Articulo Stres_Wave Lisboa 2008

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High strain dynamic testing in micropiles. Comparison of staticand dynamic test results

Oteo, C.

Dpto. Ingeniera del Terreno, A Coruna University, A Coruna, Spain

Arcos, J.L. & Gil, R.

Kronsa Internacional, S.A., Madrid, Spain

Fernandez Tadeo, C.CFT & Asociados, S.L., Barcelona, Spain

Keywords: micropile, dynamic load test, static load test

ABSTRACT: Micropiles are used for many applications, including underpinning existing foundation.Improving of technique is required to support large loads and quick control methods are demanded. Underthis project, a micropile trial site was established at Algete in Madrid (Spain). Three micropiles were executed

with 3, 5 and 7 meters length, borehole diameter of 150 mm and a steel pile external diameter/thickness of88.9/9 mm. Dynamic load testing was performed in trial area. A little hammer was developed for testing and PileDriving Analyzer (PDA) was used to investigate the ultimate capacity of the micro piles. The hammer has avariable mass between 500 Kg and 1000 Kg that can run along one meter. A static load test has been performed toestimate the bearing capacity of micropiles. The results of the testing program will show if PDA testing mayprovide accurate information in this technique. The paper, therefore, will discuss the techniques used for PDAtesting of the micropiles, and compare the results of the PDA tests to the data from static load test. The paper alsocontains a brief discussion on the construction method used and its influence on micro pile resistance. Local soilcharacteristics (silty clay with fine sand) will be taken in consideration to analyze the correlation between staticand dynamic load tests. Finally, precautions to prevent micropile damage are studied, and severalrecommendations will be given after analyzing the damages in the micropiles.

1 INTRODUCTION

Micropiling works are very common in Spain and areusedformany applicationsandsoils. Heavily reinforcedmicropiles with high axial load-carrying capacities aredemanded in the market and borehole diameter of300 mm with bearing capacity over 2500kN arepresent in building, tank and bridge foundations.

The testing of individual micropiles is oftenstipulated as part of a project specification in bigconstruction. Dynamic micropile testing can becomean integral part of a quality assurance system in thesense of confirmatory testing and a good complementor alternative to static vertical load test.

Kronsa Internacional S.A. together with A CorunaUniversity initiated in October 2006 a researchprogramme involving the construction and testingof three micropiles with 3, 5 and 7 metres length.

Dynamic load test were carried out using anspecially developed hydraulic free fall hammer. PDIequipment was used and CAPWAP analyses wereperformed.

A prediction of the Class A, without knowledgeof the static test result, was done.

In December 2007, a new micropile with 7 meterslength was installed close to the dynamic testing areaand a static load test was carried out. Simultaneously, a

new dynamic test was made in previous testedmicropiles.

This paper presents the outline of the test micropileprogramme, summarises the results of dynamic testand provides a comparison of these results with thestatic test data.

Recommendations to prevent micropile damageare also included.

2 SOIL CHARACTERIZATION OF THE TEST

AREA

The soil where the tests were carried out can bedescribed as a gravel layer about 2 meters thick,overlying silty clay with a fine sand layer with athickness greater than 20 meters.

Water level was about 1 meter deep at the momentof test execution.

3 TEST MICROPILE PROGRAMME

3.1 High strain dynamic load test

3.1.1 Micropile installationKronsa installed the micropiles by pre-drilling with aHead Hammer and using simultaneous injection of

Science, Technology and Practice, Jaime Alberto dos Santos (ed) 609


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water throughout the process. A provisional casingwas also used. The borehole diameter of the installedmicropiles was 150 millimeters.

Once pre-drilling completed, the reinforcement, acylindrical steel tube with external diameter of88.9 mm and 9 millimeter of thickness wasintroduced inside of the predrilled hole.

For grouting, a tremie pipe was inserted to thebottom of the hole to gravity fill grout, and

continued until the grout reached the top of the casing.After grouting, provisional casing was retired.

3.1.2 Hammer systemDevelopment of an automatic hydraulic free fallhammer was carried out. This hammer can bemounted in a standard retroexcavator machine. Thisallowsrapidpositioningateverymicropiletobetested.

The hammer allows repetitive blows worked by thehydraulic power of the excavator. The ram mass isformed by a compact block of 500 kg plus additionalmass that can grow progressively to 1 ton. The

maximum stroke is 1 meter (max. energy 1 mt),but this height is also adjustable.

For a proper selection of the hammer to be used fordynamic testing, Hussein et al. (1996) suggests a ramweightabout1.5%ofthestaticresistancetobeverified.In the test, static resistance mobilized was very high,and we have to use maximum drop hammer height,assuming risks with regard to micropile integrity.

3.1.3 Dynamic test resultsIn order to perform the dynamic testing the micropiletops were prepared. The micropile reinforcement

consisted of a cylindrical steel tube where weremade eight drills. On the other hand, the top ofeach pile was fitted with reinforcing bars welded tothe steel tube. Two piezoelectric accelerometers andtwo strain transducers were attached to the micropiletop with the help of the drills. A Pile Driving Analyzer,PAK model, was employed (PDI, 2004).

Cushioning was provided between the head of thepile and the hammer and it consists of a cylindricalpiece of oak wood with 10 cm of thickness.

In order to mobilize as much as possible the soilresistance, the test employed increasing hammerenergies. Testing of each micropile was performedby striking the pile initially with a low energy blow.The drop height was increased until the micropile wasmoved several centimeters. The set of the pile top wererecorded for all blows (Kormann et al. 2000).

In addition to PDA testing, Case Wave Pile

Analysis Program (CAPWAP) analyses wereperformed on all blows and micropiles in order toget additional data on pile response to loading and toconfirm the capacity obtained from PDA testing.

During testing, large displacements at the head ofthe micropile were noticeable in last blows. Whenshaft micropile was broken, the capacity went downquickly. In last blows, micropile was practically drivenwith set per blow close to 20 millimeters.

The Ultimate Capacity for 3, 5, 7 m lengthmicropile is shown graphically in Figs. 3, 4 and 5.These values are based on CAPWAP analyses.

Figure 1. Hammer system detail.

Figure 2. Hammer system and micropile with instrumentation.

Figure 3. Ultimate Capacity micropile 3 m length fromCAPWAP and stroke per blow.

610 2008 IOS Press, ISBN 978-1-58603-909-7


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3.2 Static vertical load test

3.2.1 Micropile and anchor installationA new 7 meter length micropile was installed to betested statically. This micropile was placed a fewmeters away from the three ones testeddynamically, in the same soil conditions, onDecember 4th, 2007.

Three vertical anchors were also installed with afree length of 8 m and a bulb of 15m, to avoidinteraction with the micropile.

3.2.2 Procedure for static vertical load testStatic load was applied with one hydraulic jack on thetop of a steel plate over the micropile head, and actingagainst an anchored reaction frame. The maincharacteristics of the hydraulic jack were:

* Maximum force2,243 kN;* Maximum oil pressure700 bar;* Maximum stroke160 mm.

An electric hydraulic pump was used, with the helpof a manual pump for initial small loads. Internal jackpressures have been measured by using a calibrated

digital manometer. The hydraulic jack was calibratedin a static-dynamic press model MTS in a CertifiedCalibration Laboratory.

Anchored reaction frame was a monolithic steelbeam with T shape, of sides 3.3 m and 2.95 m. It was

anchored to ground with three cement-grouted cableanchors. Anchors were pre-tensed placing the reactionframe over three short steel columns. Minimumdistance from micropile tested to columns andanchors was 1.0 m. Several steel bearing plates andone hemispherical bearing were used to get goodcontact between the hydraulic jack and reaction frame.

Two electronic and one dial displacementindicators were used to measure micropilesettlement during testing. Electronic indicators weretwo lineal potentiometers of 75 mm travel and 0.1 mmprecision. The dial indicator was 50 mm travel and0.01 mm precision. The displacement indicators werepositioned over small horizontal steel plates welded tothe micropile reinforcement steel tube under the

micropile head. Measurements of electronicindicators were taken automatically every 5 secondsusing a data-logger and a PC.

Displacement indicators were mounted attached totwo parallel reference beams 5 m long. Beams werefirmly supported to the ground at a clear distance fromthe ground anchors and reaction frame columns. Asunshade was installed to prevent direct sunlight andprecipitation from affecting the measuring andreference systems.

Load was applied in two cycles. The first one untilthe micropile nominal working load of 33 t (323 kN)was applied and the second cycle until reaching thefailure of the micropile. Load increments were of 25%nominal working load until 100%, and increments of12 t (118 kN) until reaching failure. First cycle wasdischarged in 3 equal decrements and second cycle in4 decrements. During each load interval, load was keptconstant until the rate of axial movement did notexceed 0.02 mm/5 minutes (0.25 mm/hour), with amaximum of 15 minutes (10 minutes in discharge).

3.2.3 Static vertical load test resultsThe results of static vertical load test were:

* The vertical axial displacement in the first cycleuntil nominal working load was 1.95 mm. In thedischarge, the remaining displacement was0.3 mm.



Figure 6. Hydraulic jack and reaction frame.



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* The failure of micropile arrived suddenly at load100.4 t (984 kN), when vertical axial displacementwas 17.7 mm.

* After the sudden failure, the micropile was notcapable of sustaining the failure load. Themaximum load applied while the verticaldisplacement continued to increase was 81.8 t(802 kN).

* The final discharge began when the vertical axial

displacement arrived to 43 mm. The remainingfinal displacement was 34.6 mm.

3.3 Comparison of static and dynamic test results

The records of several blows were submitted tonon-uniform pile CAPWAP analysis. The obtainedmobilized capacities are compatible with the results ofthe static load test.

The results of the static and dynamic tests havebeen plotted on Figs. 8 and 9. Fig. 8 shows blow no 8

CAPWAP analysis, where pile displacement was zero,compared with first cycle of static load test.Fig. 9 shows blow no 11 CAPWAP analysis, where

pile displacement was 6.5 mm, compared withcomplete static load test.

It can be seen that the CAPWAP analysis providedconservative load-deflection behavior compared to the

static text data until reaching about 80% of the failureload. For settlements greater than 10 mm in secondcycle the prediction is not good.

3.4 Discussion on resultsThe simulation plotted shows a remarkable agreementbetween the static load test within the service loadrange of the load-settlement curve and dynamic loadtest. It can be observed that the maximum micropilehead displacement in dynamic load test did not exceed6.5 mm in blow no 11. That might explain why thismethod encountered more difficulties in predictingpile behavior under large displacements.

Dynamic load test results are very close to the staticload test ultimate capacity even though dynamic test

did not predict the pile behavior for settlements greaterthan 20 mm.Concerning the micropile integrity, a hammer with

a heavier ram should have been used to decreasestroke. Visual shaft inspection of the micropilewhen it is extracted will allow to be made a moreaccurate commentary.

A new dynamic testing was performed in themicropiles in December 2007. An considerableresistance recuperation was recorded respect to lastblows carried out a year before. About 80% of initialfailure load was again calculated in CAPWAP

analysis.

4 CONCLUSIONS

At the current stage of the research here presented, thefollowing conclusions can be outlined:

1. The maximum provided capacities in theClass A CAPWAP analysis presented a goodagreement with the results of the static test.

2. The load-deflection behavior showed a good

agreement with the static load test within theservice load range of the load-settlement curve.

3. The results have demonstrated the potential of thehigh strain dynamic testing as a valuable tool forthe assessment of the behavior of the micropiles.

Figure 7. Static load test results at micropile.

Figure 8. First cycle of static load test and CAPWAP staticsimulation for blow no 8.

Figure 9. Static load test and CAPWAP static simulation forblow no 11.

612 2008 IOS Press, ISBN 978-1-58603-909-7


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REFERENCES

American Society for Testing and Materials. Standard TestMethod for Deep Foundations Under Static AxialCompressive Load, ASTM D 1143 M-07.

CAPWAP Manual (2006). Pile Dynamics Inc. Cleveland, Ohio.Gomez, J., Cadden A., and Webster C. (2004). Micropile

Foundations in Karst: Static and Dynamic TestingVariability, Proceedings of Fifth International Conferenceon Case Histories in Geotechnical Engineering, Paper

No. 9.05.Holeyman, A., Maertens, J., Huybrechts and Legrand, C. (2000).Results of an international pile dynamic testingprediction event, Proceedings of Sixth InternationalConference on the Application of Stress-Wave Theory toPiles: 725732.

Hussein, M., Likins, G., Rausche F. (1996). Selection of ahammer for high-strain dynamic testing of cast-in-placeshafts, Proceedings of Fifth International Conference onthe Application of Stress-Wave Theory to Piles: 759772.

Klingberg, D. and Mackenzie, P. (2000). Static and dynamictesting of the CampileA displacement, cast-in-situ pile,Selection of a hammer for high-strain dynamic testing ofcast-in-place shafts, Proceedings of Sixth InternationalConference on the Application of Stress-Wave Theory toPiles: 715718.

Kormann, A., Chamecki, P., Russo, L., Antoniutti, L., Bernardes,G. (2000). Behavior of short CFA piles in an overconsolidated clay based on static and dynamic load test,Proceedings of Sixth International Conference on theApplication of Stress-Wave Theory to Piles: 707714.

PDI (2004). Pile Dyanamics Inc. Cleveland, Ohio.Poulos, HG. (2000). Pile load test methods applications

and limitations. Libro homenaje a J. A. Jimenez Salas.Ministerio de Fomento, CEDEX y Sociedad Espanola deMecanica del Suelo e Ingeniera Geotecnica, Madrid.

William, J., Luiz, R. (2004). Comparison of static and DynamicLoad Tests Results, Proceedings of Seventh InternationalConference on the Application of Stress-Wave Theory toPiles: 527533.


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