Chin Jian Leo

Vertical dynamic response of pile embedded in layered transversely isotropic soil

The dynamic response of pile embedded in layered transversely isotropic soil and subjected to arbitrary vertical harmonic force is investigated. Based on the viscoelastic constitutive relations for a transversely isotropic medium, the dynamic governing equation of the transversely isotropic soil is obtained in cylindrical coordinates. By introducing the fictitious soil pile model and the distributed Voigt model, the governing equations of soil-pile system are also derived. Firstly, the vertical response of the soil layer is solved by using the Laplace transform technique and the separation of variables technique. Secondly, the analytical solution of velocity response in the frequency domain and its corresponding semianalytical solution of velocity response in the time domain are derived by means of inverse Fourier transform and convolution theorem. Finally, based on the obtained solutions, a parametric study has been conducted to investigate the influence of the soil anisotropy on the vertical dynamic response of pile. It can be seen that the influence of the shear modulus of soil in the vertical plane on the dynamic response of pile is more notable than the influence of the shear modulus of soil in the horizontal plane on the dynamic response of pile.

Comparison of Different Two-Dimensional Idealizations for a Geosynthetic-Reinforced Pile-Supported Embankment

AbstractEmbankment construction on soft ground has increased considerably over recent years as a result of the increase in infrastructure development activities and because of the unavailability of suitable land. Geosynthetic-reinforced pile-supported (GRPS) embankments provide an effective and reliable solution to the problem of constructing embankments over soft ground. The combination of geosynthetic reinforcement and piles can alleviate the uneven surface settlements on the embankment crest while reducing the embankment load transferred to the soft foundation soil. This paper presents a numerical analysis based on the FEM carried out on a GRPS embankment. Analysis was carried out in both a two-dimensional (2D) plane strain condition for different 2D idealizations of piles and in a three-dimensional (3D) condition. The interaction between geosynthetic and soil was taken into consideration during the analysis. The results obtained for the 2D models are compared with the 3D model results. The stress tran...

Numerical Modeling of Geotextile-Reinforced Embankments over Deep Cement Mixed Columns Incorporating Strain-Softening Behavior of Columns

Geotextile-reinforced embankments over deep cement mixed (DCM) columns are widely used for the construction of highway embankments over soft clay with low shear strength and high compressibility. Numerical modeling based on the finite element method (FEM) is widely used to investigate the behavior of these embankments during construction and serviceability, incorporating consolidation of the foundation soil over time. However, not much attention has been paid to the strain-softening behavior of DCM columns beyond yield, which is essential in ultimate limit-state computations to determine the stability of embankments during the failure of columns. This paper presents a constitutive model, which is an extension of the Mohr-Coulomb model, for the simulations of strain-softening behavior of cement admixed clays. The model is validated using triaxial test data found in the literature for cement admixed Singapore and Hong Kong marine clays and Ariake clay. A two-dimensional (2D) coupled mechanical and hydraulic numerical implementation of a geotextile-reinforced DCM column-supported (GRCS) embankment constructed over a very soft soil in Finland is carried out incorporating strain-softening behavior of DCM columns. Even though the isolated columns and overlapped column walls used in this embankment do not yield significantly under the service loads, the model simulations show good agreement with field data, confirming the capability of the 2D plane strain finite-element model in predicting the GRCS embankment behavior. Finally, the finite-element model with strain-softening DCM columns is used to investigate the progressive failure of a typical hypothetical GRCS embankment with isolated columns in a square pattern. Results clearly illustrate the bending failure mode caused by progressive softening of the DCM columns, including the plastic hinge development within the columns.

Numerical Modeling of an Embankment over Soft Ground Improved with Deep Cement Mixed Columns: Case History

AbstractThis paper describes a case history of a deep cement mixed (DCM) column–supported embankment that is part of the Ballina Bypass section of the Pacific Highway Upgrade project in Australia. Measured settlements during and after construction of the embankment were significantly greater than the predicted settlements and suggested that the DCM columns were yielded. The case history was analyzed using a finite-element model based on the coupled theory of nonlinear porous media. Two cases were analyzed: with and without the strain-softening behavior of DCM columns caused by breakage of the soil-cement structure. The computed settlements, excess pore-water pressures, and lateral deformations were compared with field measurements. Results show that there was good agreement between the measured and the computed parameters when the strain-softening behavior of the columns was included. These results clearly show that consideration of the strain softening of DCM columns in the analysis is important if yield...

The diffusion and sorption of volatile organic compounds through kaolinitic clayey soils

Laboratory experiments to estimate the effective molecular diffusion coefficient (D(e)) and sorption coefficient (K(d)) for volatile organic compounds through natural clayey soils were conducted using diffusion testing apparatus. The compounds tested were methyl ethyl ketone (MEK), toluene and trichloroethylene (TCE). The D(e) and K(d) values were determined by a curve fitting procedure. The compound losses, and the effects of porous disks used in the apparatus were significant. The transport of MEK was faster than that of TCE and toluene because of the lower sorption to the soils. The D(e) values of all the compounds were of the order of 10(-10) m(2)/s and smaller than the diffusion coefficient in pure aqueous solution at infinite dilution (D(0)), due to the tortuosity of the samples. The effects of the sample thickness on the parameter determination were not significant. Comparison to the K(d) values estimated from batch sorption tests and from organic carbon content (f(oc))-based predictions showed that the diffusion test results were intermediate between those from the other two methods. The diffusion tests use compacted soil samples and should be more relevant to in situ conditions, but the reliability of the tests is affected by large compound losses that cause uncertainties in their interpretation. It is recommended that more than one method be used to assess K(d) values.

A Thermodynamics-Based Model on the Internal Erosion of Earth Structures

The present paper describes a model of internal erosion of earth structures, based on rigorous thermodynamic principles and the theory of porous media. A particular focus of this paper is concerned with the initial stage of internal erosion, when the pore volume forms a continuous network, without the formation of macroscopic cavities or channels. The continuum approach is applicable in this case. The soil skeleton saturated by a pore fluid is treated as the superposition of three continua in interaction, with independent velocity fields. The pore fluid itself consists of a mixture of water and eroded particles. The erosion kinetics is based on the shear stress developed at the solid–fluid interface. The applicability of the model is illustrated by numerical simulations based on the finite element method. These simulations show how the phenomenon of piping can progressively arise, and preferentially in regions where hydraulic gradients are critical. Effects of mechanical degradations due to internal erosion are at the same time demonstrated.

A boundary element method for analysis of contaminant transport in porous media I: homogeneous porous media

A boundary element method is developed for the analysis of contaminant migration in porous media. The technique involves, firstly, taking the Laplace transform with respect to time then followed by a co-ordinate transform and a mathematical transform of the well-known advection-dispersion equation. The series of transforms reduce the equation into the modified Helmholz equation and this greatly facilitates the formulation of the boundary integral equation and a system of approximating algebraic boundary element equations. The algebraic equations are solved simultaneously in the transform space before being inverted numerically to obtain the concentration of the contaminant in real time and space. The application of this technique is demonstrated by some illustrative examples.

A TIME-STEPPING FINITE ELEMENT METHOD FOR ANALYSIS OF CONTAMINANT TRANSPORT IN FRACTURED POROUS MEDIA

This paper describes the development of a finite element method for analysing contaminant transport in double-porosity geomaterials using a time-stepping approach. In many cases, double-porosity models may be used to represent fractured rock formations and fissured soils. A distinctive feature of utilizing this kind of model is that it is not necessary to have an intimate knowledge of the nature, distribution and properties of individual fractures and fracture arrangement since the fracture geometry and details are considered only in an averaged or equivalent continuum sense. The flux exchange that occurs between the fluid in the fractures and in the solid matrix is represented by a linear heriditary process. This has the consequence that in order to carry the solution forward from time t to t + Δt, it is necessary to know and to store the complete contaminant history up to time t. This paper shows that all the hereditary information necessary to carry the solution forward is contained in the values of certain hereditary variables at time t so that it is not necessary to store the complete time history and consequently a more efficient numerical process can be developed.

Land subsidence caused by internal soil erosion owing to pumping confined aquifer groundwater during the deep foundation construction in Shanghai

Land subsidence is presented in many factors in different areas with urbanization. Internal soil erosion, owing to pumping confined groundwater during the deep foundation pit construction, has contributed to land subsidence. Four governing equations are presented to describe the process of internal soil erosion based on the mathematical–physical model. The finite element computation results, based on practical deep foundation pit engineering consisted of 8 layers of soil of Shanghai area, demonstrate that internal soil erosion will cause the increment of land subsidence and deformation and is related to the hydraulic gradient and the characters of the soils.

Laboratory studies of the diffusive transport of 137Cs and 60Co through potential waste repository soils.

Tests using reconstituted samples have been performed to assess the diffusive transport of (137)Cs and (60)Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion (D(e)) and sorption coefficients (K(d)) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D(e) values obtained in this study were of the order of 10(-6) cm(2) s(-1) for both species, higher than previously published data. The K(d) values from the diffusion and batch sorption tests were in reasonable agreement for (137)Cs, but an order of magnitude different for (60)Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.