Two-layer Soil Research Articles

Seismic Bearing Capacity of Strip Footing Resting on Reinforced Layered Soil Using Chaotic Particle Swarm Optimization Technique

In this paper limit equilibrium method under pseudo-static approach is applied to evaluate the ultimate bearing capacity of shallow strip footing resting on geosynthetic reinforced soils. The foundation is resting on two layered c − ϕ soils. Chaotic Particle Swarm Optimization algorithm is used to optimize the solution. Failure mechanism has been assumed as linearly varying with depth with different wedge angle. Bearing capacities evaluated for different reinforcement geometries and soil properties. Bearing capacity of shallow strip footing in two-layered soil conveyed as a single coefficient for a coincident resistance of unit weight, surcharge and cohesion. The effect of different parameters on the seismic bearing capacity coefficients is studied in details. A comparison of the present method with other theories is presented, which shows the merits of the present study. Hence, it was concluded that present method for reinforcement layered soils can be prosperously utilized in calculating the bearing capacities of geosynthetic reinforced soils.

Numerical Formulation to Evaluate the Coupling by the Soil Among Multiple Grounding Electrodes: A High-Voltage Substations Application

This article presents a methodology to calculate grounding potentials in adjacent electrodes present in high-voltage substations subjected to a short-circuit current dissipated in the main grid. The contribution of the technique is the consideration of the coupling by the soil among electrodes taking into account a two-layer soil and irregular geometry of the conductors. This problem is of special concern due to the presence of passers-by near metallic fences, not directly connected to the grounding grid, whose potentials usually have not been evaluated. In this context, the article presents a general analytical formulation aiming to estimate the potential in multiple electrodes, leading to a possible risk to human beings close to the substation. Sensitivity studies are performed in order to evaluate the effect of the soil resistivity and the distance from the faulted grid and the coupled electrode. A practical application in a substation grounding system is discussed. Finally, a huge grounding system is evaluated in order to verify the robustness of the proposed formulation.

The Deep Ground Rod in a Three Layer Soil - A New Approach

The aim of this paper is to present a practical method based on a new approach in order to compute the rod ground resistance when buried in a three-layer soil. This electrode is frequently used although it requires a mechanical hammer to bury the rod, through a three-layers soil. The originality of this new approach is a two-layer equivalent soil model replacing the initial three layer soil one, enabling the rod resistance computation to be obtained using Tagg formula for rods in a two-layer soil. The authors have used measured values available in a scientific paper, have drawn the apparent resistivity curves from those values according to IEEE Std. 80-2000, and have obtained the soil parameters using the Pirsen method. From the three- layer soil, a two- layer equivalent soil has been derived and the Tagg formula has been used to compute the deep rod resistance. No numerical methods have been used in this calculation, which just requires a practical formula. The theoretical results agree with the published measurements in the field. For rods with length of 24m, 30m and 36m the proposed method has errors of -9.4%, 0.0% and 2.6%. In order to ensure that these results are not due to redundancy, the IEEE two layer model is deduced from the resistivity curve, and the use of Tagg formula leads to errors of 22.6%, 23.8% and 12.8% for the already referred lengths, and are bigger than the ones obtained from the authors suggested method.

Comprehensive Design of an Earthing System for Broadlands Hydropower Station Switchyard

Broadlands Hydropower Project (BHP) harnesses the last hydropower potential of Kehelgamu-Maskeli Oya rivers with an installed capacity of 35 MW and 126 GWh annual energy generation. Excavation of the powerhouse switchyard site exposed to a bedrock formation with highly weathered granitic gneiss beneath a thin layer of scum soil top, resulting in an irregular high soil resistivity profile. Therefore, the main purpose of this research is to design a safe and effective grounding system for the switchyard of BHP, which can carry fault current into the ground without exceeding tolerable ground potential rise, ensuring the desired operation of protective & control devices so that not to endanger human & equipment. Owing to the nature of non-uniform high soil resistivity and limited land space for extension, this has become a great challenge. Research used two approaches, guidelines of conventional IEEE 80-2000 standards and Finite Element Method (FEM). Initially, resistivity measurement was conducted covering the entire area of concern. A soil model was prepared using orthodox horizontally stratified two-layer soil model using Sunde’s graphical technique based on measured data. Then the grounding grid was designed adhering to guidelines given in IEEE 80:2000 standard and observed high overall grid resistance, eventually exceeding the tolerable step and touch potential levels. Thereafter a soil model was prepared based on FEM which facilitates plot of accurate and smooth surface voltage distribution over the entire switchyard area. Applying fault current to these discrete finite elements and based on the first principle of Kirchhoff’s current distribution balance, the localized voltage distribution has been developed for the entire area and plotted using a self-developed MATLAB computer program. FEM model can trace the points where the touch and step potentials exceed safe limits in two-dimensional stratified grid, estimation of voltage gradients at boundary areas, which all are unable to track using conventional IEEE method. Accuracy of the model can further be increased by reducing the size of the soil element. Finally, several sensitivity studies were conducted so as to optimize the BHP switchyard grid design ensuring safe grid operation.

Experimental investigation of kinematic pile bending in layered soils using dynamic centrifuge modelling

This research provides an insight into the previously unexplored aspects of kinematic pile bending, especially for large-intensity earthquakes where the soil behaviour is highly non-linear. In this study, a series of dynamic centrifuge experiments was conducted on pile foundations embedded in a two-layered soil profile to investigate the kinematic effects on pile foundations during model earthquakes. A single pile and a closely spaced 3 × 1 row pile group were used as model pile foundations, and the soil model consisted of a soft clay underlain by dense sand. It was observed that the peak kinematic pile bending moment occurs slightly beneath the interface of the soil layers and this depth is larger for the pile group compared to a single pile. Also, the piles in a group attract lower bending moments but carry larger residual kinematic pile bending moments compared to a single pile. Furthermore, the elastic solutions available in the literature for estimating the kinematic pile bending moments are shown to yield satisfactory results only for small-intensity earthquakes, but vastly underestimate for large-intensity earthquakes, if methods are applied injudiciously. The importance of considering soil non-linearity effects and accurate determination of shear strain at the interface of layered soils during large-intensity earthquakes for a reliable assessment of kinematic pile bending moment from methods in the literature is demonstrated using dynamic centrifuge test data.

Impact of interlayer on moisture characteristics of reclaimed soil backfilled with Yellow River sediments

Underground coal mining causes land subsidence, and backfilling with Yellow River sediment is an effective reclamation technology to restore farmland in China. To date, two-layer soil reconstructed (TSR) for subsided land reclamation resulted in poor capacity to retain water. To solve this problem, multi-layered soil reconstructed (MSR), sandwiching soil interlayers between sediment, was developed as a new reclamation strategy with Yellow River sediment. In order to evaluate the impact of soil interlayer on moisture characteristics, laboratory experiments of infiltration and evaporation were conducted. Two control treatments (CK1, CK2) and four experimental treatments (T1-T4) were designed. CK1 was undamaged farmland, CK2 was conventional reconstructed two-layers soil profile (filled sediment with 40 cm soil cover). T1-T4 were multiple-layers soil profiles sandwiching different structures of soil interlayers between sediment layers. The results indicated that putting interlayers into sediment reduced water leakage and water evaporation, improved the water-holding capacity of conventional two-layer soil profiles. The total thickness of soil interlayers of 30 cm (T3 and T4) was better than 20 cm (T1 and T2) and two soil interlayers (T2) were better than one (T1) on water-holding capacity. Furthermore, the best reconstructed soil profile was T3, sandwiched two soil interlayer and the first thickness was 20 cm. This treatment had the greatest improvement on soil water holding capacity with an increase of 49.14% compared to CK2 at the end of the evaporation and was closest to CK1 (402.31 mm). This study provided experimental evidence that compares with TSR, MRS improved the moisture characteristics of backfilling with Yellow River sediment. Keywords: land reclamation, Mining subsidence, Yellow River sediment, multi-layered soil reconstructed, moisture characteristics DOI: 10.25165/j.ijabe.20201301.5418 Citation: Wang X T, Hu Z Q, Liang Y S. Impact of interlayer on moisture characteristics of reclaimed soil backfilled with Yellow River sediments. Int J Agric & Biol Eng, 2020; 13(1): 153–159.

AC interference from a faulty power line on nearby buried pipelines: influence of the surface layer soil

The study generalises the calculation method described by Lucca, devoted to the evaluation of induced voltage and current on a pipeline due to a nearby overhead power line in a fault condition, from the case of homogeneous soil to the case of a two-layer soil. By means of some examples of calculation, it is put in evidence that the homogeneous soil hypothesis can lead, in some cases, to excessively precautionary results and, in other cases, to underestimated results. Therefore, a calculation method based on the two-layer soil model seems a better approach.

DETERMINATION OF THE SOIL SOUNDING DEPTH FOR THE EARTHING RESISTANCE CALCULATION OF SUBSTATIONS 35 KV

Purpose. Determination of the minimum required sounding depth for calculation of the earthing resistance for substations with a voltage class of 35 kV. Methodology. For each ratio of electrical resistivity values of soil layers, earthing resistance was calculated with changing of the layers separation depth from 0.4 m to hmax, where hmax is the layers separation depth in a two-layer soil at which the earthing resistance value becomes the same as in a uniform soil. Results. In the experiments carried out, a family of curves was obtained that describes the effect of separation depth of soil layers for various combinations of soil electrical resistivities and geometric dimensions of the earthing arrangement. The accumulated statistical data for substations with a voltage class of 35 kV made it possible to determine the required sounding depth depending on the maximum size of the earthing arrangement and the probability of the relative resistivity falling into the corresponding range of values. An algorithm is proposed for determining the required investigation depth by Wenner method as part of the electromagnetic diagnostics of the earthing arrangement of existing substations with a voltage class of 35 kV. Originality. For the first time, a probabilistic relationship was established between the ratio of the electrical resistivity of soil layers, the size of the earthing arrangement, and the necessary depth investigation of the geological medium. As a result it has been proven that there are substations for which the required sounding depth does not exceed the maximum size of the earthing arrangement. Practical value. The use of the algorithm developed in this work allows increasing the accuracy of the earthing resistance calculation of electrical installations with voltages above 1 kV operating in a network with isolated neutral.

Effective Green-Ampt Parameters for Two-Layered Soils

AbstractThe Green-Ampt method is a physically based model for partitioning rainfall into surface runoff and infiltration. This method is widely used in infiltration practice because of its simplici...

Biogeochemical Sequestration of Phosphorus in a Two-Layer Lignocellulose-Based Soil Treatment System

AbstractOn-site wastewater treatment systems can contribute to the oversupply of phosphorus (P) to aquatic systems which represents a key factor for the development of eutrophic conditions and asso...

Analytical Solution for Axially Loaded Piles in Two-Layer Soil

AbstractAn analytical elastic continuum model is developed for the settlement of end-bearing piles in a two-layer soil over a rigid stratum. The model has its roots in the point-load solution of We...

Estimation of an Upper Bound to the Value of the Step Potentials in Two-Layered Soils from Grounding Resistance Measurements.

Due to the constant updating of regulatory standards on safety issues in electrical installations, limits are established for the maximum step potential that an installation can hold in a ground fault situation. In this paper, an upper bound to the maximum value of the step potentials arising in the soil surface when a fault takes place in a grounded electrical installation is estimated by means of a simple procedure. The direct measurement of the grounding electrode resistance together with some information about the soil resistivity and the knowledge of characteristic parameters of the electrode are used for the calculation of that upper bound. The procedure is tested at numerical simulation level by using different electrodes in several different scenarios corresponding to two-layered soils with different resistivity ratios. The dependency of the calculated upper bound with the electrode burial depth is also studied. Finally, a real case study is presented, and the results of the field measurements are shown as an example of the validity of the procedure.

Impact of interlayer on moisture characteristics of reclaimed soil backfilled with Yellow River sediments

Underground coal mining causes land subsidence, and backfilling with Yellow River sediment is an effective reclamation technology to restore farmland in China. To date, two-layer soil reconstructed (TSR) for subsided land reclamation resulted in poor capacity to retain water. To solve this problem, multi-layered soil reconstructed (MSR), sandwiching soil interlayers between sediment, was developed as a new reclamation strategy with Yellow River sediment. In order to evaluate the impact of soil interlayer on moisture characteristics, laboratory experiments of infiltration and evaporation were conducted. Two control treatments (CK1, CK2) and four experimental treatments (T1-T4) were designed. CK1 was undamaged farmland, CK2 was conventional reconstructed two-layers soil profile (filled sediment with 40 cm soil cover). T1-T4 were multiple-layers soil profiles sandwiching different structures of soil interlayers between sediment layers. The results indicated that putting interlayers into sediment reduced water leakage and water evaporation, improved the water-holding capacity of conventional two-layer soil profiles. The total thickness of soil interlayers of 30 cm (T3 and T4) was better than 20 cm (T1 and T2) and two soil interlayers (T2) were better than one (T1) on water-holding capacity. Furthermore, the best reconstructed soil profile was T3, sandwiched two soil interlayer and the first thickness was 20 cm. This treatment had the greatest improvement on soil water holding capacity with an increase of 49.14% compared to CK2 at the end of the evaporation and was closest to CK1 (402.31 mm). This study provided experimental evidence that compares with TSR, MRS improved the moisture characteristics of backfilling with Yellow River sediment. Keywords: land reclamation, Mining subsidence, Yellow River sediment, multi-layered soil reconstructed, moisture characteristics DOI: 10.25165/j.ijabe.20201301.5418 Citation: Wang X T, Hu Z Q, Liang Y S. Impact of interlayer on moisture characteristics of reclaimed soil backfilled with Yellow River sediments. Int J Agric & Biol Eng, 2020; 13(1): 153–159.

The Application of Centrifuge Test to Analyse Surface Displacement in Front of Tunnel Face

In this paper, the author performed two centrifuge model tests at the University of Science and Technology of Hong Kong for two cases where the tunnel was located at Cover over Diameter (C/D) of 1.5 and 3.3 to investigate the passive failure and deformation mechanism due to tunnelling in two-layer soils: sand and clay. The tests were carried out on a 1/100 scale miniature model. The experimental process simulated the progress of the tunnel face with a speed of 0.2 mm/s (equivalent to 15 m per day in practice), displaced to 40 mm and then stopped. Linear variable differential transformers are used to measure the ground transitions during the testing process. Particle image velocimetry technology measures the movement of soil in front of the tunnel face. The load cells were attached to one end of the tunnel face in order to obtain the passive pressure exerted on the tunnel face. As the tunnel face displaces, the ground in front of the tunnel face is shifted forward, while the ground that is far from the surface of the tunnel is pushed outward; the effect on the ground thus causes the ground to emerge, and so form a breakout. The partial failure mechanism in front of the tunnel face is similar to the localised cutting mechanism. When the observation failure mechanisms are idealised by continuous lines, the failure mechanism of the ground in front of the tunnel face is shaped like a funnel.

Effect of interference on ultimate bearing capacity of strip footings resting over multi-layered-reinforced soil

ABSTRACT In an attempt to modify the conventional approach of considering homogeneous soil underneath the foundation, a numerical investigation is carried out to evaluate the bearing capacity of two closely spaced identical strip footing on a two-layered soil system. Two different cases including strong soil overlying the weak soil and vice versa have been considered in the analysis. The analysis was carried out for various ratios of clear spacing to the footing width (s/B). The values of critical spacing (s cr ) and significant depths (H s ), beyond which the increase in ultimate bearing capacity ceases were observed. [s cr, Hs ] values were found to be [B, 3B] and [3B, 2B] for strong soil overlying weak soil and weak soil overlying strong soil mass respectively. With the inclusion of geogrid reinforcement, the s cr is obtained as 0.5B and 1.5B in strong soil overlying weak soil and weak soil overlying strong soil respectively.

Neural Computing Improvement Using Four Metaheuristic Optimizers in Bearing Capacity Analysis of Footings Settled on Two-Layer Soils

This study outlines the applicability of four metaheuristic algorithms, namely, whale optimization algorithm (WOA), league champion optimization (LCA), moth–flame optimization (MFO), and ant colony optimization (ACO), for performance improvement of an artificial neural network (ANN) in analyzing the bearing capacity of footings settled on two-layered soils. To this end, the models estimate the stability/failure of the system by taking into consideration soil key factors. The complexity of each network is optimized through a sensitivity analysis process. The performance of the ensembles is compared with a typical ANN to evaluate the efficiency of the applied optimizers. It was shown that the incorporation of the WOA, LCA, MFO, and ACO algorithms resulted in 14.49%, 13.41%, 18.30%, and 35.75% reductions in the prediction error of the ANN, respectively. Moreover, a ranking system is developed to compare the efficiency of the used models. The results revealed that the ACO–ANN performs most accurately, followed by the MFO–ANN, WOA–ANN, and LCA–ANN. Lastly, the outcomes demonstrated that the ACO–ANN can be a promising alternative to traditional methods used for analyzing the bearing capacity of two-layered soils.

A study of grounding arrangements composed by vertical electrodes for two-layered stratified soil models

This paper presents a study of a grounding arrangement proposed for transmission line (TL) towers, particularly for conditions of a stratified soil where the electric resistivity of the first layer is higher than the second medium. Techniques based on TL-theory are presented to estimate the harmonic grounding impedance of such configurations and are compared to the solution obtained with a rigorous electromagnetic (EM) method. The results show that the grounding resistance, important to low frequency frequency phenomena, and the impulse impedance, to fast transients, decrease significantly with the use of vertical electrodes. For a 230 kV TL, the critical towers can be improved by the proposed arrangement, which includes a significant economy of the conductor material applied to the grounding.

Simplified assessment of pile-head kinematic demand in layered soil

Soil-pile kinematic interaction develops as a consequence of soil movements during seismic excitations. Severe bending strains exceeding the strain limits of the pile material can then develop. The highest kinematic bending moments occur at the interface of layered soil, characterized by consecutive layers with a high Vs2/Vs1 ratio between their shear wave velocities, or at the pile-head of fixed-head piles. Seismic design codes are generally worried about the kinematic bending developing at the interface of two layers with different stiffnesses and suggest evaluating kinematic effects on strategic buildings in seismic prone areas. Recent research has shown that the kinematic demand at the pile-head is important in the design of reinforced concrete piles in soft deposits, and relatively simple formulas exist for its assessment, both in homogenous and inhomogeneous soil conditions. However, there are no simplified solutions for the assessment of the kinematic demand at the pile-head in two-layered soil with a relatively shallow interface depth. In this study a recently developed BEM-based code (KIN SP) is used for this, and the relevance of the threshold values of the average shear wave velocity parameter suggested in seismic codes is discussed. The influence of pile and soil nonlinearities is also investigated with KIN SP, which has been enhanced to account for these nonlinearities.

Two-dimensional rainfall-runoff and soil erosion model on an irregularly rilled hillslope

The spatial characteristics of rills and non-planar hillslope could dramatically alter the mechanisms of the hydrologic and geomorphic processes, which were usually neglected in previous studies. Based on a two-layer soil infiltration model, a two-dimensional (2D) diffusion wave model, and modified WEPP erosion modeling approaches, a 2D distributed hillslope runoff and soil erosion model was developed to account for these effects. The interrill and rill areas are distinguished based on the high resolution topographic data. The model is tested against experimental erosion data from a hillslope subjected to three successive rainfall events, resulting in irregular rill networks. Good agreements between the modeled and observed results are found in runoff depth, erosion and its spatial distribution. A sensitivity analysis was also performed for the related parameters and found that the sensitive parameters for interrill erosion are rainfall intensity and interrill erodibility factor, while the sensitive parameters for rill erosion are the rainfall intensity, Manning’s friction coefficient, rill erodibility factor and soil critical shear stress. Moreover, the role of the dendritic rill network and concave hillslope have been examined and both of them can greatly affect the overland flow routing and increase the rill erosion, which is not accounted for in previous models such as WEPP. These results are valuable for rainfall-runoff and soil erosion prediction on irregularly rilled hillslopes.

Incorporating stratigraphic boundary uncertainty into reliability analysis of slopes in spatially variable soils using one-dimensional conditional Markov chain model

Slope stability analysis based on different stratigraphic boundary conditions may have significant differences in terms of factor of safety (FS) and the location of the critical slip surface. Therefore, traditional reliability analysis of layered slopes that assumes the stratigraphic boundary between two soil layers being a deterministic line or surface can be misleading. This study aims to investigate the influence of the stratigraphic boundary uncertainty (SBU) arising from limited site investigation data on slope stability analysis with the consideration of soil spatial variability. A modified one-dimensional conditional Markov chain model is proposed to model the SBU. Cholesky decomposition technique is subsequently utilized to simulate the inherent spatial variability of soil properties based on the simulated stratigraphic boundary within the framework of Monte Carlo simulation. A sample example on a two-layered soil slope with limited number of boreholes shows that the proposed method can well simulate the stratigraphic boundary based on limited borehole information. The statistics of FS and probability of failure are found not to increase or decrease monotonically with the number of boreholes, but can converge to the correct results if the number of boreholes increases. Moreover, the conventional reliability analysis with an implicit assumption of deterministic stratigraphic boundary condition can significantly overestimate the reliability of the slope, but this overestimation decreases with the increase in the scale of fluctuation.