum statnamic force attained was

Statnamic load (KN) load Statnamic -3000 3500 KN and the maximum movement was 35 mm. Top load 2 m 5 m -4000 The statnamic force, top movement, velocity and top and 8 m 500 bottom shaft accelerations versus time are shown in Figures 11 m

10 and 11. Peak downward velocity was about 0.9 m/s, 0 which is within the typical range for a statnamic test. The magnitudes of the top and toe accelerations are nearly the -500 same supporting the assumption of rigid body motion. -1000 Time (msec)

Derived static load (KN) load static Derived -1500 0 100 200 300 400 500 600 1000 10 -2000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 0 Fig. 12 Statnamic and derived static load distribution in Shaft2 with time Top movement Statnamic force -1000 -10 Figure 13 compares the QLM, CRP and derived static load- movement curves for Shaft 2. The statnamic tests are -2000 -20 somewhat stiffer than static tests. Peak loads differ significantly. The load at a top movement equal to 5% of the shaft diameter gives failure loads of 1250 KN for both the Statnamic Force (KN) Force Statnamic -3000 -30 QLM and CRP tests. The maximum top movement under

Shaft Head Movement (mm) Movement Head Shaft statnamic loading was a few millimeters short of the 5% -4000 -40 criteria. However, if the minimum load value just before the Figure 10. Statnamic Force and Top Movement secondary rebound is assumed to represent a vs Time for Shaft 2.

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residual soil resistance analogous to “plunging” loads of the Time (sec) static tests, then the SUPERSAW curve coincides with the 0.0 0.1 0.2 0.3 0.4 0.5 1000 20 static results. Alternatively, the SAW yields a failure load of 1750 K. 0 0

-1000 Statnamic force -20 Movement

Load (KN)

Load (kN) -2000 -40 0 250 500 750 1000 1250 1500 1750 2000 0 Movement (mm) -3000 -60 -25

-4000 -80 -50 Fig 15. Statnamic load and movement vs time for shaft 4. -75 Time (sec) -100 QML 0.0 0.1 0.2 0.3 0.4 0.5 CRP SAW 1 60 SUPERSAW -125 40

Displacement Displacement (mm) -150

)

2 0 20 -175

0 -200

Velocity (m/s) Velocity -1 -20 -225

Velocity (m/s Acceleration Acceleration Fig. 13 Comparison between QML, CRP and -40 Derived Static for Shaft 2 -2 -60 Fig 16. Velocity and acceleration for shaft 4. Shaft 4 statnamic loading. The STN load-movement curve is shown in Figure 14 and is similar in shape to that of Shaft 2. The maximum statnamic force attained was 3800 KN; maximum movement 68 mm The STN and derived static load distribution with time are and net movement was 53 mm. Again, a large net shown in Figure 17. Usable data was returned from 4 of the movement was attained indicating full mobilization of 5 gages on the “A” side of shaft 6 but only 2 levels on the “B” dynamic and static resistance. The statnamic force, top side. Note this shaft was constructed with defects throughout movement, velocity and top acceleration versus time are the lower 40 feet, most of which took up 20 % or more of the shown in Figures 15 and 16. Peak velocity was higher than cross sectional. For example, there was a large void defect for Shaft 2 at about 1.5 m/s which is consistent with a larger along one side of the shaft 0.3048 m in diameter and 1.22 m total amount of pile movement during the statnamic loading long spanning the sister bar located at 5 m and a large soil event. inclusion defect 635 mm in diameter extending from the tip to 0.381 m above. Nevertheless, the load distribution with Load (KN) depth appears reasonable with approximately 13% of the 0 -500 -1000 -1500 -2000 -2500 -3000 -3500 -4000 20 load reaching the tip of the pile.

Figure 18 summarizes the loading history for Shaft 4. The 0 upper curve shows the first load-movement cycle for QLM testing applied at a rate of 45 KN/2.5 min. Applying 5% of -20 the diameter and plunging criteria gives a value of 1100 KN. The next curve represents loading at 90 KN/2.5 min and -40 exhibits a stiffer response but plunges more sharply and at a

Movement (mm) Movement lower failure load of 1000 KN. The derived static SAW and

-60 SUPERSAW curves are nearly identical and give failure loads of 1000 KN at 5% and 950 and 900 KN at “plunging” load respectively (using the assumption discussed in Shaft -80 2). Comparing the derived failure loads to the QLM failure Fig 14. Statnamic load-movement for shaft 4. loads for the first cycle results in under predictions of 9%

and 16% at 5% and plunging respectively. Comparing the

derived failure loads to the second QLT cycle failure loads

results in no difference based on 5% criterion and a 7.5%

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under prediction at plunging. Additional remolding of the clay CONCLUSIONS in successive loading cycles could have also caused these differences. 1. In general, the Segmental Unloading Point analysis (SUPERSAW) yielded a better correlation than the Time (sec) original UPM (SAW). 0.00 0.05 0.10 0.15 0.20 0.25 0.30 1000 2. Good correlation between static and derived static was attained if the 5% of the tip diameter criteria 0 are applied to obtain a failure load.

-1000 3. Analysis of the top and toe accelerometers supports rigid body motion for the test shafts.

-2000 4. Results indicate that sufficient load must be applied

Statnamic load (KN) load Statnamic in this type of soil to attain large permanent -3000 Top movements and fully mobilize both dynamic and 2 m static soil resistance. 5 m -4000 11 m 1000 14 m 5. Although the shafts were equal in size variation in shaft capacity was on the order of 10%. Variation 500 in capacities using static and statnamic represented less than 5% of the shaft capacity, as long as 0 sufficient energy was used to mobilize the full

-500 capacity of the shafts.

-1000 ACKNOWLEDGEMENTS Special thanks are due to ADSC member Jim Maxwell,

Derived static load (KN) load static Derived -1500 president of HUB engineering for his contribution of time and materials to this project and to graduate students Edward -2000 Garbin and Christopher Lewis for developing and Fig. 17 Statnamic and derived static loads in shaft 4 maintaining the computer programs used in this study. vs time. REFERENCES Load (KN) 0 250 500 750 1000 1250 1500 1750 2000 Hajduk, E. L., Paikowsky, S. G., Mullins, G., Lewis, C. and Hourani, 20 N. M., 2000 Proceedings of the Second International Statnamic Seminar, Tokyo, pp. 59-73. 0 Iskander, M., Kelley, S., Ealy, C. D., Roy, D., 2001. Load Tests on

-20 drilled shafts with planned defects in varved clay, Proceedings of the 80th Transportation Research Board Meeting, Washington, D.C. -40 Iskander, M., D. Roy, C. Ealy, and S. Kelley, (2001) "Class-A -60 Prediction of Construction Defects in Drilled Shafts," in Press,Transportation Research Record: Journal of the -80 Transportation Research Board, No. 1772, pp. 73-83

-100 Luttenagger, A., 2000. National Geotechnical Experimentation Site University of Massachusetts, Geotechnical Special Publication No.

-120 93, ASCE.

Displacement (mm) Displacement Middendorp, P., Bermingham, P., Kuiper B., 1992. Statnamic testing -140 of foundation piles. 4th International Conference on Stress Waves, The Hague, Balkema. -160 Middendorp, P., B. and Bielefeld, M. W., 1995. Statnamic testing -180 and stress wave phenomena. Proceedings of the First International Statnamic Seminar, pp. 123-136. -200 Seidel, J. P., 1996. A review of the analysis of statnamic tests, Fifth

-220 International Conference on the Application of Stress-wave Theory on Piles, Orlando, pp. 1038-1050. Fig 18. Comparison between QML and derived static for shaft 4.

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