Fayun Liang

Numerical analysis of composite piled raft with cushion subjected to vertical load

Abstract In order to mobilize shallow soil to participate in the interaction of piled raft foundation sufficiently, the authors extended the concept of piled raft to a new type of foundation named composite piled raft. In the system of composite piled raft, the short piles made of flexible materials were used to strengthen the shallow soft soil, while the long piles made of relatively rigid materials were used to reduce the settlements and the cushion beneath the raft was used to redistribute and adjust the stress ratio of piles to subsoil. Finite element method was applied to study the behavior of this new type of foundation subjected to vertical load. Influencing factors, which include ratio of length to diameter and elastic moduli of piles as well as thickness and elastic modulus of cushion, were studied in details. Load-sharing ratios of piles and subsoil as well as foundation settlement were also investigated in this paper. The conclusions had been successfully applied to some practical buildings in the coastal cities of China. The validity of the numerical results was examined through a seven-story building observed in situ.

Extreme scour effects on the buckling of bridge piles considering the stress history of soft clay

Scour is a natural phenomenon caused by the erosion or removal of streambed or bank material from bridge foundations due to flowing water. As the buckling capacity of bridge piles varies inversely with the square of the unsupported pile height, an extreme scour at the pile caused by floodwater could result in a buckling failure of the pile and collapse of the bridge. The common way to analyze the scour-affected buckling stability of bridge piles is to remove the scoured soil layers while possible changes in stress history of the remaining soils are ignored. In reality, however, the remaining soils undergo an unloading process due to scour, and its overconsolidation ratios are increased. In this study, an analytical model with modified lateral subgrade modulus is presented to investigate the extreme scour effect on the buckling of bridge piles in soft clay considering the stress history of the remaining soils. A case study is used to compare the calculated results by considering and ignoring the stress history effect. The results show that ignoring the stress history of the soft clay will overestimate the buckling capacity of bridge pile foundation under scour.

A Literature Review on Behavior of Scoured Piles under Bridges

Scour is a natural phenomenon caused by erosion or removal of streambed or bank material from bridge foundations due to flowing water. Literature has shown that most bridge failures are caused by scour. While extensive research has been done on the behavior of scour, only limited research has been done on how to evaluate effects of scour on piles. This paper presents a literature review on scoured piles including description and evaluation of scour at bridges, analysis methods for laterally loaded piles, and behavior of scoured piles. The review clearly shows that the behavior of scoured piles is important to the safety of bridges and requires more systematic investigation in future research. A suitable model to simulate scoured piles and a practical design method should be developed to evaluate the safety of bridges subjected to flood.

A review of bridge scour: mechanism, estimation, monitoring and countermeasures

Scour of sediments around bridge foundations by the stream is the most significant contributing factor for bridge failures. The scour failures tend to occur without prior warning and have led to fatalities and economic loss every year. A significant amount of work has been conducted on bridge scour. Such efforts can be broadly classified into two major categories, namely science driven and engineering driven. The science-driven research focuses on understanding the scour mechanism and aims to explain the cause of scour due to different factors. Meanwhile, engineering-driven research focuses on the estimation, monitoring and countermeasures of bridge scour. This paper presents a comprehensive and up-to-date literature review of bridge scour research and practice. Firstly, a brief introduction is given which includes recent cases of failures caused by bridge scour. Then, both scientific and technical research on bridge scour is reviewed, which are categorized into four aspects: macroscopic and microscopic mechanism, scour depth prediction carried out by experimental and field data, direct and remote monitoring methods and active and passive countermeasures. Finally, a summary is provided covering both experimental and computational methods for scour research. Discussion is also provided on emerging ideas to investigate bridge scour from both science and engineering perspectives.

Simplified Boundary Element Method for Kinematic Response of Single Piles in Two-Layer Soil

A simple approach is formulated to predict the elastic, kinematic pile bending during harmonic or transient excitation for a circular pile (rather than a simplified thin strip). The kinematic response of a pile embedded in two-layer soil is resolved in the frequency domain caused by the upward propagation of shear waves from the underlying bedrock. The simplified approach is generally valid to nonhomogeneous soil profiles, in light of the good comparison with the dynamic FE method and BDWF solution. It employs the soil-displacement-influence coefficients to consider the pile-soil interaction (resembling the spring constant in the BDWF) and provides conservative estimations of maximum kinematic bending moments at the soil-layer interface (with a sharper stiffness contrast). The accuracy of the approach may be improved by incorporating the interaction of soil into the soil-displacement-influence coefficients for such cases with .

Experimental observations and evaluations of formulae for local scour at pile groups in steady currents

ABSTRACT Bridge scour is recognized as one of the key factors that causes structure failures, which in turn leads to economic and life loss. In this study, flume tests of four typical arrangements of pier groups embedded in sand under steady clear water conditions were carried out to observe the process and maximum depth around piles of scour. The investigation included single pile, tandem piles, side-by-side piles, and 3 × 3 pile groups. Different conditions including different pile spacing, flow velocity, and water depth are considered. Moreover, the evaluation of design methods from the United States, New Zealand, and China was analyzed and compared through experimental and mathematical methods. The experimental results show that shielding and jetting effects are obvious in pile groups, which become less obvious with the increase of pile spacing. The dynamic process of scour around single pile and pile groups are quite different. Meanwhile, most of the predicted scour depths by these equations tend to be much larger than those from field data, which may lead to overdesign and consequently high construction cost. In addition, data from this study and some laboratory experiment data from previous work were used to derive the correction factors of a new scour prediction equation, which can be used to estimate the scour in a sand bed and agree well with the observations.

Analytical Solutions of Spherical Cavity Expansion Near a Slope due to Pile Installation

Based on the hypothesis that the penetration of a single pile can be simulated by a series of spherical cavity expansions, this paper presents an analytical solution of cavity expansion near the sloping ground. Compared with the cavity expansion in the half-space, the sloping free boundary has been taken into account as well as the horizontal free boundary. The sloping and horizontal free surfaces are considered by the introduction of a virtual image technique, the harmonic function, and the Boussinesq solution. The results show that the sloping free boundary and the variation of the inclination angle have pronounced influences on the distribution of the stress and displacement induced by the spherical cavity expansion. The present solution provides a simplified and realistic theoretical method to predict the soil behaviors around the spherical cavity near the sloping ground. The approach can also be used for the determination of the inclination angle of the slope according to the maximum permissible displacement.

Note on axisymmetric consolidation of multi-layered poroelastic soils with compressible constituents

ABSTRACT In the conventional analysis of Biot’s axisymmetric consolidation, the solid phase and the surrounding fluid were often assumed incompressible for simplicity. Such assumption in soil engineering ignored the effect of compressibility of constituents on the consolidation. In this article, the compressible fluid and solid in soil were taken into account for 3-D consolidation. The pore pressure, the displacements, and the stresses were expressed by two displacement functions, and the Laplace–Hankel transform was applied to set up the stiffness matrix between the generalized displacement and stress. The stiffness matrix consisted of negative exponential functions, ensuring that the computation is efficient and stable. Then the global stiffness matrix is extended by embracing the continuity of the interfaces and boundary conditions of soil base. The relationship between the generalized displacement and stress of the soil base reduces the number of unknowns of the global matrix. Such consideration reduced the number of unknowns of the global matrix and brought in more acceptable boundary conditions where the stiffness and the permeability of semi-infinite soil base can be taken into account. After the inversion of the Laplace–Hankel transform, the real solutions were obtained. The results show that the stiffness and permeability of the soil base can change the development consolidation and that the compressibility of solid particles has a great effect on the settlement in the beginning of consolidation.

Accuracy of three-dimensional seismic ground response analysis in time domain using nonlinear numerical simulations

To provide appropriate uses of nonlinear ground response analysis for engineering practice, a three-dimensional soil column with a distributed mass system and a time domain numerical analysis were implemented on the OpenSees simulation platform. The standard mesh of a three-dimensional soil column was suggested to be satisfied with the specified maximum frequency. The layered soil column was divided into multiple sub-soils with a different viscous damping matrix according to the shear velocities as the soil properties were significantly different. It was necessary to use a combination of other one-dimensional or three-dimensional nonlinear seismic ground analysis programs to confirm the applicability of nonlinear seismic ground motion response analysis procedures in soft soil or for strong earthquakes. The accuracy of the three-dimensional soil column finite element method was verified by dynamic centrifuge model testing under different peak accelerations of the earthquake. As a result, nonlinear seismic ground motion response analysis procedures were improved in this study. The accuracy and efficiency of the three-dimensional seismic ground response analysis can be adapted to the requirements of engineering practice.

Numerical Study on the Performance of Bio-inspired Bridge Attachments as Local Scour Countermeasures with Attack Angles

Scour is a natural phenomenon caused by erosion or removal of streambed or riverbank materials around bridge foundations by the stream. To reduce the scour depth around bridge foundations, local scour countermeasures are designed. During the past decades, a significant amount of countermeasure types and concepts has been proposed. Different from the traditional countermeasure concept of riverbed attachments, a new protection method named bio-inspired bridge attachments is provided in the present study. To evaluate the performance of this method, numerical simulation using Computational Fluid Dynamic (CFD) with scour initiation criteria was involved. This scour countermeasure can provide good protection of riverbed and is more economical than the traditional methods. To make full use of this concept, the preliminary design guideline of bridge attachments as countermeasures has also been carried out.