BLOCK VIBRATION TEST ON PILE FOUNDATION

Presented by

Abhisekh Saha

INDIAN INSTITUTE OF TECHNOLOGY, GUWAHATI

• Pile foundation generally used in weak soil for supporting massiveand important structure such as offshore towers, Nuclear power plantas well as used for machine foundation.

• Block vibration test is conducted on the pile foundation for theanalyze of the response of the structure if dynamic load applied.

• In my study two types of vibration are discussed

• Vertical vibration test

• Horizontal vibration test

i)Force acting above c.g. and

ii)Force acting below the c.g. pile cap loading system

Vertical vibration test

• For identifying thedifferent soil layersthree boreholeswere made andsamples werecollected.

• Based on differentlaboratoryobservations andfield results the soilwas divided into six

layers.

*Test conducted by B.Manna and Prof. Baidya at IIT kharagpur

• For the test bored cast in situ two concrete piles were constructed.The diameter and length of the pile were 0.45m and 22mrespectively.

• A mechanical oscillator was used as a source of the vibration. In orderto connect the load and oscillator to the pile head a steel cover platewas connected on the pile.

• The oscillator consists of two shafts so arranged that they rotate inopposite direction at the same speed when one of them is driven bya motor through a flexible shaft.

• Initially the mass on each shaft is balanced when angle is zero. Whenangle is set to a value, mass on the shaft become eccentric.

• The oscillator connected to a motor which speed was controlled by aspeed control unit.

• The acceleration pickup and theassociated vibration meter wasattached vertically on the plate overthe pile head.

• The pile was subjected to verticalvibration and amplitudes weremeasured at different frequencies foreach eccentric setting.

• Test were conducted for threedifferent static loads (8, 10 and 15KN) and four different excitingmoments (0.278, 0.366, 0.45 and0.529 N m) for each pile

Test Results

• From the recorded frequency andvertical amplitude a set of responsecurve were obtained for each staticload.

• Observation:

i)As the exciting momentincreases the peak is increased i.e.amplitude increases but the naturalfrequency decreases. So the peakshifting towards left.

ii)The natural frequency of pile 1is more than pile 2.

• The effect of static loads on the

dynamic response is also shown:

• Observations:

As we increased the load both frequency and resonant amplitude decreases.

Comparison with Novak’s Solution• Novak(1974) expression for stiffness and damping constant of single

pile

……..(1) …………(2)

• Where Vs= shear wave velocity in soil the soil layer at the pile tip,

• Ep= Young’s modulus of the pile,

• r0 and A= radius and cross sectional area of pile;

• fw1 and fw2 =dimensionless stiffness and damping parameters of pile.The stiffness parameter fw1 and damping parameter fw2 depend onthe ratio of Young’s modulus of pile Ep to shear modulus of soil Gs andthe ratio of length to radius of the pile

• Damped natural frequency and resonant amplitude of pile in verticalvibration is given by

...…..(3) …………(4)

• Where natural frequency fn=1/2π√(kw1/ms); damping ratio,

D=cw1/2√(kw

1.ms); ms =mass of external static load; m =mass of theeccentric rotating part; and e =eccentricity of rotating part.

• Theoretical response curves are computed from the above equations.The shear modulus variation at different soil layer assumed to beparabolic.

• Comparison of theoretical resultsobtained from Novak’s solution andexperimental result presented.

• From the figure it can be seen thatNovak’s model overestimate bothnatural frequency and amplitude ofthe pile.

• For example at static load of 8 KN andeccentric moment of 0.278 N m, thenatural frequency and amplitudes byexperimental values of pile 2 are 50 Hzand 0.004 mm and the Novak’s modelare 320 Hz and 0.0185 mm,respectively.

• The possible reason for this type of response are

If we look the soil profile the top two layer are verysoft and weak layer (SPT value is very less). So, these twolayer may not contribute for supporting the external load.These two layer act as a normal vertical static load andtotal vertical load is calculate by adding this component tothe external applied load. This may cause low naturalfrequency and amplitude.

Another reason may be the total external staticload is very small compared to its full capacity. This maycause the effective length i.e., the length of the pile up towhich load is transferred, much smaller than the actuallength of the pile. In the Novak’s model it is assumed thatthe load is transferred up to the full length of the pile. Sothis model calculates the stiffness for the full length of pileand it gives a very high value of stiffness resulting in veryhigh natural frequency.

Horizontal Vibration Test

• Boring operation wereconducted to identify the soilprofile and for collecting soilsamples.

• Based on field and laboratoryobservation soil stratum wereclassified into three differentcategories as per Unified SoilClassification System (USCS).

• Cross hole test was alsoconducted to finding the shearmodulus at different depth. *Test conducted by B.Manna and Prof. Baidya at Hanger

adjacent to IIT kharagpur

• Two different types of coupled vibration tests were conducted:

• type 1- horizontal exciting force acting above the c.g ;

Zc = 0.40 m and Ze = 0.44 m and

• type 2 – horizontal excitation force acting below the c.g of the pile cap loading system.

Zc=0.64 m and Ze = -0.24 m.

Where Zc=The height of the c.g. above base and Ze=the height of the horizontal excitation force above the c.g.

• The direction of moment generated at the c.g of the pile-cap loadingsystem were different for vibration test type 1 and type 2. Howeverthe magnitudes of the exciting force remain constant. Clockwise orpositive moments were produced at the c.g for type 1 but for type 2anticlockwise or negative moment were generated.

• The horizontal component was measured using one accelerationpickup attached at the side of the loading system at the level ofcenter of gravity and rocking amplitudes were measuredsimultaneously by another acceleration pickup mounted vertically atthe top on the axis of pile cap. The frequency and the amplitudesdata were recorded for both horizontal and rocking motion.

• The test were conducted for four different eccentric moments (0.187,0.78, 0.366, 0.450 Nm ).

Test Result

• The dynamic response ofgroup piles under differentexcitation level for bothhorizontal and rockingmotion are plotted.

• Important observationsfrom the graphs are

Contd.• Two resonant peaks are observed

for both horizontal and rockingcomponent. The second peak iswell separated from first one.

• The first peak is more dominantthan the second for horizontalmotion.

• The second peak is moredominant in case of rockingmotion in both cases.

• As the excitation momentincreases the amplitude increasesbut natural frequency decreases.

Theory Vs Experiment

• The comparison ofhorizontal and rockingresponse betweenexperimental results andanalytical results areshown.

• The continuumapproaches developed byNovak et al. (1978) areused to analyse thedynamic behaviour.

• The important observations are

• For both the both i.e. first and second modes of vibration theoreticalmodel predicts reasonably well.

• In prediction of rocking amplitude analytical results gives slightlylower value for the second mode of vibration (type 1). The accuracyof this model largely depend on the proper boundary zoneparameters and soil-pile separation length.

• The mechanism involved for the development of the boundary zoneparameters and soil pile separation lengths with different frequenciesare very complex.

Conclusion

• This study describes the different field experiments conducted onsingle and group pile subjected to vertical vibration and horizontalexcitation. Both vertical and horizontal excitation results arecompared with the theoretical responses.

• For the prediction of response using Novak’s plane strain model wasfound unsatisfactory for the vertical vibration.

• Novak’s continuum approach was found effective for prediction ofresults for horizontal excitation if boundary zone parameter and soilpile separation properly implemented which are very complex innature.

References• B. Manna and D.K. Baidya, “Vertical Vibration of Full-Scale Pile—Analytical

and Experimental Study” Journal of Geotechnical and Geo environmental Engineering, Vol. 135, No. 10,October 1, 2009, doi:10.1061/ASCEGT.1943-5606.0000110

• Manna Bappaditya and Baidya Dilip Kumar, “The Nonlinear Coupled Response of Single and Group Piles under Various Horizontal Excitations: Experimental and Theoretical Study,” Geotechnical Testing Journal, Vol. 35, No. 6, 2012, pp. 1–15, doi:10.1520/ GTJ20120088

• Bahaa El Sharnouby and Milos Novak, “Dynamic experiments with group of piles,” Journal of Geotechnical Engineering, Vol. 110, No. 6, June, 1984

• H. El-Marsafawi, Y. C. Han and M. Novak, “Dynamic experiments on two group of piles,” Journal of Geotechnical Engineering, Vol. 118, No . 4, April, 1992

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