Saturated Ground Research Articles

Influences of buried depth and grain size distribution on seepage erosion in granular soils around tunnel by coupled CFD-DEM approach

For tunnels built in the saturated silty sand ground, fine particles may be migrated into tunnels through seams of tunnel segmental joints and then seepage erosion is triggered, which may induce ground settlement. However, the process from fine particles erosion to the stress redistribution and soil properties’ change surrounding the tunnel and ground settlement has not been clarified up to now. For this purpose, five numerical tests of seepage erosion in granular soils around the tunnel are conducted using the Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) coupling method. The influences of buried depth and grain size distribution (GSD) of gap graded soils (mainly controlled by the fines content and mean particle size ratio from coarse to fine) on the seepage erosion around the tunnel are investigated. Eroded mass, fines loss mode, surface vertical displacement, stress redistribution, fabric anisotropy, soil behavior and water pressure around the tunnel during the seepage erosion process for five tests are presented and compared. The following results can be upscaled to the practical tunnel engineering, such as: (1) the number of fines loss, the eroded zone and the ground settlement increase with buried depth and mean particle size ratio; (2) the earth pressure near the crack significantly increases due to the stress redistribution induced by fines loss, and the stress redistributed area expands with buried depth; (3) the strength and stiffness of granular soils around the crack are significantly reduced by the seepage erosion. All results revealed that the CFD-DEM simulations provide a new sight on understanding the mechanics of tunnel seepage erosion from a microscopic perspective.

Experimental study on seismic vibration control of an offshore wind turbine with TMD considering soil liquefaction effect

Wind energy is clean and sustainable. Taiwan is establishing offshore wind farms using wind turbines in the Taiwan Strait. However, these are located in an earthquake-prone area with sandy seabed conditions. To ensure their safety and reliability, the turbines’ support structure must be protected against wind, waves, and seismic loads. Tuned mass dampers (TMDs) are commonly employed to reduce structural vibrations. A TMD is more simply incorporated into turbine structures than are other energy dissipation devices. In this study, a 1:25-scale test model with a TMD was constructed and subjected to shaking table tests to experimentally simulate the dynamic behavior of a typical 5-MW wind turbine with a jacket-type support structure and pile foundation. The scaled-down wind turbine model has a nacelle without rotating blades; therefore, the aerodynamic and rotational effects due to the rotating blades were ignored in this study. A large laminar shear box filled with saturated sandy ground was used to simulate the typical seabed conditions of Taiwanese offshore wind farms. The TMD system was designed to be tuned the first-mode frequency of the test model. Two ground accelerations, selected by considering wind farm site condition and near-fault characteristics, were used for excitation in the test. The responses of the test model with and without the TMD system were compared, and the influence of soil liquefaction on the effectiveness of TMD vibration control was addressed.

Exploration of Ground Vibration due to Trains Moving on the Nearly Saturated Ground

A study of the effects of degree of saturation on the nearly saturated ground dynamic response caused by moving of high-speed trains is presented. The soil medium is assumed as an isotropic nearly ...

Numerical simulation based heuristic investigation of inertia-induced phenomena and theoretical solution based verification by the damped wave equation for the dynamic deformation of saturated soil based on the u–w–p governing equation

The theoretical and numerical solutions for the one-dimensional dynamic deformation of saturated ground were obtained using the u–w–p governing equation. The numerical analysis was performed based on the finite element method, and the occurrence of superficially peculiar phenomena, i.e., S-shaped settlement–time relation, non-harmonic oscillation in the settlement/pressure–time relation in highly permeable soil, and inconsistency in the vertical load and immediate pressure was identified heuristically. The theoretical solution of the damped wave equation was derived from the original governing equation expressed as the superposition of several modes with different eigenvalues, and this solution could successfully explain the phenomena observed heuristically. It was noted that the phenomena were induced by the presence of inertia, because the static solution and u–p analysis did not exhibit such phenomena. The verification of the analysis was conducted owing to the correspondence of the theoretical solutions of the damped wave equation and the u–w–p analysis results.

Optimal design of wave barriers in dry and saturated poroelastic grounds using Covariance Matrix Adaptation Evolution Strategy

In this paper, Covariance Matrix Adaption Evolution Strategy (CMA-ES) is used to optimally design soft and hard wave barriers in half-space soil medium subjected to moving train loadings. Special focus is given to two-phase saturated porous medium of soil and the differences between the barrier characteristics in saturated and dry media are highlighted. Wave propagation caused by moving trains in saturated ground is modeled using Biot’s poroelastodynamic theory and time-domain finite element (FE) numerical simulations. The poroelastodynamic solution is then coupled to CMA-ES as the fitness function of the optimal design procedure. The obtained results show that the optimal barrier characteristics and amplitude reduction spectra (ARS) depend on the ground saturation as well as on the lading frequency and barrier material. It is observed that open trenches, as air-filled barriers, have a very good screening effect in both dry and saturated grounds and their optimal shape has a vertical rectangular configuration. On the other hand, hard barriers such as concrete, have less screening effects and their optimal shape takes slab-like pattern. The effectiveness of double trenches is also explored and it is found that optimal double barriers have better screening effect as compared to that of single barriers.

1g model tests of piled-raft foundation subjected to high-frequency vertical vibration loads

The piled-raft foundation (PRF) has been used in high-speed railway in China. If the design and construction of the foundation is done without taking proper precautions, then the vibration loads induced by the high speed trains may cause excessive settlement of the foundation, significantly affecting the stability and safety of trains. To carefully examine the settlement mechanisms of PRFs constructed in different grounds subjected train loads, systematic 1 g model tests were conducted. The dynamic responses of PRFs in dry loose sand, medium dense sand, and saturated medium dense sand ground, subjected to different frequencies of sinusoidal vibration loads, were investigated. Additionally, reinforcement effect of cement-treated partial ground improvement (PGI) for dry loose sand ground, a commonly used countermeasure against settlement of PRF, was also investigated via model tests. It is found that the dynamic loading frequency affected the settlement of PRF, and the pattern of PGI reinforced at different depths of pile also significantly affected the settlement. The model test also showed that load-sharing ratio between pile and raft also changed significantly during vibration loading, implying that extensive precaution should be taken in design and construction of PRFs for high-speed railway constructed in soft ground.

Centrifuge modelling for seismic response of single pile for wind turbine subjected to lateral load

The soil around the pile foundations of offshore wind turbines (OWTs) would suffer from earthquakes which are widely believed to undermine the bearing capacity and serviceability of foundations. The non-intermittent lateral environmental loads in the ocean (e.g., wave, wind, and current) continuously act on OWTs. However, the seismic response of pile foundations and the soil around is less clear when considering the OWTs are subjected to lateral environmental loads. This paper presents two sets of centrifuge modelling tests on single piles, with a prototype diameter (D) of 1.5 m and a first-order natural frequency highly comparable to that of a prototype wind turbine. Two model piles with the same dimension and stiffness, but one with a lateral load on the pile head, were both installed in a dry sand ground and a saturated sand ground. Owing to the fact that the lateral load increased the initial static shear stresses of the soil in front of the pile, the development of excess pore pressure was inhibited and the permanent ground lateral displacement was accumulated during shaking. The existence of the lateral load led to a pile head displacement as much as 34 cm and increased the bending moment along the pile up to 6 MN·m. Moreover, the seismic response of piles and soils were compared between the dry sand ground and saturated sand ground to clarify the influence of excess pore pressure. It was also found that the residual bending moment after unloading the pile was still very large up to 17 MN·m because of the rebalanced soil-pile interaction. These findings highlight the necessity to consider the lateral load effect in the seismic design of pile foundations for OWTs.

Influences on Liquefaction-Induced Damage of Pore Water Seepage into an Unsaturated Surface Layer

Various studies have examined soil liquefaction and the resultant structure damage. The 1995 Southern Hyogo Prefecture Earthquake, a near-field earthquake, caused significant damage when the ground was liquified due to the rapidly increased pore water pressure in several cycles of major motions. Therefore, the effect of pore water movement during earthquakes has been assumed to be limited, and liquefaction has mainly been evaluated in undrained conditions. Additionally, the ground and building settlement or inclination caused by liquefaction are deemed to result from pore water drainage after earthquakes. Meanwhile, in the 2011 Tohoku Earthquake, off the Pacific Coast, a subduction-zone earthquake, long-duration motions were observed for over 300 s with frequent aftershocks. Long-duration motions with frequent aftershocks are also anticipated in a future Nankai Trough Earthquake. The effect of pore water movement not only after but during an earthquake should be considered in cases where pore water pressure gradually increases in long-duration motion. The movement of pore water during and after an earthquake typically results in simultaneous dissipation and buildup of water pressure, as well as volumetric changes associated with settlement and lateral spreading. Such effects must reasonably be considered in liquefaction evaluation and building damage prediction. This research focuses on pore water seepage into the unsaturated surface layer caused by the movement of pore water. Seepage experiments were performed based on parameters such as height of test ground, ground surface permeability, and liquefaction duration. In the tests, water pressure when the saturated ground below the groundwater level is fully liquified was applied to the bottom of the specimen representing an unsaturated surface layer. Seepage behaviors into the unsaturated surface layer were then evaluated based on the experiment data. The results show that the water level rises due to pore water seepage from the liquefied ground into the unsaturated surface layer right above the liquefied ground. For this reason, a ground shallower than the original groundwater level can be liquified.

CPT correlations for thermal properties of soils

Obtaining in situ thermal properties of soils is often difficult and time-consuming. Here, cone penetration test (CPT) correlations are proposed and validated for thermal properties of saturated ground, i.e. thermal conductivity and volumetric heat capacity, giving continuous profiles of the parameters, in a substantially reduced time. The proposed correlations utilise the characteristics of existing CPT correlations. The volumetric heat capacity correlations show good agreement with laboratory hot disc tests, and the thermal conductivity correlations proved successful for a range of soil types, including organic soils, clays and sands, although with a reasonable scatter. Empirical adjustment was required for the thermal conductivity of soils showing high (normalised) cone resistance. Utilising thermal CPT (T-CPT)-derived thermal conductivity point values in conjunction with the thermal conductivity correlations offers accurate and continuous site-specific profiles.

Spatial analysis of groundwater flowing artesian condition using machine learning techniques

This study introduces a methodology for modeling the groundwater artesian condition (AC) in a specific arid region of southern Iraq using five machine learning techniques; the stochastic gradient boosting (SGB), classification and regression trees (CART), random forest (RF), support vector machine (SVM), and k nearest neighbor (kNN). To this aim, an inventory map of flowing and non-flowing groundwater wells along with six explanatory factors were used. The chosen factors were the distance to faults (FDIS), faults density (FDEN), lineament density (LDEN), aquifer saturated thickness (ST), well depths (WD), and ground surface elevation. Examining the spatial pattern of flowing wells using the average nearest neighbor approach showed that the distribution of these wells is a cluster (average nearest ratio = 0.48). Testing the powerful factors in building the models using the Boruta package indicated that all factors play a role in controlling the emergence of AC. Five statistical error measures were employed to examine the predictive capability of the models used namely, Accuracy, kappa, receiver operating characteristic curve (ROC), sensitivity, and specificity. Applying the ML models in the R statistical package and its related caret package showed that the competent model was RF, followed by SGB and kNN. The worst performance models were CART and SVM. Accordingly, RF was used as the reference model for the probability mapping of AC in the study area, indicating the 29% (618 km̏) of the study area included in the predicted high AC and this zone mainly distribute in the eastern part and a small spot in the north. The AC probability map developed in the study could be used for drilling successful artesian flowing wells with minimal efforts and cost and thus, for efficient managing of this important system.

Dynamic magnetization in non-Hermitian quantum spin systems

We report a global effect induced by the local complex field, associated with the spin-exchange interaction. High-order exceptional point up to ($N+1$)-level coalescence is created at the critical local complex field applied to the $N$-size quantum spin chain. The ($N+1$)-order coalescent level is a saturated ferromagnetic ground state in the isotropic spin system. Remarkably, the final state always approaches the ground state for an arbitrary initial state with any number of spin flips; even if the initial state is orthogonal to the ground state. Furthermore, the switch of macroscopic magnetization is solely driven by the time and forms a hysteresis loop in the time domain. The retentivity and coercivity of the hysteresis loop mainly rely on the non-Hermiticity. Our findings highlight the cooperation of non-Hermiticity and the interaction in quantum spin system, suggest a dynamical framework to realize magnetization, and thus pave the way for the non-Hermitian quantum spin system.

Analysis of Landscape Impact on Post-Earthquake, Tsunami, and Liquefaction Disasters in Palu City, Central Sulawesi

Sulawesi is an island that is affected by three main plate basins namely the Pacific Ocean plate, the Indo-Australian plate, and the Eurasian continent plate which produces several faults and causes intensive seismicity and other disasters in the region. Palu City as the Capital of Central Sulawesi Province is a disaster-prone area such as earthquake, tsunami and liquefaction. The impact of the disaster caused huge losses especially since the area has a high population. The importance of disaster mitigation plans in the area so that casualties, damage to public facilities, and other losses can be minimized. The mitigation plan consists of several stages namely the preparation, analysis, synthesis, and making the plan recommendations. Overall, Palu City is prone to disasters. The area that has a high level of vulnerability to 3 disasters has a total area of 446 hectares. Whereas for areas that have a high level of vulnerability to 2 disasters and 1 disaster has a total area of 3364 hectares and 3873 hectares respectively. There are 3 zones based on potential disasters in Palu City. Zone A is a zone that is prone to earthquake hazards. The total area of Zone A is 9100 hectares. Zone B is a zone for mangrove forest planting areas. The total area of Zone B is 572 hectares. Zone C is a zone that is prone to liquefaction hazards. The plant is expected to be able to absorb saturated ground water and reduce soil movement so it does not shift. The total area of Zone C is 1556 hectares. Disaster-prone areas are recommended to be relocated to safe areas in accordance with the recommended relocation locations. The recommended relocation area is on a flat slope of 6093 hectares and on a gentle slope of 5557 hectares.

Ground subsidence due to the backfill pressure in tunnel boring machine

Abstract. In shield tunnel boring machine constructions, backfill pressure affects surface settlement and the stability of other underground structures nearby. Therefore, it is essential to pre-calculate backfill pressure in the design stage. This study examines and compares the seven internationally known backfill pressure calculation theories and calculates and analyzes the backfill pressures in six virtual ground conditions. The calculated backfill pressure increased as the ground cover increased, but the increase rate decreased. Also, this study performs a numerical analysis to identify the impact on ground settlement and tunnel crown settlement. In the end, settlement was more impacted by face pressure than backfill pressure in the unsaturated and saturated ground conditions. Also, as the ground cover increased, the impact of backfill pressure decreased, and as the applied face pressure decreased, the impact of backfill pressure increased.

A theory of loosening earth pressure above a shallow tunnel in unsaturated ground

SummaryA theory is proposed to evaluate the loosening earth pressure (vertical earth pressure after excavation) acting on a shallow tunnel in unsaturated ground with an arbitrary groundwater level. The theory is developed based on the limit equilibrium theory, combining soil–water characteristic curves, Mohr–Coulomb failure criteria, and effective stress for unsaturated soils. The proposed theory is applied to predict the vertical distribution of loosening earth pressure in unsaturated ground, which shows a significant difference from that in saturated ground. In unsaturated ground, suction contributes to the increase in effective loosening earth pressure and shear resistance. The remarkable effects of groundwater depth, soil type, and scale of overburden height and trapdoor width on loosening earth pressure are also revealed. Based on the soil–water characteristic curve, the degree of saturation decreases, which causes wet density to decrease and the total and effective loosening earth pressures to have contrary tendencies. Moreover, effective loosening earth pressures vary with soil type as the degree of saturation varies. The total loosening earth pressures are, however, very similar regardless of soil type, because wet density and shear resistance have similar tendencies. The proposed theory provides a valid model for loosening earth pressure in unsaturated ground that will be useful for shallow tunnel excavations.

Shaking table tests on reinforced soil retaining walls subjected to the combined effects of rainfall and earthquakes

Earthquakes and rainfall often occur together, and their combined effects on reinforced soil retaining walls (RSRW) are not investigated in detail. In this paper, shaking table tests were carried out in model-scale to investigate seismic performance of RSRW subjected to the combined forces of rainfall and earthquakes. Five cases with different degrees of saturation for the model ground were considered for comparison and each wall was shaken by a horizontal motion with a peak acceleration of 0.5 g. Various mechanical quantities, such as wall deformation, horizontal and vertical effective earth pressures, pore water pressure, horizontal acceleration and reinforcement strain were monitored carefully. It was found that wall displacement and reinforcement strain at naturally dry and saturated states are larger than those at unsaturated state, meaning that suction does have an enhancement effect on the seismic performance of RSRW. Furthermore, a special film force sensor that can measure effective pressure in saturated ground accurately was used to measure the effective earth pressure along the facing panel of RSRW, based on which, the seismic performance was discussed in a systematical way.

Guilette Receives Inaugural CCA Conservationist of the Year Award

Guilette Receives Inaugural CCA Conservationist of the Year Award

Ferromagnetism in quantum dot plaquettes

Following the recent claimed obsevation of Nagaoka ferromagnetism in finite size quantum dot plaquettes, a general theoretical analysis is warranted in order to ascertain in rather generic terms which arrangements of a small number of quantum dots can produce saturated ferromagnetic ground states and under which constraints on interaction and inter-dot tunneling in the plaquette. This is particularly necessary since Nagaoka ferromagnetism is fragile and arises only under rather special conditions. We test the robustness of ground state ferromagnetism in the presence of a long-range Coulomb interaction and long-range as well as short-range interdot hopping by modeling a wide range of different plaquette geometries accessible by arranging a few (~4) quantum dots in a controlled manner. We find that ferromagnetism is robust to the presence of long range Coulomb interactions, and we develop conditions constraining the tunneling strength such that the ground state is ferromagnetic. Additionally, we predict the presence of a partially spin-polarized ferromagnetic state for 4 electrons in a Y-shaped 4-quantum dot plaquette. Finally, we consider 4 electrons in a ring of 5 dots. This does not satisfy the Nagaoka condition, however, we show that the ground state is spin one for strong, but not infinite, onsite interaction. Thus, even though Nagaoka's theorem does not apply, the ground state for the finite system with one hole in a ring of 5 dots is partially ferromagnetic. We provide detailed fully analytical results for the existence or not of ferromagnetic ground states in several quantum dot geometries which can be studied in currently available coupled quantum dot systems.

Vibration of layered saturated ground with a tunnel subjected to an underground moving load

The transmission of vibrations within multi-layered saturated ground subjected to an underground moving load in a tunnel is investigated theoretically. A two-dimensional model of a layered saturated half-space with an embedded beam is established first. The beam simulating the tunnel is located between two horizontal soil layers with a moving point load acting upon it. The half-space soil is assumed as Biot’s poroelastic medium and the transmission and reflection matrices (TRM) method is applied to consider the wave propagation in the layered soil. Combined with the surface boundary conditions and the continuity conditions between the soil layers and the beam, the governing equations of the ground-beam coupled system are solved by the Fourier transform. Finally, the dynamic response of the layered saturated ground in the time-space domain is obtained by using the fast Fourier transform (FFT) method. Three different soil cases are selected to study the influences of the soft/hard interlayer on dynamic response. The displacement as well as the velocity and acceleration of the surface vibration are analyzed. The displacement amplitude spectra at different depths are presented to find the attenuation law of the ground vibration along depth. The numerical results show that both the stiffness and the embedded depth of the interlayer have impacts on the critical velocity of the coupled system and the surface vibration. When the vibration propagates from the tunnel to ground surface, the high-frequency components decrease and the low-frequency components increase.

Saturated Ground Vibration Analysis Based on a Three-Dimensional Coupled Train-Track-Soil Interaction Model

A novel three-dimensional (3D) coupled train-track-soil interaction model is developed based on the multi-body simulation (MBS) principle and finite element modeling (FEM) theory using LS-DYNA. The novel model is capable of determining the highspeed effects of trains on track and foundation. The soils in this model are treated as saturated media. The wheel-rail dynamic interactions under the track irregularity are developed based on the Hertz contact theory. This model was validated by comparing its numerical results with experimental results obtained from field measurements and a good agreement was established. The one-layered saturated soil model is firstly developed to investigate the vibration responses of pore water pressures, effective and total stresses, and displacements of soils under different train speeds and soil moduli. The multi-layered soils with and without piles are then developed to highlight the influences of multi-layered soils and piles on the ground vibration responses. The effects of water on the train-track dynamic interactions are also presented. The original insight from this study provides a new and better understanding into saturated ground vibration responses in high-speed railway systems using slab tracks in practice. This insight will help track engineers to inspect, maintain, and improve soil conditions effectively, resulting in a seamless railway operation.

Influence of dilution on magnetization properties of geometrically frustrated magnetic systems: Effective-field theory cluster approximations on kagome lattice

The influence of the site dilution on the magnetization properties of the antiferromagnetic spin-1/2 Ising model on the kagome lattice is systematically investigated using various n-site cluster effective-field theory approximations up to the cluster size n=12. It is shown that, regardless of the cluster approximation, the site dilution of the system leads, in addition to the existence of the standard saturated ground state, to the formation of four nontrivial plateau ground states together with five single-point ground states that separate them in the zero temperature limit. It is also shown that, while magnetization properties of the saturated ground state and two single-point ground states are stable with respect to the used cluster approximation, the magnetization properties of all four nontrivial plateau ground states and three single-point ground states that separate them strongly depend on the used approximation and have tendency to approach each other with increasing of the cluster approximation.