Low-strain Pile Integrity Testing Research Articles

Analytical solution for vertical vibration of partially embedded large-diameter floating pile

A rigorous analytical model for partially embedded large-diameter floating pile (PLFP) is devised based on a three-dimensional continuum pile and soil model. The analytical solutions to the dynamic response of PLFP are obtained using Fourier transformation and variable separation. Then, the accuracy of the obtained dynamic impedance and velocity is verified by comparing the derived values with existing solutions and the measured data of the experimental model. Finally, parameter analyses of dynamic impedance and velocity response are conducted. The results indicate the following. The use of a one-dimensional pile model to calculate the dynamic impedance of PLFP results in the overestimation of resonance amplitude and frequency. Defect detection is more difficult to implement in a partially embedded pile than in a fully embedded pile. In practice, the suitable position for a signal receiver installed on a pile head during low-strain pile integrity testing is 0.6r0.

Large-scale model test for detecting pile defects using the parallel seismic method

Although the parallel seismic (PS) method has been successfully applied to determine the unknown foundation depth of buildings and bridges, the ability to detect defects in pile foundations has not been verified. In this paper, three large-scale model piles with artificial defects are designed and constructed in multilayered ground. The capability of cross-hole sonic logging, low-strain pile integrity testing and PS method to detect the defects of pile foundation is compared. The effects of the pile-to-borehole distance and the excitation position on the stress wave transmission are analyzed in detail. Finally, the abnormal characteristics of stress waves corresponding to different types of defects are summarized. The results show that pile defects affect the difference in the stress wave impedance by the PS test, and the features of the first arrival waves can reflect the type of defect. When the distance between the borehole and pile is within 0.5 m, the wave form and amplitude of the first arrival waves can be used to distinguish the defects of necking, breaking and sedimentation. Increasing the pile-to-borehole distance by more than 1 m is not conducive to identifying defects. When the pile defect position is close to the side of the shortest path of stress wave transmission, the abnormal characteristics of the first arrival waves are much clearer than when the defect position is far from the side; otherwise, the feature of the defects will be greatly weakened.

Towards Intelligent Interpretation of Low Strain Pile Integrity Testing Results Using Machine Learning Techniques.

Low strain pile integrity testing (LSPIT), due to its simplicity and low cost, is one of the most popular NDE methods used in pile foundation construction. While performing LSPIT in the field is generally quite simple and quick, determining the integrity of the test piles by analyzing and interpreting the test signals (reflectograms) is still a manual process performed by experienced experts only. For foundation construction sites where the number of piles to be tested is large, it may take days before the expert can complete interpreting all of the piles and delivering the integrity assessment report. Techniques that can automate test signal interpretation, thus shortening the LSPIT’s turnaround time, are of great business value and are in great need. Motivated by this need, in this paper, we develop a computer-aided reflectogram interpretation (CARI) methodology that can interpret a large number of LSPIT signals quickly and consistently. The methodology, built on advanced signal processing and machine learning technologies, can be used to assist the experts in performing both qualitative and quantitative interpretation of LSPIT signals. Specifically, the methodology can ease experts’ interpretation burden by screening all test piles quickly and identifying a small number of suspected piles for experts to perform manual, in-depth interpretation. We demonstrate the methodology’s effectiveness using the LSPIT signals collected from a number of real-world pile construction sites. The proposed methodology can potentially enhance LSPIT and make it even more efficient and effective in quality control of deep foundation construction.

Three-dimensional effects in low-strain integrity testing of piles: analytical solution

The interpretation of low-strain integrity tests of piles is commonly based on methods developed around the one-dimensional wave propagation theory. In reality, waves resulting from the impact of a hammer on a pile head propagate in three dimensions, and the validity of the plane-front assumption is rather questionable for cases where the size of the hammer is small relative to that of the pile. This paper presents an analytical model of the dynamic response of a pile to an impact load on its head, considering propagation of waves in both vertical and radial directions. The proposed formulation applies to a pile of finite length embedded in multilayered elastic soil, and allows for considering both shape and material pile defects, by reducing locally the radius of the pile cross section or the Young’s modulus of its material. Arithmetic examples are used to depict the effect of high-frequency interferences on the interpretation of pile integrity tests, which can only be accounted for in the three-dimensional formulation of the problem, and lead to practical suggestions for the interpretation of such tests.

Comparison of Three Different Methods for Pile Integrity Testing on a Cylindrical Homogeneous Polyamide Specimen

Three different methods for pile integrity testing are proposed to compare on a cylindrical homogeneous polyamide specimen. The methods are low strain pile integrity testing, multichannel pile integrity testing and testing with a shaker system. Since the low strain pile integrity testing is well-established and standardized method, the results from it are used as a reference for other two methods.

A study on the application of the parallel seismic method in pile testing

Parallel seismic testing of pile is theoretically studied using the ray path theory and the elastodynamic finite integration technique (EFIT). According to the basic principles and the testing process, the practical pile model is simplified as a space axisymmetric problem, and the definite solution can be obtained with given boundary and initial conditions. The EFIT is a simple numerical tool and employed in this study. A more accurate method for determining the pile length is presented based on ray path theory, the effects of pile-to-borehole distance and other parameters on the testing results are numerically analyzed in detail. In addition, parallel seismic testing is compared with the low-strain pile integrity testing with respect to the pile with a cap. The former has good performance in determining the pile length. Also, neck or expansion part in the pile body is well able to be identified.

Effects of the Source on Wave Propagation in Pile Integrity Testing

One-dimensional stress wave theory is widely used to analyze quantitatively the reflections in low-strain integrity testing of piles. However, a point or disk loading produces body and Rayleigh waves near the pile top. The multireflections of these waves from the lateral surface of a pile are present in the wave field near the pile top. Effects of three-dimensional waves on the near field responses are obvious. These effects can be interpreted erroneously by an inexperienced user as “noises” or “pile anomalies.” To investigate wave propagation in the longitudinal direction, the behavior of the waves in the far field (some distance below the pile top) is studied by theoretical analysis of the longitudinal modes in free cylinders and numerical simulations. The wave pattern at the pile top is analyzed based on the response of an elastic half-space to a harmonic disk loading. The results show that when the ratio of the characteristic length of an impact pulse to the cylinder radius is large enough, the components of Rayleigh waves in the wave field at the pile top are diminished; the waves in the far field behave approximately as plane waves; the responses at positions between 1/2R and 3/4R from the pile axis are less affected by the multireflections. The results from numerical simulations support the practical recommendation to use a ratio of characteristic wavelength to pile radius larger than four. Under this condition, the reflections from the far field (say deeper than two pile diameters) can be analyzed from the responses at receiver positions about 0.6R from the pile axis based on one-dimensional stress wave theory.

Early stage elastic wave velocity of concrete piles

The low-strain pile integrity testing method is used for quality control of new piles as well as length and integrity check for old piles. If no other methods for calibration are available, the accuracy and reliability of the techniques depend on a good estimation of the elastic wave velocity. In practice it is assumed that piles can be tested 7 days after casting and that the velocity is almost constant in all piles of one kind in a construction project. Recent measurements at the BAM test site in Horstwalde, Germany, showed that the 7 day criterion is not generally applicable.