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

Oteo, C.

Dpto. Ingenierıa 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 executedwith 3, 5 and 7 meters length, borehole diameter of 150mm and a steel pile external diameter/thickness of88.9/9mm. 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 500Kg and 1000Kg that can run along onemeter. 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 areused formanyapplicationsandsoils.Heavily reinforcedmicropiles with high axial load-carrying capacities aredemanded in the market and borehole diameter of300mm 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 TESTAREA

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

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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 of88.9mm 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, andcontinued 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. Thisallows rapidpositioningat everymicropile tobe tested.

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. Themaximum stroke is 1 meter (max. energy 1m�t),but this height is also adjustable.

For a proper selection of the hammer to be used fordynamic testing, Hussein et al. (1996) suggests a ramweight about1.5%of the static resistance tobeverified.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 reinforcementconsisted 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 micropiletopwith the help of the drills. A Pile DrivingAnalyzer,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 topwererecorded for all blows (Kormann et al. 2000).

In addition to PDA testing, Case Wave PileAnalysis 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,micropilewas practically drivenwith set per blow close to 20 millimeters.

The Ultimate Capacity for 3, 5, 7m 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 3m length fromCAPWAP and stroke per blow.

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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 8m 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 force¼ 2,243 kN;* Maximum oil pressure¼ 700 bar;* Maximum stroke¼ 160mm.

An electric hydraulic pumpwas used, with the helpof a manual pump for initial small loads. Internal jackpressures have been measured by using a calibrateddigital 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 3.3m and 2.95m. 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 1.0m. 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 75mm travel and 0.1mmprecision. The dial indicator was 50mm travel and0.01mm precision. The displacement indicators werepositioned over small horizontal steel plates welded tothe micropile reinforcement steel tube under themicropile 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 5m 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, loadwas keptconstant until the rate of axial movement did notexceed 0.02mm/5 minutes (0.25mm/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 1.95mm. In thedischarge, the remaining displacement was0.3mm.

Figure 4. Ultimate Capacity micropile 5m length fromCAPWAP and stroke per blow.

Figure 5. Ultimate Capacity micropile 7m length fromCAPWAP and stroke per blow.

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 17.7mm.

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

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 compatiblewith 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 8CAPWAPanalysis, where pile displacement was zero,compared with first cycle of static load test.

Fig. 9 shows blow no 11 CAPWAP analysis, wherepile displacement was 6.5mm, 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 10mm in secondcycle the prediction is not good.

3.4 Discussion on results

The 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 exceed6.5mm 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 testdid not predict the pile behavior for settlements greaterthan 20mm.

Concerning the micropile integrity, a hammer witha 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 CAPWAPanalysis.

4 CONCLUSIONS

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

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

2. The load-deflection behavior showed a goodagreement 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.

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