In differ from ”Driving Formulas”, that basically consider hammer and pile as stiff masses and the reaction from the soil as static, the dynamic pile testing exploits the fact that a drop hammer in impact with the pile top induces an elastic stress wave, that moves down the pile with a characteristic wave speed for the specific material.
Step by step the pile is downward put into movement along the pile axis, and thereby ad first the skin friction is mobilized and later on the toe resistance. The skin friction generates stress waves due to the movement partly as a tension wave downwards in the pile simultaneously with the initial wave, partly as a compression wave upwards in the pile.
At the pile toe a reflected tension- or compression wave is likewise generated depending on the size of the toe resistance in comparison with the size of the initial compression amplitude in the arriving compression wave. Appearing irregularities the cross section of the pile will also create reflecting waves.
The test method is made in the way that one can separate the downward bound initial wave from the upward bound reflection wave. The principle of the method is that one measure the size of the amplitude in the initial wave moving down the pile and making the pile move. Simultaneously one measure the amplitude of the reflected stress wave generated by the reaction of the soil on the movement of the pile partly along the surface of the pile partly at the pile toe.
In principle the change of the initial wave to the reflected stress wave is caused by changes in the shape of the pile or the outer forces reaction on the pile. Because one measures the amplitudes in both the initial wave and the reflection wave one can calculate the size of the reaction.
The outer resistance consists of both dynamic and static forces and the aim of the analysis methods is among other aims to separate these from each other.
In General
The measurement is carried out directly on the pile, which has the advantage, that it is unnecessary to estimate the efficiency of the piling equipment.
PDA tests can be done during installation of the pile or during restriking. It is important to emphasize that piles often get an increased bearing capacity after installation, primarily in cohesive soils.
Restriking with PDA-test after a suitable period of time (minimum 3 days – or after a weekend) will therefore give better information regarding a piles bearing capacity in a long term.
By a proper interpretation of the test results a good compliance between the dynamic bearing capacity and the corresponding static bearing capacity normally will be good, if the tests are done at the same time.
Piling Equipment
When making the PDA-tests the piling equipment used for installation of the piles can be used for the restriking. When using other piling equipment it must be assured that it is capable of delivering sufficient energy to the pile making it move permanently minimum about 3 mm per blow.
Piling equipment is mobilized, mounted, handled and demobilized by the piling contractor. Rammeudstyr mobiliseres, klargøres, betjenes og demobiliseres af rammeentreprenøren. A member of the crew on the piling rig must be at service to cp test a/s during the tests.
Test Equipment
Precast concrete pile: På to modstående glatte sider mindst 2 × pælediameteren under pæletoppen monteres parvis instrumentering bestående af henholdsvis en straintransducer og et accelerometer. Transducerne monteres symmetrisk på begge sider af pælen for at kompensere for bøjning ved evt. skæve slag. Se under afsnittet ”Til dig der vil vide mere” under boreskabeloner.
Der kan benyttes flere transducere på samme pæl, og placeringen af transducere afhænger af pæletypen. Du kan se de forskellige boreskabeloner under afsnittet ”Til dig der vil vide mere”.
Signalerne opsamles og kalibreres i en ‘analyser’ (special-computer). For hvert slag registreres således kraft og partikelhastighed som funktion af tiden. Måleudstyr mobiliseres, klargøres, betjenes og demobiliseres af cp test a/s.
Preparing the pile
Test during restriking:
Pæle til test skal have en fri, ubeskadiget pæletop stående minimum 2,5 á 3 × pælediameteren over terræn og intakte (evt. planslebne) sideflader på målestedet fri for f.eks. olie, fedt og bitumen. Hvor det ikke er muligt, skal pæletop frigraves, så 2,5 á 3 × pælediameteren er fri under pæletoppen. Udgravningen skal tørholdes i et sådant omfang, at den frigravede pæl og måleudstyr ikke er under vand. Målestedet forberedes for montage af måleudstyr ved forboring og isætning af slagankre i betonpæle eller ved gennemboring/gevindskæring i stålpæle.
Test during installation:
Pæle til test skal have en fri, ubeskadiget pæletop og intakte (evt. planslebne) sideflader på målestedet fri for f.eks. olie, fedt og bitumen. Rammemaskine skal være til rådighed for hævning af pæletop ved montering af transducere umiddelbart før indramningen starter. Kabler og ”blæksprutter” skal fastgøres forsvarligt til pælen, så de ikke beskadiges under indramningen af pælen. Forberedelse af målested som ovenfor. Pælene må ikke rammes så dybt, at måleudstyret rammes under terræn eller evt. vandspejl.
Hammer efficiency test:
Hammeren testes på en pæl, der er rammet til en sådan modstand, og er af en sådan dimension, at den kan modstå påvirkningen fra hammeren ved den krævede maksimale energioverførsel, og målestedet ikke kommer under terræn eller vandspejl under målingen. Målingen gennemføres ved at lade hammeren arbejde i serier på ca. 10 slag pr. faldhøjde/energitrin fra laveste indstilling til højeste og igen fra højeste til laveste. Herved kan unøjagtigheder i evt. manuel aflæsning af faldhøjder elimineres ved en korrelationsanalyse mellem faldhøjde og kvadratet på anslagshastigheden.
Målestedet forberedes for montage af måleudstyr ved forboring og isætning af slagankre i betonpæle eller ved gennemboring/gevindskæring i stålpæle af cp test a/s. Evt. frigravning og anden klargøring af pæle foretages af rammeentreprenøren.
Contact our experts on PDA for further information:
Test- and analysis engineer Test- and analysis engineer
Piet Bendixen Morten Bernhard Christensen
Mobile: +45 4012 4079 Mobile: +45 2082 3999
piet@cptest.dk morten@cptest.dk
