Pile Integrity Testing Developments

Transcription

1 927 Pile Integrity Testing Developments Développements du contrôle de pieu par les méthodes impulsionelles A.J.G. Schellingerhout 1 Profound BV A.J. van Rietschoten-Rietveld Profound BV ABSTRACT Low strain integrity testing is a widely used method for the quality control of concrete piles. The detection of pile defects with this method strongly depends on the length of the generated stress wave. However, when the stress wave is too short the wave propagation will be a three dimensional wave propagation which is conflicting with the standard signal interpretation method that is based on the one dimensional wave theory. This paper gives an overview of a number of three dimensional effects and proposes two parameters to determine the validity of the interpretation method based on the one dimensional wave theory. Based on this, analysis guidelines can be derived for the optimized use of hammers in order to stay in the realm of the standard low strain integrity testing interpretation method. RÉSUMÉ La méthode de contrôle d'intégrité des pieux par réflexion ou impédance est largement utilisée pour l'auscultation d'un pieu en béton. La détection des défauts des pieux avec cette méthode dépend fortement de la longueur de l'onde plane générée. Toutefois, lorsque l'onde plane est trop courte, sa propagation sera une propagation tridimensionnelle, ce qui est en conflit avec la méthode standard d'interprétation du signal, qui est basée sur la théorie d'onde unidimensionnelle. Cet article donne un aperçu d'un certain nombre d'effets tridimensionnels et propose deux paramètres afin de déterminer la validité de la méthode d'interprétation basée sur la théorie d'onde unidimensionnelle. En se basant sur cette théorie, les règles d'analyse peuvent être obtenues pour l'utilisation optimisée des marteaux, afin de rester dans le domaine de la méthode standard d'interprétation du contrôle de pieu par les méthodes impulsionelles. Keywords: low strain integrity testing, one dimensional wave equation, three dimensional effects, stress wave, shock wave, impulse hammer, pile integrity testing 1 INTRODUCTION Pile quality is very often determined by performing a low strain integrity test directly after pile driving or within days after installation of cast-in-situ piles. Testing can point out which piles require further examination. The measurement is performed by hitting the pile head with a hand-held hammer and measuring the response of the head with an accelerometer. For each pile multiple signals should be acquired for proper evaluation. Nowadays there is a large 1 Corresponding Author.

2 928 choice of reliable measuring equipment which can be used to perform these measurements on the construction site. The measured signals are interpreted using the one dimensional wave theory. The low strain integrity test is a quick, nondestructive test to detect pile defects which can possibly reduce the bearing capacity. This method measures the integrity of the pile and cannot be used to estimate the bearing capacity of piles. The sensitivity of the measurement method is dependent on the length of the hammer generated stress wave. Defects that are of a smaller size than the generated wave length are reduced in amplitude in the measured signal, as is discussed in Schellingerhout [1]. In general, the blow should therefore be a sharp and narrow pulse with a high frequency content. When the generated wave is too short, the one dimensional approximation is no longer valid and cannot be used any longer for the interpretation, as three dimensional effects will occur. Several three dimensional wave effects must be reduced for an accurate analysis. 2 ORIGINS OF THREE DIMENSIONAL EFFECTS A number of three dimensional effects can occur while performing a sonic integrity test. These can be caused by the geometrical dispersion, pile diameter changes or the stress wave length. 2.1 Geometrical dispersion The wave velocity in a slender pile is given by the equation: E c = (1) ρ with: c = the wave velocity [m/s] E = Young s modules [Pa] ρ = density [kg/m 3 ] Pochhammer [2] and Chree formulated an analytic solution for the wave propagation in a semi-infinite elastic cylindrical rod. This analytic solution shows a geometric dispersion of the wave velocity and leads to a reduction of the wave velocity for higher frequencies. This reduction has a small dependency on the Poisson constant of the material. The results of these calculations are shown in Benetar [3]. Wave lengths of four times the pile diameter have about a 10 % reduced wave velocity compared to the wave velocity given in equation (1). At even shorter wave lengths the reduction is about 40 %. 2.2 Pile diameter changes A pile defect is detected in an integrity test signal, because the wave reflects on a change in pile impedance. The biggest change in impedance is obviously the end of a free pile or fixed end, which leads to the full reflection of the traveling wave. During the interpretation the impedance change is usually attributed to a change in pile diameter (area) and not to a change in material parameters. A change in pile diameter can lead to three dimensional effects which where studied in small plastic piles by Schellingerhout [4]. This study shows that these three dimensional effects lead to an increase in the reflected amplitude of the wave improving the detection of defects in the signal. This result is positive for the discovery of defects, but results in an overestimation of the size of the defect in the one dimensional wave theory. 2.3 Stress wave length The stress wave is generated in the pile by the blow of a small hammer. The diameter of the hand held hammer is much smaller than the diameter of the pile. This leads to strong three dimensional effects which were studied by Chow et al. [5] in three dimensional finite element models as well as in field studies. One of their conclusions is not to place the acceleration sensor in the vicinity of the place of impact of the hammer, as is also prescribed in the French code on integrity testing NF P [6]. A transition distance is necessary to minimize these effects especially for large piles.

3 929 3 PARAMETERS All day to day measurements are still interpreted with the one dimensional wave theory. To be sure this interpretation is valid, two parameters are relevant. 3.1 Parameter t50% The first parameter is t50% which defines the hammer blow duration at half of the maximum value, as is shown in figure 1. Usually the soil damping is low at the pile top and in that case, the velocity signal can also be used for this measurement of t50%. Thus it is possible to obtain this parameter from integrity tests without an instrumented force hammer. The duration is the preferred parameter, because this is the most relevant parameter for the impact of the hammer on the pile head. c t50% 3 D = (2) D 3D = cross over parameter c = wave velocity [m/s] t50% = blow duration when the loading pulse exceeds 50% of the max. value [s] D = pile diameter [m] This 3D parameter is the ratio of the stress wave length to the diameter of the pile. For square piles an effective diameter is suggested which has the same area as an equivalent round pile. The impulse of the hammer blow can be given by: F( t) d t = 2 m h (3) Pile head velocity as function of time t50% = 0.30 ms F(t) m h = force as a function of time on the pile head [N] = mass of the hammer [kg] = impact velocity of the hammer [m/s] [mm/s] Equation (3) is valid when there are no energy losses during the impact. The energy loss in the plastic of the hammer head and the loss caused by the generated stress wave can be neglected which makes this equation a sufficiently accurate approximation time [ms] Figure 1. The hammer blow on a pile and the definition of t50%. The definition of the parameter t50% has also been described in the Dutch CUR code on Sonic Integrity Testing of concrete foundation piles [7] D parameter The second parameter is given in the following equation: With equation (2) and (3) and a low soil damping at the pile top, the pile head velocity can be approximated by: 8 1 m vimpact π 3D ρ D (4) h vp 3 v p m h = pile head velocity [m/s] = mass of the hammer [kg] = impact velocity of the hammer [m/s] ρ = density [kg/m 3 ] D = pile diameter [m]

4 930 Equation (4) shows that the pile head velocity v p strongly depends on the pile diameter. The t50% of the hammer impact can be derived from mathematical models of the hand held hammer. If the hammer is modeled with a mass spring combination, the t50% is about 1/3 of the resonance time and independent of the impact velocity. A better approximation for t50% can be obtained by using the Hertz theory of impact which predicts a dependency on the impact duration with the impact velocity. This theory is also given in Timosheko [8]. t50% [ms] Figure 2. Parameter t50% as a function of the hammer impact velocity. This measurement is executed with a 0.65 kg hammer with nylon caps on a precast concrete pile with a smooth surface. The continuous line is the Hertz prediction. 4 DISCUSSION t50% as a function of hammer impact velocity [m/s] Hertz theory ( ) -0.2 measurements The proposed parameter 3D can be used to obtain an optimal detection of defects and at the same time to verify the validity of the use of the one dimensional interpretation method. The geometric effects suggest that the 3D parameter must be between 3 and 4 which is not a very sharp optimum. If the 3D parameter is significantly higher than 4, a reduced detection of defects will occur but the one dimensional wave theory can be applied. The results of Chow et al.[5] must be reviewed with a constant 3D parameter, because their calculations used a constant input pulse duration while changing the diameter of the pile. The previously discussed geometric effects affect their calculations as well. This analysis also suggests an optimum value for 3D between 3 and 4. Their conclusion about the transition distance between the point of impact of the hammer and the placement of the accelerometer remains unchanged. Only t50% can be optimized to obtain the optimum value for 3D. This implies a range of hammers with a different impact duration, because this value is relatively constant for a hammer. A large diameter pile needs a hammer generating a longer t50%. Equation (4) also shows that the hammer mass must increase to have an acceptable pile head velocity v p. The range of impact velocities is limited by the user, so an increase in hammer mass is helpful for obtaining a good signal to noise ratio. However, there will be a reduction in amplitude because a constant value of v p would result in a hammer too heavy to be convenient for field applications. The t50% can also be increased by electronic filtering of the measured signal. This method leads to suboptimal results, because the resulting acceleration levels must be higher, which therefore results in a lower quality signal. However, most important is that a longer t50% during the impact needs a less stiff hammer, which as a result has a much bigger contact area. This reduces the stress levels in hammer and pile and also reduces the three dimensional effects as studied in Chow [5]. Pile : /10/2010 v = 7.1 mm/s t50% = 0.31 ms c = 4200 m/s l = 8.60 m fil = 0.10 ms exp : 2 V 7.93 auto Figure 3. A measured integrity test signal with a t50% of 0.31 ms. The filter has a t50% of 0.10 ms.

5 931 5 CONCLUSIONS REFERENCES The parameters t50% and 3D can be calculated for every integrity test. These parameters give information about the applicability of the one dimensional wave theory and, if optimized during the measurement, maximize the detection of pile defects in the generated signal. The quality of an integrity test can be improved by using the optimal hammer for a pile diameter. This will lead to better signals, which are the basis for further interpretation and signal comparison by an experienced engineer. Engineers can also make use of additional analysis software. As discussed, using the optimal hammer is a much better approach than adjusting the duration of the hammer blow by electronic filtering in the post processing of the signal. This latter method leads to much higher acceleration levels which reduces the signal quality. To cover a range of pile diameters several hammers are necessary. [1] A.J.G. Schellingerhout and T.K. Muller, Detection Limits of Integrity Testing, Proceedings of the fifth International Conference On The Application of Stress- Wave Theory to Piles (1996), [2] L. Pochhammer, Biegung des Kreiscylinders- Fortpflanzungs-Geschwindigkeit kleiner Schwingungen in einem Kreiscylinder. Journal für die reine angewandte Mathematik 81 (1876), [3] A. Benatar, D. Rittel, A.L. Yarin, Theoretical and experimental analysis of longitudinal wave propagation in cylindrical viscoelastic rods, Journal of the Mechanics and Physics of solids 51 (2003), [4] A.J.G. Schellingerhout, Quantifying pile defects by integrity testing. Proceedings of the fourth International Conference On The Application of Stress-Wave Theory to Piles (1992), [5] Y.K. Chow, K.K. Phoon and W.F. Chow, Three- Dimensional Stress Wave Analysis of Pile Integrity Tests, Proceedings of the seventh International Conference On The Application of Stress-wave Theory to Piles (2004), [6] Association Francaise de Normalisation, NFP Auscultation d'un élément de fondation. Partie IV - Méthode par impédance, AFNOR (Mars 1994). [7] CURNET, CUR-aanbeveling AA109 Akoestisch doormeten van betonnen funderingspalen (in Dutch), CURNET (2007). [8] S.P. Timoshenko, J.N. Goodier, Theory of Elasticity, p , McGraw-Hill, Singapore, 3 rd edition 1982.