Xuanming Ding

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.

Influence of Particle Breakage on Critical State Line of Rockfill Material

AbstractThe influences of particle breakage on the position of the critical state line (CSL) were systematically investigated in this paper through a series of large-scale triaxial compression tests on Tacheng rockfill material (TRM). It was found that the critical-state stress ratio of TRM (i.e., the gradient of the CSL in the p−q space) was approximately regarded as a constant. In the e−logp space, the CSL of TRM descended with a decrease in the initial void ratio, whereas the gradient of the CSL was constant. A procedure was established for evaluating the critical state point at a same particle breakage, which comprised the breakage critical state line (BCSL). An increase of the particle breakage led to not only a vertical translation but also a rotation on the BCSL of TRM in the e−logp space, which was similar to the observation of Dog’s Bay sand. Consequently, the initial gradation (or the corresponding initial void ratio) was the dominant factor that affected the position of the CSL of TRM in the e−...

Field Tests on Bearing Characteristics of X-Section Pile Composite Foundation

AbstractTo investigate the behavior of X-section cast-in-place concrete piles (XCC pile), a series of static load tests for piled foundation are conducted on the basis of a soft soil reinforcement engineering for a sewage treatment plant in the north of Nanjing, China. The testing results are presented in load-settlement curves, pile-soil stress ratios, distributions of skin friction (side friction) and axial force, and load-sharing between side resistance and end-bearing capacity. Comparative analysis between an XCC pile and a circular section concrete pile (circular pile) with the same cross-sectional area indicates that the XCC pile with its increased perimeter can improve the vertical-bearing capacity by 20% because of the larger skin friction. Also, the XCC pile shows increasing pile-soil stress ratio and reduces settlement. The existing design standards for traditional piles can be referenced by the XCC pile composite foundation, because the axial force and skin friction distribution are the same as...

Performances of Large-Diameter Cast-in-Place Concrete Pipe Piles and Pile Groups under Lateral Loads

AbstractLarge-diameter cast-in-place concrete pipe (PCC) pile is widely used for pile foundation and pile-supported embankment over soft clay in China. However, studies on PCC pile-soil reactions (p-y curves) or performance of the pile groups under lateral load are not widely reported. A large-scale model test of a single PCC pile under lateral load is carried out, and its lateral bearing capacity, bending moment, and p-y curves are measured and analyzed. The deflection and bending moment of this PCC pile is calculated by the p-y curves method and modified pile modulus using LPILE software. Three-dimensional numerical analyses are conducted using ABAQUS software. The reliability and accuracy of the numerical simulation model are verified by comparing the results of the large-scale model test with the LPILE calculation. The distribution of deflection and bending moment along the pile and pile group efficiencies of the PCC piles are comparatively analyzed with those of a drilled shaft, which has the same co...

Effects of the Tip Location on Single Piles Subjected to Surcharge and Axial Loads

When applying axial load on piles subjected to negative skin friction (NSF), the yielded NSF is gradually eliminated. The process is notably influenced by the tip location (Y) and still a lack of understanding. This paper reports three-dimensional numerical simulations with tip locations Y = 1.00 pile diameter (D), 0.25D, 0.00D, and −1.00D. It is found that, against expectations, the dragload and NSF are not proportionally related to the tip location. When maximum dragload (P max) is eventually eliminated due to an axial load, there is also a negative crest of the skin friction, indicating that NSF still exists based on the criterion of the dragload reduction. The side resistance of the piles with Y = 1.00D and 0.25D is almost fully mobilised, which is demonstrated by the increment of end resistance that greatly increases with the larger axial loads. However, the side resistance of the piles with Y = 0.00D and −1.00D has a potential capacity to carry more loads with continued displacement since the increment of end resistance increases almost linearly with axial load. Therefore, when designing the pile foundation, the inclusion of the NSF should be governed by the amount of axial load to be resisted.

Vertical Vibration of a Pipe Pile in Viscoelastic Soil Considering the Three-Dimensional Wave Effect of Soil

AbstractAn analytical solution is developed in this paper to investigate the vertical vibration of a pipe pile in viscoelastic soil. The soil is assumed to be a homogeneous and isotropic layer. The pipe pile is considered as a one-dimensional Euler rod. Considering both the vertical and radial displacements of the outer and inner soils and the soil pile–coupled vibration, the dynamic equilibrium equations of the soil and pile are established. The dilatations of the outer and inner soil are obtained by differential transformation on the equations of soil and the variable separation method. Then, the vertical and radial displacements of the outer and inner soils are obtained. The displacement response and impedance function of the pipe pile are derived using the continuity assumption of the displacement and stress between the piles and the soils. Numerical examples are presented to analyze the vibration characteristics of the pile.

Time-domain solution for transient dynamic response of a large-diameter thin-walled pipe pile

The propagation of stress waves in a large-diameter pipe pile for low strain dynamic testing cannot be explained properly by traditional 1D wave theories. A new computational model is established to obtain a wave equation that can describe the dynamic response of a large-diameter thin-walled pipe pile to a transient point load during a low strain integrity test. An analytical solution in the time domain is deduced using the separation of variables and variation of constant methods. The validity of this new solution is verified by an existing analytical solution under free boundary conditions. The results of this time domain solution are also compared with the results of a frequency domain solution and field test data. The comparisons indicate that the new solution agrees well with the results of previous solutions. Parametric studies using the new solution with reference to a case study are also carried out. The results show that the mode number affects the accuracy of the dynamic response. A mode number greater than 10 is required to enable the calculated dynamic responses to be independent of the mode number. The dynamic response is also greatly affected by soil properties. The larger the side resistance, the smaller the displacement response and the smaller the reflected velocity wave crest. The displacement increases as the stress waves propagate along the pile when the pile shaft is free. The incident waves of displacement and velocity responses of the pile are not the same among different points in the circumferential direction on the pile top. However, the arrival time and peak value of the pile tip reflected waves are almost the same among different points on the pile top.

Three-Dimensional Effects in Low-Strain Integrity Testing of Large Diameter Pipe Piles

AbstractThe interpretation of low-strain integrity testing performed on piles is commonly based on methods developed from the one-dimensional wave propagation theory. However, stress waves generated from the impact of the hammer on the head of a pipe pile propagate not only along the vertical, but also the circumferential and radial directions. One-dimensional methods that ignore these waves may underestimate the amplitude of the incident wave, and fail to predict the development of high-frequency interferences that may compromise the assessment of the integrity, particularly of large-diameter pipe piles. To account for these three-dimensional effects, the authors formulate a solution for determining the vertical vibration response along the cross-section of the pipe pile head to an impact load, which robustly accounts for coupling of pipe pile and viscoelastic soil vibrations. Presentation of the method is followed by a discussion on identifying the mechanisms that underlie body and surface stress-wave p...

Horizontal Vibration of a Large-Diameter Pipe Pile in Viscoelastic Soil

An analytical solution is developed in this paper to investigate the horizontal dynamic response of a large-diameter pipe pile in viscoelastic soil layer. Potential functions are applied to decouple the governing equations of the outer and inner soil. The analytical solutions of the outer and inner soil are obtained by the method of separation of variables. The horizontal dynamic response and complex dynamic stiffnesses of the pipe pile are then obtained based on the continuity conditions between the pile and the outer and inner soil. To verify the validity of the solution, the derived solution in this study is compared with an existing solution for a solid pile. Numerical examples are presented to analyze the vibration characteristics of the pile and illustrate the effects of major parameters on the stiffness and damping properties.

In-Situ Tests on Cast-in-Place Concrete X-Section Pile for Bearing Capacity of Single-Pile Composite Foundation

The static load tests for pile composite foundation of X-section pile and circular section pile in the same area are conducted on trial pile in soft soil reinforcement engineering of a sewage treatment plant in the North of Nanjing, China. The load-settlement curves, pile-soil stress ratios and distributions of axial force are obtained. The comparison of test results on single-pile composite foundation of the two indicates that the vertical bearing capacity of X-section pile is 20% greater than that of the circular section pile with the same use of concrete because of the larger side friction. The results also show that the pile-soil stress ratio of X-section pile is higher than that of circular section pile, and the settlement of circular section pile is larger than that of X-section pile under the same loading. The axial forces of both X-section pile and circular section pile decrease from the pile top to the bottom. Therefore, in the same area, the bearing capacity of single-pile composite foundation of X-section pile is better than that of circular section pile. The in-situ test results are very significant for the theory study and engineering application of X-section pile composite foundation.