Soil Strata Research Articles

Method of Line for Modeling of Grounding Electrodes Buried in Stratified Multilayer Soil

An efficient full-wave semi-analytical method based on the method of line is developed for rigorous modeling of grounding electrodes buried in stratified multilayer soil. As compared to the existing full-wave modeling techniques, the proposed method is extremely computationally efficient while enables modeling of any number of vertical and/or horizontal soil layers. Furthermore, the proposed method is not prone to any stability or convergence numerical issue. The efficiency and accuracy of the proposed method is evaluated using the cases of horizontal and vertical grounding electrodes buried in vertically or horizontally stratified multilayer soil.

Assessment of Frequency-Dependent Parameters of Stratified Soils for Lightning Response of Earth Electrodes

Typically, the frequency dependence of soil parameters is predicted by models or formulas that assume a uniform soil, although it usually presents a stratified structure. This work develops analytical expressions that allow to evaluating of the errors associated with the uniform soil hypothesis, in comparison with the hypothesis of a two-layered soil, taking as reference a field methodology for measuring the variation of soil parameters with frequency. The results suggest that in the case of a stratified soil, the frequency dependence of its parameters can be considered individually for each layer.

Forest trees as a natural metamaterial for surface wave attenuation in stratified soils

Forest trees have been shown to act as a natural metamaterial to attenuate elastic waves. However, the existing studies assumed that forest trees are located on homogeneous soils and did not consider the soil stratification that widely exists in natural strata. This study investigates the forest trees as a natural metamaterial to manipulate the propagation of elastic waves in stratified soils. A semi-analytical method is proposed to calculate the dynamic responses of a layered half-space coupled with arbitrary number of trees. A close-form Green’s function for a harmonic line load on a layered half-space is derived by the thin layer method, and subsequently used to determine the multiple scattered wavefield induced by forest trees as surface oscillators. The attenuation performance of forest trees on stratified soils is systematically investigated. Trees with a graded array of heights can provide a wide attenuation zone at frequencies below 80 Hz. Since the higher surface wave modes in a layered half-space can propagate forward without being affected by trees, the attenuation efficiency of trees on stratified soils is significantly lower than that of trees on corresponding homogeneous soils. The performance uncertainty induced by the randomness of the height, sizes, and spacing of trees is subsequently investigated through a Monte Carlo simulation. The forest trees can still obtain a significant attenuation efficiency at frequencies below 80 Hz even the randomness of tree properties being considered. The randomness in height and radius leads to significant uncertainty in attenuation performance, while the randomness in spacing results in reasonable uncertainty.

Laterally loaded piles and pile groups partially embedded in transversely isotropic fractional viscoelastic saturated soils

This paper investigates the time-dependent behaviour of laterally loaded piles and pile groups partially embedded in stratified transversely isotropic (TI) saturated soils. The partially embedded single pile in stratified soils is regarded as a Timoshenko beam, and its total stiffness matrix can be assembled by using the finite element method (FEM). Based on the fractional viscoelastic model for solving non-axisymmetric consolidation of stratified cross-anisotropic media developed before, the solution for a partially embedded single pile is obtained by combining the boundary element method (BEM) and the extended precise integration solution of the media. Considering the interaction between piles in a pile group and the load distribution from the pile cap to pile tops, the solution for the partially embedded pile group is derived by extending the solution of single piles. The accuracy and efficiency of the proposed solution is verified with existing literatures, and some examples are designed to investigate the effects of the embedment ratio, the transverse isotropy, the slenderness ratio, the fractional derivative order, the flexibility coefficient and the pile spacing. Numerical results show that the embedment ratio and fractional derivative order are essential parameters that significantly affect the rate of the time-dependent response of the partially embedded lateral loaded piles.

Risk assessment of surface conductor jetting installation during deep-water drilling in sloping seabed

With the increasing exploration and development area of deep-water oil and gas fields in the South China Sea, well construction operations in the deep-water sloping seabed area become more and more frequent, so it is of great significance to accurately assess the safety of jetting well construction in the sloping seabed. Based on the characteristics of jetting well construction technology, the soil disturbance model was established and the wellhead instability mechanism in sloping seabed was studied. It was found that the jetting flow rate and fluid range were the main factors to determine the disturbance intensity and disturbance range to sloping seabed. The risk of jetting well construction in the sloping seabed was also studied based on the geologic models of various forms of sloping seabed. The finite element strength reduction method was used to analyze the influence of seabed dip angle, soil strength and soil stratification on sloping seabed stability, and the sloping seabed instability evolution law under different jetting disturbance parameters was obtained. Through the assessment of the risk of jetting well construction in the sloping seabed, the selection range of well locations was expanded, which could aid in ensuring the security of deep-water oil and gas field construction.

The response of layered seabed to ice gouging: Sand over clay

Travelling icebergs may threaten the structural integrity of the offshore pipelines in any territories they can reach. Arctic offshore pipelines are usually buried for physical protection against ice gouging. The safe and cost-effective burial depth of these pipelines needs to be determined by accurately assessing the subgouge soil deformation and the ice keel-seabed contact forces. Layered seabed soil strata found in many geographical locations add to the complexity of assessing the ice keel-seabed interaction process. The currently used design codes have been developed for uniform soil strata-an assumption that may affect the accuracy and cost-effectiveness of the design. In this study, the significance of layered sand over clay seabed in the ice gouging process was investigated by performing large deformation finite element (LDFE) analysis using Coupled Eulerian-Lagrangian (CEL) approach. A comprehensive parametric study was conducted to assess the influence of ice gouging characteristics and soil properties on the subgouge soil deformation, the developed plastic shear strains, and the resultant ice keel reaction forces. The results showed that the seabed response to ice gouging largely depends on the soil strata, where the layers’ interaction effects can alter the usual response expected from a uniform seabed.

Response characteristic of crane-wharf interaction system: Numerical simulation and global sensitivity analysis

Crane, an important platform to carry and move cargo in the ports, can evidently affect the overall response of pile-supported wharf system. In many earlier analytical studies, the crane was simply represented by a lumped mass. Such modeling is unable to realistically replicate the salient response characteristics of crane-wharf interaction system. To investigate such interaction effects, a three-dimensional (3D) finite element (FE) model of a typical crane-wharf system is developed. As such, the static/dynamic response of pile-supported wharf with/without crane are systematically studied and directly compared. In addition, global sensitivity analysis is conducted to explore the effects of input parameter uncertainties on global response of crane-wharf system. From the computed results, it is shown that: (1) both the static and dynamic response are considerably dependent on the crane-wharf structure as an integral global system; (2) the bending moment and displacement of pile are more sensitive to the friction angle of soil stratum near pile tip; and (3) the interaction effect among various uncertainty parameters becomes more significant with consideration of crane. Overall, the 3D FE analysis technique and the derived insights provide a useful reference and guidance for evaluation of similar crane-wharf interaction system under static/dynamic loading conditions.

Analysis and Intelligent Prediction for Displacement of Stratum and Tunnel Lining by Shield Tunnel Excavation in Complex Geological Conditions: A Case Study

This paper presents the analysis and intelligent prediction for the displacement of stratum and tunnel lining of Qingdao Metro Line 4 by earth pressure balance (EPB) shield tunnel excavation in complex strata. When the tunnel is excavated in different stratum sections, the tunneling parameters of shield machine are systematically analyzed and compared, and the vertical displacement of the tunnel crown and the horizontal convergence deformation on both sides are investigated. When the tunnel body passes through the soft soil stratum and rock stratum, the curves of the vertical displacement of the stratum surface with time are respectively discussed. A machine learning method for predicting stratum surface deformation induced by shield tunnel excavation in complex strata is developed, where extreme learning machine (ELM), particle swarm optimization (PSO) algorithm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k$ </tex-math></inline-formula> -fold cross-validation method are comprehensively considered. 65 data samples are collected from the field monitoring data of Qingdao Metro Line 4 and each data sample includes seven input values and one output value. The developed PSO-ELM has good prediction performance for stratum surface vertical displacement due to shield tunnel excavation. The case study in this work can provide a practical reference for similar tunneling projects.

An Analytical Propagation Model Based on Dyadic Green’s Functions for TTE Communications in an Arbitrary Stratified Soil

In this work, we present a novel analytical model for the magnetic field in through-the-Earth (TTE) communications. The modeling approach is based on dyadic Green’s functions to compute impressed and scattered fields in a multilayered structure. Its performance is assessed through comparisons with previous models, measurements, and simulations of magnetic field. The great advantages of the proposed approach in relation to the previous ones are its flexibility in the positioning and direction of antennas and the unlimited number of layers of soil. Moreover, this approach can be more accurate than classical ones in scenarios where the soil has a high conductivity below the antenna in an uplink scenario.

Numerical Study on Seismic Response of Offshore Wind Turbine Monopile in Multi-Layered Soil Profile of Arabian Sea

The seismic performance of monopiled offshore wind turbine (OWT) structures was evaluated numerically. The aim was to analyze offshore wind farm sites on complicated layered seabed with high seismicity. Following pile soil analysis (PISA) model, three-dimensional (3D) numerical evaluations were undertaken for two separate locations under two independent seismic events. The effects of pile diameter, depth, site impact owing to transverse soil layering, uni-directional and multi-directional seismic loading and seismic acceleration magnitude are presented. Dynamic impedance depth variation and site response analysis via lateral displacement, lateral soil response and Fourier response amplitude are explored in frequency and time domain. In a seismic zone with stratified soil, pile diameter has a greater influence. Variations in soil profile affect wind turbine performance and seismic sensitivity. This research will give a strong platform for later studies to recommend a safe wind farm site based on simulation results.

Modified pseudo-dynamic based soil-structure interaction of caisson with a novel 3D failure wedge

Passive resistance computation for caissons embedded in layered soil is a complicated problem. In the present study, modified pseudo-dynamic method of analysis has been used to determine both amplification of earthquake waves through the soil strata and the resulting passive earth pressure on the embedded structure. The failure wedge has been idealized based on the results of experimental and numerical studies by previous researchers as log-spiral in vertical plane and oval in the horizontal plane. Earthquake waves are assumed to propagate harmonically in vertically upward direction through layered strata considering modified pseudo-dynamic approach with the entire failure wedge assumed to be lying in a single soil layer. Both, dry and submerged soil conditions, have been considered in the present study. The effect of increase in volume of failure wedge compared to corresponding plane strain condition on earth pressure coefficient has also been discussed in the current study. The earth pressure coefficient due to unit weight component has been reported. The earth pressure coefficient obtained from the current study for structures of different widths are found to be smaller in magnitude than corresponding pseudo-static analysis results.

Estimation of elastic settlement for skirted strip foundations subjected to vertical loading

Settlement calculations are an essential part of the design of offshore foundations, although most of the existing studies on skirted foundations focus on bearing capacity. In this study, finite element analysis is used to evaluate the elastic settlement (vertical displacement) of skirted strip foundations of arbitrary stiffness embedded in either a homogeneous or nonhomogeneous soil stratum with finite layer depth. Graphical solutions are presented, which enable the determination of the drained and undrained settlements at the center and edge of the skirted foundations. Closed-formed expressions that approximate the numerical solutions well are also proposed. Numerical examples are provided to illustrate how the solutions and expressions can be readily applied in hand calculation for preliminary design purpose.

Influence of transient flow during infiltration and isotropic/anisotropic matric suction on the passive/active lateral earth pressures of partially saturated soils

In this study, the influence of transient flow on the active and passive lateral earth pressures of variably saturated backfills is thoroughly evaluated employing the lower bound theorems of the finite element limit analysis with second-order cone programming. In order to account for the tempo-spatial variations of suction stress within the soil stratum during infiltration, a closed-form solution derived for one-dimensional transient flow through variably saturated porous media is adopted. The unsaturated ground condition is simulated by incorporating the well-established unified effective stress approach into the soil yield function. The soil medium is considered to be both isotropic and anisotropic, due primarily to the various distributions of suction stress along different directions within the partially saturated porous medium. In order to model the soil suction stress anisotropy, an iterative procedure is employed by differentiating between the mobilized suction stress within the horizontal and vertical planes. In general, the influence of the transient flow condition is magnified for cohesive soils compared with granular backfills. In either case, such an effect is more noticeable for the active lateral earth pressure as compared to the passive counterpart. In the anisotropic condition, while the suction stress anisotropy within the soil medium has fairly negligible influence on the contribution of transient flow to the lateral earth pressure coefficient in the active state, the influence of elapsed infiltration time on the passive earth pressure coefficient turns out to be more at play at lower anisotropy ratios.

Applying Imaging Technique to Investigate Effects of Solute Concentrations and Gas Injection Rates on Gas Bubble Generation

This work investigated the bubble size variation under various aqueous conditions, including saline and surfactant solutions, with different gas injection rates using a commercial bubble analyzer. The results show that salt and surfactant can minimize bubble size with increasing solute concentration, and the surfactant outperforms salt. In addition, the critical coalescence concentrations (CCC) of salt and surfactant could be determined at 20 g/l and 20 mg/l, respectively, over which bubble mean size cannot be further reduced. On the other hand, the gas injection rate, compared to the solute concentration, has minor effects on bubble size variation. Nonetheless, there is a critical coalescence injection rate (CCIR) of 30 ml/min for surfactant solution, over which the standard deviation of the bubble size distribution (BSD) cannot be further increased. In principle, this work improves the accuracy and efficiency of the bubble analyzer. It also presents a sound understanding of two influential factors rather than a single-factor controlling bubble size. Most importantly, it is the first time to observe and propose the concept of CCIR to describe how the gas injection rate influences the standard deviation of BSD. Based on those results and findings, it is able to conclude that bubble size control, whose mechanism has been previously identified as the bubble collision and coalescence rather than the surface tension of water solutions, is actually dominated by not only the solute concentration but also the gas injection rate when a porous air sparger is used to generate bubbles. It is expected that this work could contribute to laboratory modeling bubbly flow in a porous medium in order to bring more insights into the mechanism of soil gas leaking through soil strata.

Site specific seismic hazard analysis of monumental site Dharahara, Kathmandu, Nepal

This study was carried out to estimate the seismic requirement for analyzing and designing the monumental structure, Dharahara, in Kathmandu, Nepal. A probabilistic seismic hazard analysis of the study region was performed to develop the elastic response spectrum for the bedrock site. The obtained spectrum was compared with the existing seismic codes, NBC 105:2020 and IS 1893:2016, to estimate the target response spectrum. One-dimensional equivalent linear (EQL) and non-linear (NL) ground response analyses were performed using DEEPSOIL v7, incorporating a suite of ground motions. The three borehole logs exhibited very soft soil in the region. Due to the presence of soft soil strata in the region under study, strength-corrected modulus reduction and damping curves were adopted in the current study to overcome the effect of larger shear strain. The average response spectra from EQL analyses predicted higher values than NL analyses. Finally, a comparison of the proposed design response spectrum with the code spectra was performed. Comparisons revealed that the mean EQL and NL outputs fall in between the design spectra of similar soil classes in NBC 105:2020 and IS 1893:2016 seismic codes.

Two-Layer numerical model of soil moisture dynamics: Model assessment and Bayesian uncertainty estimation.

A two-layer model based on the integrated form of Richards' equation (RE) was recently developed to simulate the soil water movement in the roots layer and the vadose zone with a relatively shallow and dynamic water table. The model simulates thickness-averaged volumetric water content and matric suction as opposed to point values and was numerically verified for three soil textures using HYDRUS as a benchmark. However, the strengths and limitations of the two-layer model and its performance in stratified soils and under actual field conditions have not been tested. This study further examined the two-layer model using two numerical verification experiments and, most importantly, tested its performance at site-level under actual, highly variable hydroclimate conditions. Moreover, model parameters were estimated and uncertainty and sources of errors were quantified using a Bayesian framework. First, the two-layer model was evaluated for 231 soil textures under varying soil layer thicknesses with a uniform soil profile. Second, the two-layer model was assessed for stratified conditions where the top and bottom soil layers have contrasting hydraulic conductivities. The model was evaluated by comparing soil moisture and flux estimates to those from the HYDRUS model. Last, a case study of model application using data from a Soil Climate Analysis Network (SCAN) site was presented. Bayesian Monte Carlo (BMC) method was implemented for model calibration and quantifying sources of uncertainty under real hydroclimate and soil conditions. For a homogeneous soil profile, the two-layer model generally had excellent performance in estimating volumetric water content and fluxes, while the model performance slightly declined with increasing layer thickness and coarser textured soils. The model configurations regarding layer thicknesses and soil textures that generate accurate soil moisture and flux estimations were further suggested. With the two layers of contrasting permeability, model-simulated soil moisture contents and fluxes agreed well with those computed by HYDRUS, indicating that the two-layer model accurately handles the water flow dynamics around the layer interface. In the field application, given the highly variable hydroclimate conditions, the two-layer model combined with the BMC method showed good agreement with the observed average soil moisture of the root zone and the vadose zone below (RMSE <0.021 during calibration and <0.023 during validation periods). The contribution of parametric uncertainty to the total model uncertainty was too small compared to other sources. The numerical tests and the site level application showed that the two-layer model can reliably simulate thickness-averaged soil moisture and estimate fluxes in the vadose zone under various soil and hydroclimate conditions. Results also indicated that the BMC method could be a robust framework for vadose zone hydraulic parameters identification and model uncertainty estimation.

THE EFFECT OF SOIL STABILIZATION AND REINFORCEMENT ON THE STABILITY OF EMBANKMENT ON SOFT SOIL

The construction of toll roads in South Sumatra faces two problems. First, most of the road sections are built on soft soil deposits, so the soil must be improved to speed up consolidation process and to improve bearing capacity. Second, the embankment required to reach the design elevation of the toll road is quite high so that reinforcement is needed to improve slope stability. This paper contains the results of a study on the effect of soil improvement using PVD and vacuum pressure, installation of geotextile layers and pile at the toe of embankment slopes, on the stability of the embankment. The analysis were carried out using data obtained from the construction of the Kayu Agung – Palembang toll road Section 1A, including geometry data, soil stratification, and configuration of PVD installation and soil reinforcement. Slope stability analysis was carried out using the Morgenstern &amp; Price method which is integrated in SLOPE/W program. The results of the analysis show that the safe embankment height for the original soil condition is 3.5 m while the required height of embankment was 6 m. Analysis made for the 6 m high embankment using soil properties after ground improvement with PVD and vacuum pressure shows the FoS is still below 1.5. The presence of geotextile layers and pile improved the performance of the embankment and increased the FoS to 2.410. The slope is still in a safe condition with FoS of 1.762 after the construction of toll road and traffic load which induces a combined load of 35 kPa.

Sensitivitätsanalyse der unterschiedlichen Einflussfaktoren auf numerische Untersuchungen von Hangrutschungen

AbstractCalculation approaches, shear strength approaches, material parameters and external influences have a significant impact on the results of slope stability analyses. This influence can be quantified as influence on the safety factor in sensitivity analyses. As expected, a lower shear strength of the soil or a higher groundwater flow through a slope lead to lower factors of safety. In the two case studies considered here, it is evident that the parameter with the highest sensitivity of the safety factor is not unique. This most influencing parameter depends on the boundary conditions and the geometry of the landslide. In addition to the safety factor, the above parameters also strongly influence the geometry of the failure mechanism. Here, for example, different permeability coefficients for stratified soil and the consideration of a nonlinear shear strength criterium show significant changes in the failure geometry. Therefore, the modelled failure geometry has to be considered in addition to an certain safety factor for the validation of a landslide modelling.

Use of multifractal parameters to determine soil particle size distribution and erodibility of a physical soil crust in the Loess Plateau, China

Understanding the knowledge of multifractal analysis on particle size distribution (PSD) of soil physical crusts is indispensable for progressing soil surface erosion mode. In this study, hoeing tillage (20 cm long × 15 cm wide × 5 cm deep) were conducted to simulate classic tillage mode in Yangling, China to obtain physical crusts under artificial rainfall (60 mm h−1 intensity with three durations (5, 15, and 30 min)). We defined crusts formed in ridges as structural crust (Cst), formed by infiltration of runoff in hoes as sediment-containing depositional crust (Cscd), and formed by the extraction of runoff as sediment-free depositional crust (Csfd). Soil stratification methods were used to explore physical crust particle sorting. Results represented that the sand content of Cst, Cscd, and Csfd decreased, whereas clay and silt contents increased after rainfall compared to undisturbed soil. There was no significant difference between Cscd and Csfd under short rainfall duration. Three crust types became gradually concentrated in areas with higher density with increasing rainfall duration according to six common parameters in the generalised dimension spectrum (D0, D1, D2, D1/D0, Dv(D-10–D0), and Ds(D0–D10)). Multifractal singularity spectrum (α0, Av, Fv) showed that PSD was most symmetrical in Csfd, followed by Cscd and Cst. Erodibility index KEPIC was largest for Cst, followed by Csfd then Cscd. The relationship between KEPIC and multifractal parameters were significant. Spatial difference caused by tillage activities and rainfall events determine the heterogeneous formation mode, particle size distribution and erodibility of physical crust.

DEM-continuum mechanics coupling simulation of cutting reinforced concrete pile by shield machine

Based on the discrete element method (DEM) - continuum mechanics coupling method, a numerical simulation analysis is carried out to model cutting some existing bridge piles that have invaded Beijing Metro Line 12. Firstly, the macroscopic parameters of the reinforced concrete pile and the soil stratum where the shield tunneling machine traversed are calibrated using uniaxial compression and Brazilian test through a series of numerical tests. In this paper, the procedure of building up a DEM-continuum mechanics coupling model is presented, then the model is applied to simulate the process of cutting the pile foundation using the earth pressure balanced (EPB) shield machine. The influences of different advancing parameters on the displacement of the pile bottom, the thrust force, and the torque of the EPB shield machine are investigated. The simulation-calculated results, such as thrust force and torque of the shield machine, are in good agreement with the theoretical method in previous studies and the corresponding monitored data in the field. Finally, a method based on the thrust of a single cutter is proposed to improve the theoretical model so that it can consider the effects of cutting bridge piles.