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.     

Pile Driving Analysis for Top Hammering and Bottom Hammering

Piles are used for platform foundations and other offshore structures. Pile driving performance is predicted and analyzed using the wave equation analysis method. In general, the hammering point can be any part of the pile and the same analyses used for hammering at the top of the pile (top hammering) can be used for pile driving by hammering at the bottom of the pile (bottom hammering). Based on the numerical analyses in this research including residual stresses in the pile, there is little difference between the predictions of pile penetration per hammer blow by single- or multiple-blow analyses when soil resistance is low, such as 10 blows/m. The same is true for top hammering and bottom hammering when soil resistance is low. However, when soil resistance is high compared to that of the pile-hammer system, single-blow analysis predicts early refusal for top hammering and unrealistically high pile penetration for bottom hammering. Therefore, multiple-blow analysis, which considers residual stresses, should be used for better understanding of realistic pile driving performance and predictions. Additionally, this study shows that gravity is another controlling factor for pile driving in low-resistance soil.     

Wave Propagation in a Pipe Pile for Low-Strain Integrity Testing

This paper presents an analytical solution methodology for a tubular structure subjected to a transient point loading in low-strain integrity testing. The three-dimensional effects on the pile head and the applicability of plane-section assumption are the main problems in low-strain integrity testing on a large-diameter tubular structure, such as a pipe pile. The propagation of stress waves in a tubular structure cannot be expressed by one-dimensional wave theory on the basis of plane-section assumption. This paper establishes the computational model of a large-diameter tubular structure with a variable wave impedance section, where the soil resistance is simulated by the Winkler model, and the exciting force is simulated with semisinusoidal impulse. The defects are classified into the change in the wall thickness and Young’s modulus. Combining the boundary and initial conditions, a frequency-domain analytical solution of a three-dimensional wave equation is deduced from the Fourier transform method and the separation of variables methods. On the basis of the frequency-domain analytic solution, the time-domain response is obtained from the inverse Fourier transform method. The three-dimensional finite-element models are used to verify the validity of analytical solutions for both an intact and a defective pipe pile. The analytical solutions obtained from frequency domain are compared with the finite-element method (FEM) results on both pipe piles in this paper, including the velocity time history, peak value, incident time arrival, and reflected wave crests. A case study is shown and the characteristics of velocity response time history on the top of an intact and a defective pile are investigated. The comparisons show that the analytical solution derived in this paper is reliable for application in the integrity testing on a tubular structure.     

Characterization of Heterogeneous Soils Using Surface Waves: Homogenization and Numerical Modeling

Seismic surface waves are well-adapted to study the elastic parameters, and hence the mechanical properties, of soils. The aim herein is to evaluate whether Rayleigh waves in heterogeneous soils may be used to estimate average elastic parameters and to determine how these parameters are influenced by heterogeneities. The heterogeneous medium, underlain by a homogeneous half-space, is considered as a homogeneous matrix with one or several types of randomly distributed inclusions (with a normal distribution) in the matrix. Seismic waves generated by surface loads and propagating in this medium were calculated using the finite element method (FEM) and then compared with single- and multiple-scattering homogenization methods. For the FEM calculation, special care was taken to reduce numerical dispersion through the use of elements smaller than 1/20 of the dominant wavelength. The group and phase velocity dispersion curves were measured and inverted in order to obtain the effective shear wave velocity of the heterogeneous medium. The results show a clear dependence of the wave velocity with respect to the nature, concentration, and size of the inclusions. The dependence with respect to the nature and concentration of inclusions coincides with that obtained from a multiple-scattering homogenization method up to an inclusion concentration of approximately 50%.     

Coupling Behavior of Wire Ropes Subjected to Tensile Impulses

To determine the stress state of a wire rope is tedious although analytical solution of a simple rope subjected to static load is available. While facing the problems involving complex ropes, it is usual practice to take approximations based upon the concepts of an average stress state for the constitutive ropes or for every wire. For a statically loaded cable superimposed with a tensile impulse, practically in sudden lifting of a heavy weight, the coupled axial-shearing strain waves in the cable has rarely been studied and explored through analytical approaches. Based on Costello’s force-deformation relationship and elastic wave propagation theory, analysis procedures and results are presented in this paper. Time-dependent coupled axial-torsional displacements and axial-shearing strain waves in a simple straight wire rope, due to a longitudinal impact at one end, are obtained. At the instance of the strike, a pair of coupled primary axial-torsional waves is created and begins to travel in the cable independently with different speed. Meanwhile, a coupling induced secondary torsional wave and an axial wave were observed to travel with the primary axial wave and the primary torsional wave, respectively. Phenomenon such as the traveling, reflections from ends, and intersections of the primary waves as well as the secondary wave are presented. Information provided in this paper would be useful in the study of unexpected overstress and/or fatigue problems.     

Physical Properties of Plywood and Waferboard as Pile Cushion Materials

The use of wave equation software (e.g., GRLWEAP) to determine driving stresses of piles requires the input of pile cushion thickness and elastic modulus. Up to this point in time, the values of pile cushion thickness and elastic modulus after a number of blows have been assumed and input into the program for an “end of drive” analysis. Based on the results of this laboratory study, two graphs and an equation have been developed that can be used to determine the thickness and elastic modulus, after a number of blows, for plywood and waferboard pile cushions.     

Apparent Roughness in Wave–Current Flow: Implication for Coastal Studies

High turbulence intensities generated by waves in the wave bottom boundary layer affect the mean current velocity and should be taken into account for calculation of currents in the presence of waves. This influence of the wave-induced turbulence on the mean current can be schematized by introducing an “apparent” bed roughness, which is larger than the physical bottom roughness. Apparent bed roughness is defined here as roughness that provides the same depth-mean velocity for current alone configuration as for the wave–current flow. A one-dimensional vertical “k–l” turbulence closure model that allows detailed time dependent flow modeling has been applied for apparent roughness computations. The domain of variable parameters is chosen according to the Israeli near-shore conditions. An approximate expression for apparent bed roughness calculations as a function of wave and current parameters based on this turbulence closure model is derived. Simulation of flow patterns on the Tel Aviv coast using the three-dimensional Costal and Marine Engineering Research Institute flow model and implementing apparent roughness maps, calculated by the approximate expression, has been performed.     

Impulse Response Evaluation of Foundations using Multiple Geophones

A new method to nondestructively evaluate existing deep foundations is described for the situation where access to the top of a deep foundation is prevented by an intervening structure. The test consists of striking a surface of a structure and simultaneously recording velocities with vertical geophones at several locations on an impacted surface. Arranging the geophones at different distances from both the impact location and the source of surface wave reflections allows one to minimize the interfering effects of surface waves on recognizing the compression wave reflections from an underlying deep foundation. The processed data can be evaluated as either a conventional sonic echo or impulse response test. Results of field tests of the system are described for groups of shafts at the National Geotechnical Experimentation Site at Northwestern University. Results of these tests indicate that there are preferential locations for the geophones that enhance the interpretation of the condition of an underlying drilled shaft. A case study is presented that describes the application of the method to evaluate possible concrete deterioration of existing bridge piers.     

Bottom–up and top–down influences on auditory scene analysis: Evidence from event-related brain potentials.

The physiological processes underlying the segregation of concurrent sounds were investigated through the use of event-related brain potentials. The stimuli were complex sounds containing multiple harmonics, one of which could be mistuned so that it was no longer an integer multiple of the fundamental. Perception of concurrent auditory objects increased with degree of mistuning and was accompanied by negative and positive waves that peaked at 180 and 400 ms poststimulus, respectively. The negative wave, referred to as object-related negativity, was present during passive listening, but the positive wave was not. These findings indicate bottom–up and top–down influences during auditory scene analysis. Brain electrical source analyses showed that distinguishing simultaneous auditory objects involved a widely distributed neural network that included auditory cortices, the medial temporal lobe, and posterior association cortices. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Improved Evaluation of Equivalent Top-Down Load-Displacement Curve from a Bottom-Up Pile Load Test

A modified procedure is presented in this study to evaluate the equivalent top-down load-displacement curve in a bottom-up pile load test considering elastic shortening. On the basis of the results of a parametric study on a bored pile in normally consolidated cohesive soils under undrained conditions, varying shear strength distribution and pile slenderness ratio, it was concluded that the pile shortening caused by the skin-friction component of the load in a top-down test can be related to the measured elastic shortening in a bottom-up test. A λ-factor is used to define this relationship, that is, the ratio of the top-down to bottom-up pile shortening. The factor λ = 1.0 is used for the case of a pile in soil with uniform shear strength profile, λ = 2.0 for linear profiles, 1.0<λ<2.0 for nonlinear profiles varying above linear, and λ>2.0 for nonlinear profiles varying below linear. In addition, the method suggests taking the corresponding readings of the skin-friction load component from the upward displacement curve of the top of the pile, which is a closer approximation to rigid pile displacement than the bottom when corrections for elastic pile shortening are to be applied. Assuming a fully mobilized skin-friction, a logarithmic relation for the factor λ to the normalized area under the shear strength profile was generally formulated and is limited to the assumptions on which they were derived. The suggested procedure in this study has produced the equivalent top-down load-displacement curves that are in close agreement with the measured top-down curve, as validated in the case studies.