Academic literature on the topic 'Finitely conducting soil'

A current generation type return stroke model which can take into account the possible modifications of the return stroke properties by the soil conductivity at the strike point of the lightning flash is introduced. The model is also capable of incorporating the reflection of the current at the ground end of the return stroke channel. In this paper, this return stroke model is used to investigate (a) the effect of the ground conductivity at the strike point on the source electromagnetic fields generated by return strokes and (b) the effect of current reflection at ground level on the electromagnetic field generated by return strokes. The source electromagnetic fields are the electromagnetic fields generated by lightning flashes calculated in such a way that they are not distorted by propagation effects. The results obtained show that the ground conductivity at the strike point does not significantly influence the return stroke current peak or the radiation field peak for ground conductivities higher than about 0.001 S/m. However, strike points with very poor conductivities (lower than 0.001 S/m) would result in a decrease of the peak electric field. In contrast to the peak values of the lightning current and the electric field, the peak values of the time derivatives of the lightning current and electric field are significantly reduced when the strike point of the lightning flash is located over a finitely conducting ground. The inclusion of the current reflection at ground level influences significantly the saturation of the close electric fields. The current reflection also gives rise to residual electric fields, a difference in the field levels generated by the dart leader and the return stroke. The residual field decreases as the fraction of the reflected current decreases.

This paper proposes representative constitutive relationship equations of dredging and reclamation soft soil in Korea. The marine soft soils were sampled at 23 dredged-reclaimed construction sites in the Busan, Gwangyang, and Incheon regions in Korea; then, laboratory tests were carried out. The consolidation property was classified as LL = 60% for Busan and Gwangyang marine soft soil and LL = 30% for Incheon marine soft soil by conducting basic physical property tests and consolidation tests. Busan soft soil showed a slightly higher consolidation settlement property than Gwangyang soft soil. Incheon soft soil showed the lowest consolidation settlement property among the three regions. In particular, 77 consolidation simulations were carried out at a high void ratio using the centrifugal experiment to realize high water content and in-field stress conditions. The constitutive relationship equations of each of the 23 specimens were analyzed with regard to the void ratio–effective stress and void ratio–permeability coefficient through the back analysis of finite consolidation theory from the experimental results. The constitutive relationship equation for Korean soft soil was determined to be a reasonable power function equation. The representative constitutive relationships for soft soils in the three regions were estimated using six equations, which were classified by physical and consolidation properties. The representative constitutive equations were compared to those in previous studies on high void ratio conditions of marine soft soil, and the results showed a similar range.

In order to attain a satisfactory level of safety and stability in the construction of structures on weak soil, one of the best solutions can be soil improvement. The addition of a certain percentage of some materials to the soil may compensate for its deficiency. Cement is a suitable material to be used for stabilization and modification of a wide variety of soils. By using this material, the engineering properties of soil can be improved. In this study, the effect of soil stabilization with cement on the bearing capacity of a shallow foundation was studied by employing finite element method. The material properties were obtained by conducting experimental tests on cement-stabilized sand. Cement varying from 2% to 8% by soil dry weight was added for stabilization. The effect of reinforced soil block dimensions, foundation width and cement content were investigated. From the results, it can be figured out that by stabilizing the soil below the foundation to certain dimensions with the necessary cement content, the bearing capacity of the foundation will increase to an acceptable level.

As an effective way of passive damping, isolation technology has been widely used in all types of building structures. Currently, for its theoretical analysis, it usually follows the rigid foundation assumption and ignores soil-structure interaction, which results in calculation results distortion in conducting seismic response analysis. In this paper, three-dimensional finite element method is used to establish finite element analysis model of large chassis single-tower base isolation structure which considers and do not consider soil-structure interaction. The calculation results show that: after considering soil-structure interaction, the dynamic characteristics of the isolation structure, and seismic response are subject to varying degrees of impact.

This thesis simulated the principle of the fast heat conductivity testing instrument, introduced how to use the finite element method to calculate two-dimensional unstable heat conduction condition. When establish the mathematical model, the article simplifies the soil temperature field as the two-dimensional non-stable heat conduction problem. Through computation it can get the soil temperature field at any moment in the running time and the plan uniform temperature lines, that also may obtain the change of temperature about one point in the process. The method is simple and credible. These solutions of these questions are the foundation of research the heat conduction in the ground and the temperature field.

A one-dimensional model for the consolidation of thawing soils is formulated in terms of large-strain consolidation and heat-transfer equations. The model integrates heat transfer due to conduction, phase change, and advection. The hydromechanical behaviour is modelled by large-strain consolidation theory. The equations are coupled in a moving boundary scheme developed in Lagrangian coordinates. Finite strains are allowed and nonlinear effective stress – void ratio – hydraulic conductivity relationships are proposed to characterize the thawing soil properties. Initial conditions and boundary conditions are presented with special consideration for the moving boundary condition at the thaw front developed in terms of large-strain consolidation. The proposed model is applied and compared with small-strain thaw consolidation theory in a theoretical working example of a thawing fine-grained soil sample. The modelling results are presented in terms of temperature, thaw penetration, settlements, void ratio, and excess pore-water pressures.

Numerical implementation of an anisotropic constitutive model for clays (SANICLAY) is presented. Moreover, a case study in which a soil embankment is placed on a K0-consolidated over-consolidated clay is analyzed by conducting an elastoplastic fully-coupled finite element analysis. It is shown that anisotropy has significant impact on the ground settlement caused by the placement of soil embankment and on the pore pressure generation and dissipation within the foundation soil. The simulations using SANICLAY favorably compare with the field measurements of ground settlement and pore pressure. The drawbacks of the use of an isotropic elastoplastic model (Cam Clay) are also demonstrated.

Because of evapo-transpiration, compacted loess road embankments were considered to be in a partially saturated state in both arid and semi-arid regions. Based on previous studies and the theory of unsaturated soil mechanics, a numerical analysis of rainfall infiltration in a compacted loess road embankment was conducted. The transient seepage characteristics and moisture migration patterns of the moisture in the embankment were analysed. The results showed that after precipitation, the moisture profile of the compacted loess could be separated into three zones .The data also showed that: under the effect of gravity, the water continued to migrate into the embankment after the rainfall had ended. In time, the saturated zone became partially saturated as the moisture content decreased, whereas the moisture content in the conducting and humid zones increased and the wetting front moved downward. The data also showed that the depth of the conducting and humid zones increased in time, but that the moisture content in the conducting zone increased along a linear gradient with depth, while the moisture content in the humid zone decreased in a similar manner.

In this paper, a method has been developed for determining the nonlinear heat-conducting characteristics of the soil. Two-layer container complexes were created, the side faces of which are thermally insulated, so the 1D thermal conductivity equation is used. The temperature sensor is placed at the junction of two media, and a mixed boundary value problem is solved in each region. In order to provide the inverse coefficient problem with initial data, two temperature sensors are used: one sensor was placed at the open boundary of the container and recorded the soil temperature at this boundary, and the second sensor was placed a short distance from the boundary, which recorded the air temperature. The measurements were carried out in the time interval (0,4tmax). First, the initial-boundary problem of heat conduction with nonlinear coefficients is studied by the finite difference method. Two types of difference schemes are constructed: linearized and nonlinear. The linearized difference scheme is implemented numerically by the scalar Thomas method, and the nonlinear difference problem is solved by the Newton method. The solution of a linearized difference problem was taken as the initial approximation of Newton’s method. To find the thermophysical parameters, the corresponding functional is minimized using the gradient descent method. In addition, all thermophysical characteristics (8 coefficients) were found for a two-layer container with sand and chernozem.

The lightning return stroke forms one of the severest natural sources of electromagnetic interference for ground-based and airborne systems. Many physical fields are involved in this complex physical phenomenon. Several pertinent aspects are somewhat unclear, and it is not practical to conduct the field measurements to resolve them. One such important aspect, which is of practical relevance, is the influence of soil's electrical properties on the stroke current evolution and the fields in the soil. It formed the genesis of the present work. The collection of the required data from on-field measurements would be nearly impossible, and hence suitable theoretical approach was considered. For that, an appropriate model for the return stroke is necessary. Among different models for the lightning return stroke, only the 'Self-consistent return stroke' model is found to be suitable. This model employs a macroscopic electrical representation of the underlying physical phenomenon and accounts for the associated dynamic electric field to emulate the stroke current evolution. However, in the past works, only perfectly conducting earth was considered, and it relied on the time-domain thin-wire formulation to evaluate the associated dynamic electromagnetic fields. On the other hand, a more realistic representation of the soil, with its frequency-dependent and non-linear parameters, is required for the present work. This necessitated a suitable adoption of the domain-based 'Finite difference time domain' (FDTD) method for field computation. It turned out that, in an FDTD framework, the modeling of the channel and its corona sheath, soil-ionization, and soil-dispersion is a challenging exercise. For the simulation, a straight vertical channel of 5 km is considered. A complex-frequency-based PML (perfectly matched layer) is employed to truncate the problem domain. The high aspect ratio of the channel does not permit the application of standard FDTD update equations with a realistic spatial discretization. The conventional subcell approach, generally used to model thin-wire structure in an FDTD framework, was also not usable for two reasons. Firstly, the channel has a dynamic conductivity, and secondly, the presence of corona-sheath surrounding the channel produces a typical field profile in the region. The channel in the soil and the non-linear ionization around it also posed a similar problem. A ‘Modified subcell approach’ was developed to handle the lightning channel, which is one of the essential contributions of the present work. In the ‘Modified subcell approach’, the spatial field variation is computed at each time step, taking into account all the relevant field contributions in the respective region. The radial current produced by the charge deposited in the corona sheath is also be taken into account separately. The frequency-dependent conductivity and permittivity of the soil require a convolution in time domain formulation. This would require a repeated calculation of the integral over each cell, a forbidden task. Based on one of the recent literature, a suitable simplification is adopted, thereby drastically minimizing the computational requirement. The soil ionization, a strongly field-dependent phenomenon, required a different set of developments. Each cell is divided into subgrids to account for the local field variation and the dynamic conductivity profile. The developed FDTD formulation is deployed to investigate the role of soil's electrical properties on the stroke current evolution and the field in the soil using the self-consistent return stroke model. For the first time, it is shown that the soil's electrical conductivity has some noticeable influence on the stroke current magnitude (up to about 45 %), and the ionization phenomenon in soil tends to reduce this influence . It is shown that the current magnitude varies most for a low magnitude fast-rising current as the soil ionization is minimal for these cases. On the other hand, for high-level slow-rising currents, the ionization process significantly matures, and as a result, the dependence of current magnitude on soil resistivity is reduced substantially. It is noted that the effect of soil permittivity and the frequency-dependent soil parameters on the return-stroke current is minimal. From the results of the detailed simulation, it is found that soil resistivity also affects the field in the soil significantly. The field in the air is increased with decreasing soil resistivity, and the increase is primarily due to the increase in channel current magnitude. For the field in the soil, in addition to modulating the field magnitude, soil resistivity also affects the temporal nature, with the field becoming peakier for lower resistivity. A comparison of the computed field demonstrates that the field is underestimated significantly by the prevalent quasi-static approach, and the difference increases with the radial distance from the channel. The frequency-dependency of the soil's conductivity, and permittivity to a lesser extent, significantly reduces the field in the soil. It is also seen that the current concentration near the surface due to skin-effect is altered at later periods by the field produced by the channel current. The presence of a second layer of lower resistivity at a shallow depth, on the other hand, effectively controls the current and field in the top layer. It is also shown that the field for a strike to a mountain can depend significantly on the mountain height. In summary, significant contributions have been made in the present work towards the FDTD formulations for modeling lightning phenomena and assessing the soil’s electrical parameters on lightning stroke current evolution and the resulting field .

The numerical modeling of spudcan penetration involves technical challenges posed by large soil deformation coupled with significant material non-linearity. The Lagrangian approach commonly used for solid stress analysis often does not work well with large deformations, resulting in premature termination of the analysis. Recently, the Arbitrary Langrangian Eulerian (ALE) and the Eulerian methods have been used in spudcan analysis to overcome problems caused by the soil flow and large deformation. However, most of the reported studies are based on total stress analysis and therefore shed no light on the excess pore pressures generated during spudcan installation. As a result, much remains unknown about the long-term behaviour of spudcans in the ground, which is affected by the dissipation of excess pore pressures. This paper reports an effective-stress finite element analysis of spudcan installation in an over-consolidated (OC) soft clay. The Eulerian analysis was conducted using ABAQUS/ Explicit, with the effective stress constitutive models coded via the material subroutine VUMAT. The results demonstrated the feasibility of conducting effective-stress finite element analysis for undrained spudcan penetration in OC clays. The paper discusses the flow mechanism, stable cavity depths and bearing capacity factors when spudcan installation occurs in various OC soils. It was found that the pore pressure build-up concentrates in a bulb-shaped zone surrounding the spudcan. The size of the pore pressure bulb increases with increasing penetration. The maximum excess pore pressure, which is generated near the spudcan tip, is predominantly controlled by the undrained shear strength at the tip level.

The U. S. Nuclear Regulatory Commission (NRC) is conducting a research program to investigate technical issues concerning the dry cask storage systems of spent nuclear fuel by conducting confirmatory research for establishing criteria and review guidelines for the seismic behavior of these systems. The program focuses on developing 3-D finite element analysis models that address the dynamic coupling of a module/cask, a flexible concrete pad, and an underlying soil/rock foundation, in particular, the soil-structure-interaction. Parametric analyses of the coupled models are performed to include variations in module/cask geometry, site seismicity, underlying soil properties, and cask/pad interface friction. The analyses performed include: 1) a rectangular dry cask module typical of Transnuclear West design at a site in Western USA where high seismicity is expected; 2) a cylindrical dry cask typical of Holtec design at a site in Eastern USA where low seismicity is expected; and 3) a cylindrical dry cask typical of Holtec design at a site in Western USA with medium high seismicity. The paper includes assumptions made in seismic analyses performed, results, and conclusions.

Observed from the earthquake disasters occurred over the decades in Taiwan, the deformation of near surface soil was the major cause lead to damages of underground structures or pipe lines; for instance, the damage of diversion tunnel of Shih-Kang Dam in Chichi earthquake is a typical case. To study the process of fault propagation as well as the associated soil and structure deformation during a fault offset event, model experiments of simulating thrust fault offset were set up, in which non-cohesive sands was adopted simulating near surface soil. The results, obtained from experiment studies and numerical analyses based on finite element method were then compared to further explore the behavior of soil, structure during faulting process. The soil deformation obtained from numerical analysis complies with the outcome from model experiments. In the near future, when conducting a risk evaluation for earthquake-induced damage on underground structure, a numerical stimulation can provides helpful quantity analysis and can serve as a handy tool for the earthquake resistance design.

Abstract Ground-heat transfer is tightly coupled with soil-moisture transfer. The coupling is threefold: heat is transferred by thermal conduction and by moisture transfer; the thermal properties of soil are strong functions of the moisture content; and moisture phase change includes latent heat effects and changes in thermal and hydraulic properties. A heat and moisture transfer model was developed to study the ground-coupled heat and moisture transfer from buildings. The model also includes detailed considerations of the atmospheric boundary conditions, including precipitation. Solutions for the soil temperature distribution are obtained using a finite element procedure. The model compared well with the seasonal variation of measured ground temperatures.

Abstract In this paper the effects of moisture on the heat transfer from two basic types of building foundations, a slab-on-grade and a basement, are examined. A two-dimensional finite element heat and moisture transfer program is used to show the effects of precipitation, soil type, foundation insulation, water table depth, and freezing on the heat transfer from the building foundation. Comparisons are made with a simple heat conduction model to illustrate the dependency of the soil thermal conductivity on moisture content.

Abstract Suction caissons have emerged as a viable solution for the foundations of offshore wind turbines, which are gaining momentum worldwide as an alternate energy source. When used in a multi-bucket jacket system, the system capacity is often governed by the uplift capacity of the windward bucket foundation. Seabed conditions at offshore windfarm sites often comprise dense sand where the soil response may be drained, partially drained or undrained depending on the loading regime, the foundation dimensions and the soil conditions. Given the large difference in uplift capacity of caissons for these different drainage conditions, predicting the behavior of a suction caisson under a range of drainage conditions becomes a paramount concern. Consequently, this paper presents the findings of a coupled finite element investigation of the monotonic uplift response of the windward caisson of a multi-bucket jacket system in a typical dense silica sand for a range of drainage conditions. The study adopts a Hypoplastic soil constitutive model capable of simulating the stress-strain-strength behavior of dense sand. This choice is justified by conducting a comparative study with other soil models — namely the Mohr Coulomb and bounding surface sand models — to determine the most efficient soil failure model to capture the complex undrained behavior of dense sand. The numerical predictions made in this study are verified by recreating the test conditions adopted in centrifuge tests previously conducted at the University of Western Australia, and demonstrating that the capacity from numerical analysis is consistent with the test results. The Hypoplastic soil constitutive model also provides an efficient method to produce accurate load capacity transition curves from an undrained to a drained soil state.

This paper provides the methods and results associated with an engineering assessment for a project involving pile driving adjacent to an active 6-inch (152 mm) nominal diameter gas pipeline. The pile driving was associated with the expansion of the I-95 Highway located in Daytona Beach, Florida. The work involved analysis, metallurgical field evaluation, and measurement of strain and acceleration in the pipe during the pile driving. The analysis involved using finite element methods to predict stresses in the pipe using acceleration loads provided during a previous pile driving exercise. Using a range of soil stiffness values, the calculated bending stresses in the pipeline ranged from 50 to 2,000 psi (0.3 to 13.8 MPa). Even with the most compliant soils, the stress was relatively low compared to the hoop stress created by an internal pressure of 500 psi (3.4 MPa). The metallurgical field investigation involved careful inspection of the pipe quality, including field replication and determining the carbon content of one weld. The strain measurements indicated that the stress levels in the pipe were below design stress limits and that the short-term pile driving loads did not inflict serious injury to the line. Findings of the investigation indicated that the pipe had been well-maintained over its 40 year life and that no measurable corrosion was present. This project demonstrates the benefits derived in using a range of engineering disciplines and capabilities to ensure safety in conducting potentially-damaging activities adjacent to active gas pipelines.

Abstract A light-weight gravity installed plate anchor (L-GIPLA) is put forward in the present study to secure offshore floating structures, such as floating wind turbines, floating net-cages, wave and tidal energy converters, and floating oil and gas platforms. The L-GIPLA is installed with the aid of a booster, which can be connected at the tail of the anchor during dynamically installation and retrieved after installation. The L-GIPLA owns the advantages of dynamically installation, deep penetration depth and high capacity efficiency. In the present study, the dynamic installation process of the L-GIPLA with a booster in clay is modelled by conducting three-dimensional large deformation analysis in commercial software ANSYS CFX, which is a computational fluid dynamics (CFD) software based on the finite volume method (FVM). Non-Newtonian fluid model, incorporating the strain-rate and strain-softening effects, is used to simulate the undrained clayey soil. Various factors influencing the final penetration depth of the anchor have been studied. These factors include the soil strength characterizations (strain-rate and strain-softening effects), and impact velocity. The results show that the L-GIPLA can obtain a relatively deep penetration depth in the seabed, indicating the L-GIPLA is an efficient alternative in offshore engineering.

Abstract The present paper presents a framework for holistic optimization of monopile foundations for a full offshore windfarm. The framework has been developed in-house at Wood Thilsted Partners and has been used for design of several full-scale wind farms including Vineyard Wind, the first commercial scale wind farm in the US. The backbone of the code is a beam finite-element based structural analysis code that performs all necessary design checks according to the governing standards. A direct coupling to various 3D FE-software packages is in place for detailed analyses of non-trivial structural details such as bolted flange connections and welded attachments. Three dimensional FE is also used for position-specific 3D soil analysis for calibration and validation of the non-linear soil-structure interaction springs serving as boundary conditions for the model. Installation and drivability assessments are similarly included in the design flow based on the beam model and validated subsequently via parameterized 3D models. Everything is integrated in a fully automated optimization framework that can perform a full optimization sweep including automatic generation of design reports and drawing material ready for certification for a full windfarm of several hundred individual positions in a few hours. Representative examples illustrate how the framework provides the possibility for conducting large scale sensitivity studies, such as diameter and clustering studies allowing for holistic cost-optimization across all positions in a windfarm.