Two-layer Soil Research Articles

Construction of a two-layer soil model from earth conductivity maps

Construction of a two-layer soil model from earth conductivity maps

Optimization of substation grounding grid design for horizontal and vertical multilayer and uniform soil condition using Simulated Annealing method.

Grounding systems are critical in safeguarding people and equipment from power system failures. A grounding system’s principal goal is to offer the lowest impedance path for undesired fault current. Optimization of the grounding grid designs is important in satisfying the minimum cost of the grounding system and safeguarding those people who work in the surrounding area of the grounded installations. Currently, there is no systematic guidance or standard for grounding grid designs that include two-layer soil and its effects on grounding grid systems, particularly vertically layered soil. Furthermore, while numerous studies have been conducted on optimization, relatively limited study has been done on the problem of optimizing the grounding grid in two-layer soil, particularly in vertical soil structures. This paper presents the results of optimization for substation grounding systems using the Simulated Annealing (SA) algorithm in different soil conditions which conforms to the safety requirements of the grounding system. Practical features of grounding grids in various soil conditions discussed in this paper (uniform soil, two-layer horizontal soil, and two-layer vertical soil) are considered during problem formulation and solution algorithm. The proposed algorithm’s results show that the number of grid conductors in the X and Y directions (Nx and Ny), as well as vertical rods (Nr), can be optimized from initial numbers of 35% for uniform soil, 57% for horizontal two-layer soil for ρ1> ρ2, and 33% for horizontal two-layer soil for ρ1< ρ2, and 29% for vertical two-layer soil structure. In other words, the proposed technique would be able to utilize square and rectangle-shaped grounding grids with a number of grid conductors and vertical rods to be implemented in uniform, two-layer horizontal and vertical soil structure, depending on the resistivity of the soil layer.

Seismic bearing capacity of shallow strip footing embedded in slope resting on two-layered soil

Abstract In this paper, the limit equilibrium method with the pseudo-static approach is developed in the evaluation of the influence of slope on the bearing capacity of a shallow foundation. Particle swarm optimisation (PSO) technique is applied to optimise the solution. Minimum bearing capacity coefficients of shallow foundation near slopes are presented in the form of a design table for practical use in geotechnical engineering. It has been shown that the seismic bearing capacity coefficients reduce considerably with an increase in seismic coefficient. Be sides, the magnitude of bearing capacity coefficients decreases further with an increase in slope inclination.

Site-specific response of a 5 MW offshore wind turbine for Gujarat Coast of India

In India, the coastline of Gujarat is known to have a high potential for offshore wind energy. However, the region is highly seismically active with large magnitude earthquakes experienced in the past. The aim of the work is to investigate the site-specific response of an offshore wind turbine (OWT) for assessment of suitability of installation of OWT along the Gujarat coast. Seismic hazard analyses of 44 sites are performed using deterministic approach and response spectrum matched acceleration time histories at all these sites are obtained. Monopile for the wind turbine is designed considering a two-layered soil profile. As per specifications of National Renewable Energy Laboratory (NREL), three-dimensional finite element model of 5 MW OWT along with the soil domain is developed in the software ABAQUS considering material nonlinearity. Earthquake time histories combined with wind and wave load are being employed for the analyses. It is observed that the sites lying between Kachchh and Saurashtra regions are most vulnerable whereas sites along the western coast of Saurashtra are found to be least vulnerable. Further, seismic fragility curves for the OWT are derived for different damage states in order to evaluate the probability of damage under various seismic scenarios.

Effect of Non-Homogeneous Soil Characteristics on Substation Grounding-Grid Performances: A Review

Designing an effective grounding system for AC substations needs predetermination of ground resistance and ground potential distribution caused by fault current’s presence in the ground. Therefore, it is necessary to have a suitable grounding grid structure in the soil properties in which the grid is buried. Though the soil composition where the grounding grid is located is typically non-homogeneous, the soil is often presumed to be homogeneous due to the complexities of grounding system analysis in non-homogeneous soil. This assumption will lead to inaccuracies in the computation of ground resistance and ground potentials. Although extensive research has been done on non-homogeneous soil structure, comprehensive literature on grounding system performance in non-homogeneous soil is yet to be reviewed. Thus, this paper reviews the effect of non-homogeneous soil on the grounding system, with different soil characteristics in horizontal and vertical two-layer soil structure and the horizontal three-layer soil structure. In addition, the effect of design parameters on the grounding performance in non-homogeneous soil conditions for non-transient fault conditions is also studied. The significance of this study is that it provides a comprehensive review of grounding performance as grounding design changes and their effects as soil layers and their corresponding features change. This knowledge will be useful in developing safe grounding designs in non-homogeneous soil.

Transient analysis of grounding electrodes in a two-layer soil considering frequency dependence of soil

Transient analysis of grounding electrodes in a two-layer soil considering frequency dependence of soil

Numerical Analysis of 1-D Consolidation of Two-layered Soils with Different Initial Excess Pore Water Pressure

Numerical Analysis of 1-D Consolidation of Two-layered Soils with Different Initial Excess Pore Water Pressure

Capillary Barriers during Rainfall Events in Pyroclastic Deposits of the Vesuvian Area

In the present paper, the capillary barrier formation at the interface between soil layers, which is characterized by textural discontinuities, has been analyzed. This mechanism has been investigated by means of a finite element model of a two-layer soil stratification. The two considered formations, belonging to the pyroclastic succession of the “Pomici di Base” Plinian eruption (22 ka, Santacroce et al., 2008) of the Somma–Vesuvius volcano, are affected by shallow instability phenomena likely caused by progressive saturation during the rainfall events. This mechanism could be compatible with the formation of capillary barriers at the interface between layers of different grain size distributions during infiltration. One-dimensional infiltration into the stratified soil was parametrically simulated considering rainfall events of increasing intensity and duration. The variations in the suction and degree of saturation over time allowed for the evaluation of stability variations in the layers, which were assumed as part of stratified unsaturated infinite slopes.

Efficient slope reliability analysis and risk assessment based on multiple Kriging metamodels

Kriging metamodel is an effective method for slope reliability analysis because of its flexibility and accuracy in data interpolation. However, the accuracy of traditional Kriging metamodel needs to be improved when facing complex limit state functions in slope stability problems. In addition, traditional Kriging metamodel cannot provide the key component—slope failure consequence—for risk assessment of slope failure, which greatly hinders the application of the Kriging method. This paper develops a multiple Kriging (MK) metamodels method to address the above problems within the framework of limit equilibrium method. The proposed MK metamodels are developed by simultaneously establishing a Kriging metamodel for each potential slip surface of a slope, with a global optimization search for the correlation parameters by the genetic algorithm. Slope stability analysis is then efficiently performed on the MK metamodels to obtain the associated factor of safety (FS) and locate the critical slip surface (CSS). Since the FS and the associated CSS are available, slope reliability analysis and risk assessment can be conducted with ease, thereby circumvent the abovementioned issues. For illustration and validation, the proposed model is applied to two examples, including a two-layered cohesive soil slope and a practical case of the Congress Street cut slope in Chicago. Both the influence of regression models and correlation models on the accuracy of the MK metamodels are discussed. The results show that the proposed model is more accurate than the conventional model in estimating the slope failure probability. More importantly, it is shown that the proposed model can effectively assess the risk of slope failure, which greatly extends the application scope of the Kriging based method.

Evaluation of the WEAP model in simulating subbasin hydrology in the Central Rift Valley basin, Ethiopia

BackgroundThe subbasin hydrologic behaviors have been altered by many natural and anthropologic factors such as climate change and land development activities. Model-based assessment can be used to simulate both natural hydrological processes, human-induced effects, and management strategies on water resources. For the Ketar subbasin, the WEAP (water evaluation and planning) hydrologic model was developed that aimed at (1) evaluating the application of the WEAP model in the Ketar subbasin, (2) evaluating the demonstration of the WEAP model using model efficiency evaluation criteria, and (3) simulating hydrological processes of the subbasin using the WEAP model.MethodsWEAP-based soil moisture method (rainfall-runoff) hydrology routine is comprised of a lumped, one-dimensional, two-layer soil water accounting that uses empirical functions to designate evapotranspiration, surface runoff, interflow, and deep percolation for a sub-unit at root zone. A catchment is considered as the smallest hydrologic response unit. The catchment’s surface hydrological balance is typically estimated by discretizing the catchment into multiple land uses for which water balance is estimated at root zone.ResultsThe monthly measured and simulated streamflow statistics showed a positive strong relationship with R2 of 0.82, NSE of 0.80, and IA of 0.95; and with R2 of 0.91, NSE of 0.91, and IA of 0.98 for calibration and validation periods respectively. Similarly, the mean monthly measured and simulated streamflow showed an agreement with R2 of 0.99, NSE of 0.97, and IA of 0.99, and R2 of 0.94, NSE of 0.93, and IA of 0.93 for the periods of calibration and validation respectively.ConclusionThe model has demonstrated the capability to represent the hydrologic dynamics of the subbasin both at monthly and mean monthly periods. In general, the overall model performance evaluation statistics show a very good agreement between measured and simulated streamflow at the outlet of the subbasin.

Effects of sliding liquefaction on homogeneous loess landslides in western China

Sliding liquefaction is considered to be the cause of high-speed and long-distance sliding of some homogeneous loess landslides in western China. However, there is still a lack of necessary experimental research and analysis on the effects of sliding liquefaction on these landslides. In this work, the effects of sliding liquefaction on irrigation-induced, high-speed and long-distance loess landslides on the South Jingyang Tableland area in China are studied by performing large-scale ring shear tests and using the sled mode. The results are as follows. (1) There are two kinds of long-runout sliding modes of loess landslides on the South Jingyang Tableland: sliding along the terrace surface and sliding within the saturated terrace alluvium, which is associated with sliding liquefaction. Both sliding modes can lead to long-runout sliding. (2) There are some differences in the inclination of the sliding surface between the two sliding modes. Based on the inclination of the sliding surface, the corresponding sliding mode can be distinguished. (3) Under the two sliding modes, the large shear mechanical properties of the two-layer soil composed of loess and alluvial sandy silt show significant differences. The friction between the loess and dry terrace alluvium increases with increasing normal stress and shear rate, while the friction between the loess and saturated terrace alluvium presents the opposite trend. The results show that the sliding distances under different sliding modes present opposite trends with the change in sliding speed. (4) Based on the test results from the ring shear tests and the morphological characteristics of the sliding surface, the sliding mode and sliding distance of a loess landslide can be identified and predicted.

Ultimate Bearing Capacity of Rigid Footing on Two-Layered Soils of Sand–Clay

Abstract This study widely investigates the ultimate bearing capacity of a rigid footing on the free surface of sand overlying clay using the rigid-plastic finite-element method (RPFEM). Interface ...

Centrifuge and numerical modelling of the seismic response of tunnels in two-layered soils

Soil deposits inevitably affect the seismic performance of underground structures, and underground tunnels are commonly constructed on sites composed of layered soil deposits. In this study, dynamic centrifuge tests and numerical modelling are conducted to enhance the fundamental understanding of the kinematic soil-tunnel interaction for tunnels constructed in layered soils that consist of muddy clay and silty clay. The site seismic response and tunnel seismic performance between the centrifuge and numerical modelling are compared. The acceleration time histories, amplification factors of the input motions, and the Fourier spectra between free field and structural field are experimentally and numerically investigated. For comparative purposes, parametric studies of tunnels in a single muddy clay and a single silty clay using verified numerical modelling are carried out. In addition, the dynamic bending moment and axial forces obtained by centrifuge and numerical modelling are compared with the solutions predicted by analytical methods in prior studies.

Bearing Capacity Factor of Circular Footings on Two-layered Clay Soils

Geotechnical engineers often deal with layered foundation soils. In this case, the soil bearing capacity assessment using the conventional bearing capacity theory based on the upper layer properties introduces significant inaccuracies if the top layer thickness is comparable to the rigid footing width placed on the soil surface. Under undrained conditions the cohesion increases almost linearly with depth. A few theoretical studies have been proposed in the literature in order to incorporate the cohesion variation with depth in the computation of the ultimate bearing capacity of the strip and circular footings. Rigorous solutions to the problem of circular footings resting on layered clays with linear increase of cohesion do not appear to exist. In this paper, numerical computations using FLAC code are carried out to assess the vertical bearing capacity beneath rough rigid circular footing resting on two-layered clays of both homogeneous and linearly increasing shear strength profiles. The bearing capacity calculation results which depend on the top layer thickness, the two-layered clays strength ratio and the cohesion increase rates with depth are presented in both tables and graphs, and compared with previously published results available in the literature. The critical depth for circular footing is found significantly less than for strip footing. Doi: 10.28991/cej-2021-03091689 Full Text: PDF

Radiative transfer technique for retrieving the radiative properties of agricultural soils

The knowledge of soil optical properties is very important for agriculture production and decision making in selecting and managing land to cultivate. These properties can be retrieved from the radiative transfer technique if appropriate method is used to solve the direct problem of the radiative transfer equation. The radiative transfer equation describes the propagation of the radiation in the soil. Thus, understanding the interactions of the radiation with agricultural soil is an active research area. In this paper, the reflectance characteristics of typical agricultural soil are retrieved. The soil, considered as a semitransparent plane parallel two-layered media with the refractive index mismatch, is illuminated at the top by a collimated radiation. This refractive index mismatch causes reflection and refraction to occur at the soil boundaries and interfaces. The two layered agricultural soil radiative properties and the scattering phase function are determined from the Mie theory assuming that the soil layers consist of only clay particles. With these radiative properties, the equation of radiative transfer is solved using the discrete spherical harmonics method (DSHM) under Marshak boundary conditions. The computational results are in agreement with existing literature results on two-layered media. The two-layered agricultural soil is then studied with a constant or spatial variation of the refractive index. The effects of soil radiative properties and the scattering phase function on the radiation characteristics are discussed. The anisotropy of the soil reflectance is analyzed and results demonstrate that a predominantly backscattering soil reflects more radiation than the predominantly forward scattering and Rayleigh scattering soil. These findings are applicable in the context of agricultural soil as it provides insight for developing optical equipment for quick soil properties measurement before crop planting.

Global sensitivity analysis for a prediction model of soil solute transfer into surface runoff

The solute transfer from soil to surface runoff has been recognized as a major source of non-point source pollution, and it is of great significance to predict the solute transfer process using a modeling approach. Tong et al. (2010) developed a two-layer soil mixing model with a soil mixing layer and a layer of ponding-runoff water on soil surface. The model has three critical parameters, i.e., depth dmix of the soil mixing layer and incomplete mixing parameters α and γ that determine solute concentration in the ponding-runoff layer and the soil layer beneath the soil mixing layer, respectively. Using the variance-based Sobol’s methods, a global sensitivity analysis (GSA) was performed for this model to determine importance or influence of the three parameters to the model prediction of solute transfer from soil to surface runoff αCw(t). The GSA evaluated the time-varying first order index (FOI), second order index (SOI), third order index (DOI), total order index (TOI), and TOI-FOI for cases 1, 2 and 3, i.e., initially unsaturated soil with controlled sub-surface drainage, initially saturated soil with controlled sub-surface drainage, and initially saturated soil with restricted sub-surface drainage. The FOIs of α and dmix varied oppositely, and the interactions between α and dmix became stronger with time for three cases. For cases 1 and 2, γ was important for the predicted αCw(t), which was the most important at later time, and it had strong interactions with α and dmix. γ was found to be the most sensitive and important parameter for the predicted αCw(t) all the time for case 1. However, in case 3, γ was not sensitive to the predicted αCw(t), with almost no interactions of γ with α or dmix, where γ can be set as a constant.

Numerical Study of Laterally Loaded Piles in Soft Clay Overlying Dense Sand

This paper presents the results of three dimensional finite element analysis of laterally loaded pile groups of configuration 1×1, 2×1 and 3×1, embedded in two-layered soil consisting of soft clay at liquid limit overlying dense sand using Plaxis 3D. Effects of variation in pile length (L) and clay layer thickness (h) on lateral capacity and bending moment profile of pile foundations were evaluated by employing different values of pile length to diameter ratio (L/D) and ratio of clay layer thickness to pile length (h/L) in the analysis. Obtained results indicated that the lateral capacity reduces non-linearly with increase in clay layer thickness. Larger decrease was observed in group piles. A non-dimensional parameter Fx ratio was defined to compare lateral capacity in layered soil to that in dense sand, for which a generalized expression was derived in terms of h/L ratio and number of piles in a group. Group effect on lateral resistance and maximum bending moment was observed to become insignificant for clay layer thickness exceeding 40% of pile length. For a fixed value of clay layer thickness, lateral capacity and bending moment in a single pile increased significantly with increase in pile length only up to an optimum embedment depth in sand layer which was found to be equal to three times pile diameter and 0.21 times pile length for pile with L/D 15. Scale effect on lateral capacity has also been studied and discussed. Doi: 10.28991/cej-2021-03091686 Full Text: PDF

Study on Tunnel Face Failure Mechanism in Two-Layer Soils

Study on Tunnel Face Failure Mechanism in Two-Layer Soils

Numerical Analysis of Settlement for Unusual Footing (L-Shape) on Layered Soil

Transfer the load from the foundation to the soil causes deformation that soil and surface of the soil near the foundation. Results of the settlement or deformation occurring under the foundation cause destroyed of structure built on it, due to the soil fail. This paper presents a numerical analysis to study the settlement of unusual footing (L-shape) on a two-layered soil using finite element program (ELPLA). Two cases for layered soil were studied, the first case of a layer of sand over the clayey soil, and the second case of clayey soil over the sand soil. The footing (L shape) having different depth from the surface of soil (0, 0.5, 1.0, 1.5, 2.0 and 2.5m) and the loading was (0, 50, 100, 150, 200 and 250 kN/m2) for two cases of layered soil are analyzed. The results for this analysis show that the settlement increase with depth of footing for each load, and when the loading increase the settlement also increase. Curves for load – settlement were almost similar for all depths of footing. Settlements values various between two cases , when sandy soil over clayey soil the settlements was high than when clayey soil over sandy soil.

Evaluation of the soil layering and soil lens effect on the ground-borne vibrations induced by twin metro tunnels

This study aims to examine the effects of surrounding soil layers and furthermore lens properties on induced vibrations by underground railway twin tunnels and comparing with the single tunnel. First, the predicted models for the vibration of the underground tunnel(s) are verified against in situ measurements. The tunnel(s) are assumed to be located in two-layer soil, and the influence of upper soil layer thickness, elastic modulus, and soil layers’ inclination angle (for three values of 5, 10, and 15 degrees) on surface vibrations are investigated. Furthermore, the influence of the presence of soft and stiff soil lens with various thicknesses in several levels respect to the tunnel(s) and the effect of the inclination angle of the soil lens (for two values of 5 and 15 degrees) on the surface vibrations is examined. The results indicate that the layered region around the single and twin tunnels increases the vibrations level up to 10 dB respect to the uniform soil layer. Also, the effect of soil lens with larger thickness is remarkable. The maximum inclination angle of 15 degrees for soil layers and soil lens respectively cause in 9 and 5 dB increase in the surface vibrations level.