Tunnel Erosion Research Articles

Genesis and evolution mechanism of loess tunnels in the Loess Plateau, China

Loess tunnels are very common in the Loess Plateau, and they pose unique geological threats. Loess tunnels are often difficult to detect and control due to their concealment and sudden appearance. Thus far, research on the genesis and evolution of loess tunnels remains scarce. In this paper, the genesis and evolution mechanism of the loess tunnels in the Loess Plateau is studied in depth, and the location, shape, and size information are obtained via field investigations. The potential correlations between the loess sediment, the basic physical properties (depth, water content, particle size composition, collapsibility coefficient, and self-weight collapsibility coefficient), and the tunnel density are inferred based on the Pearson’s correlation coefficients and tests on the physical and mechanical properties of the loess sediments. In addition, spatial statistical modelling is employed to justify and predict the observed spatial distribution of the loess tunnels assuming Gaussian Markov random fields. The formation of loess tunnels is due to a combination of factors, including the formation thickness, soil properties, joints and fissures, topography, hydrogeology, and climatic conditions. The thickness of the loess, loess sediment properties, and their spatial relationship jointly determine the material basis of the formation of the loess tunnels. The loess tunnels at different depths have different main controlling factors that are hierarchical by depth. The evolution process of loess tunnels can be divided into five stages: the incubation stage, formation stage, development stage, failure stage, and withered stage. The characteristics of each stage are discussed in detail. Our work provides novel insights into subsurface erosion from the aspect of soil tunnels. It improves our understanding of hill slope geomorphological evolution and also provides effective techniques for tunnel erosion control.

Investigating the Susceptibility of Tunnel Erosion in Southern Nigeria using Integrated Geophysical Methods

There has been a growing incidence of soil subsidence in various regions of Anambra State, located in southern Nigeria. These subsidence events primarily stem from the formation of natural tunnels or soil pipes within the subsurface. To delve deeper into this issue, the present study employed two geophysical techniques, namely the Very Low Frequency Electromagnetic (VLF-EM) method and the Electric Resistivity Method utilizing the Dipole-Dipole array. These methods were utilized to investigate the characteristics and spatial distribution of soil pipes at two specific sites: Awka Site 1 and Awka Site 2, both situated within Anambra State, Nigeria. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed. On each profile, the inphase and outphase were collected using the Abem Wadi Meter after a confirmed connection to the external satellite, while nine dipole-dipole profiles were carried on the same location. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed (Figures 3 and 4). The data analysis from the VLF-EM survey reveals that a significant portion (80%) of the low conducting zones observed in the pseudosection originates from the top of the profile, indicating a downward trend in the formation of soil piping. The VLF-EM result also inferred that the subsurface voids in the study areas extend vertically downward up to 10 meters, with an average horizontal extension exceeding 0.5 meters. This highlights the extensive spatial reach of the subsurface voids and emphasizes their potential impact on the surrounding environment. The dipole-dipole survey conducted in the study area identified six distinct structures, with the eroded formation being particularly noteworthy. This structure, characterized by resistivity ranging from 1200–30000 Ωm, plays a significant role in creating favorable conditions for soil piping. Furthermore, the presence of strong dispersive soils increases the likelihood of soil piping occurrences within this structure.

Investigation into the Chemical Characteristics of Soils near Sinkholes Situated in Anambra State, Nigeria

In this comprehensive study, we delve into the chemical properties of soil in the proximity of sinkholes located in Anambra state of Nigeria. Three distinct sinkhole sites were selected for examination: Awka site 1 (6.2232°N and 7.0824°E), Awka site 2 (6.2220°N and 7.0819°E), and Agulu (6.0941°N and 7.0203°E). For a comprehensive analysis, 24 soil samples were meticulously collected and subjected to thorough analysis. These samples comprised of 15 specimens obtained from the immediate vicinity of the sinkholes, while 9 samples were procured from locations situated at least 2 kilometers away from the sinkhole sites. The study focused on investigating various parameters, namely pH levels, Organic Carbon (OC) content, Organic Matter (OM) content, Aluminum (Al) levels, Hydrogen (H) levels, Total Nitrogen (TN) content, Magnesium (Mg) levels, Potassium (K) levels, Sodium (Na) levels, Calcium (Ca) levels, Effective Cation Exchange Capacity (ECEC), Base Saturation, and soil texture. The obtained results revealed that the study areas predominantly exhibited a sandy composition with a notably low clay content. Furthermore, the analysis indicated low hydrogen values, while sodium levels were observed to be relatively high. Consequently, certain areas, particularly those situated farther away from the sinkhole site, exhibited a reduced amount of exchangeable bases and effective cation exchange capacity. This phenomenon potentially resulted in leaching and dispersion within the soil, leading to inadequate water infiltration and subsequent run-off. Notably, this process may have contributed to the formation of tunnel erosion, ultimately resulting in the emergence of sinkholes.

Current-induced scouring around a submarine pipeline using a multi-phase flow model with different inter-phase drag models

This paper numerically investigates scouring beneath a submarine pipeline induced by the current. The simulations use an Eulerian–Eulerian multi-phase model for sediment transport developed in OpenFOAM (the open-source Computational Fluid Dynamic toolbox), which can capture fluid–particle–structure interactions. The model implements the Hybrid Fictitious Domain-Immersed Boundary (HFD-IB) method for representing the pipeline structure in the computational domain. The simulated flow fields are validated by experimental data for the developed flow around a pipeline over scoured beds, resulting in favorable results for velocity profiles. Further, three inter-phase drag models, the EF (Engelund and Fredsøe, 1976), RZ (Richardson and Zaki, 1997), and ERZ (Lee and Huang, 2018) are applied and subsequently examined in simulating pipeline scouring, particularly the effects on the flow field, sediment transport, and the resulting scour profile during the onset and tunnel erosion stages. The models produce consistent results with the measured scour profile. All models show that buoyancy is dominant in driving the sand movements within and near the bed surface in both stages. Meanwhile, the drag force is dominant for sand transport above the bed where the inter-phase relative velocity is high. However, the three models differently capture the physical behaviors of the particle response time and relative velocity near the bed surface and within the bed, which impact the computed drag force and, thus, the resulting scour profile.

Examining tunnel erosion using vertical electrical sounding and ground magnetic geophysical techniques in Awka, Nigeria

Examining tunnel erosion using vertical electrical sounding and ground magnetic geophysical techniques in Awka, Nigeria

Scour at River-Crossing Cylindrical Structures in Degrading Channels

Channel degradation can expose river-crossing cylindrical structures (e.g., pipelines, canals, and tunnels) that are completely buried in the riverbed, altering their surrounding flow patterns to cause scour that threatens their structural safety. This paper reports a case study of scour at a river-crossing canal in a degrading river. Based on the historical topography and hydrological data, the cause, development process, and performance of countermeasures for scour at the river-crossing canal were analyzed, and recommendations for scour protection are proposed. The findings from 53 experiments of the scour at a river-crossing cylindrical structure in a degrading channel are presented. The experimental results indicate that there are three scour development modes at a river-crossing cylindrical structure in a degrading channel: (1) the exposure of the buried cylindrical structures induces downstream scour, with no tunnel erosion (i.e., scour below the bottom of the cylindrical structure) occurring at the cylindrical structure, (2) the riverbed degradation downstream of the cylindrical structure increases the water head over the cylindrical structure and seepage below the cylindrical structure, inducing tunnel erosion at the cylindrical structure, and (3) tunnel erosion is induced when a large-scale bedform trough approaches the cylindrical structure. Based on the experimental data, the critical conditions of each scour development mode were determined. The effects of the flow intensity, dimensionless water depth, and bed material roughness on the equilibrium scour depth were analyzed, and predictors for estimating the equilibrium scour depth at a river-crossing cylindrical structure in a degrading channel are proposed.

Numerical investigation of characteristics and mechanism of tunnel erosion of loess with coupled CFD and DEM method

Tunnel erosion of loess can cause geological disasters such as landslide and debris flow, as well as threaten the safety of civil engineering structures. Understanding the mechanism and evolution law of tunnel erosion is not only helpful to analyze the occurrence and development of gully erosion, but also conducive to the guidance of engineering practice. Although many scholars have carried out statistical analysis on loess tunnel erosion based on field investigation, quantitative analysis and visualization at the micro level are lacking. Because the combination of computational fluid dynamics (CFD) and discrete element method (DEM) has great advantages in studying the seepage erosion process. This manuscript adopts the CFD-DEM coupling method, and the virtual cement method is used to transform the failure of cemented matrix into the reduction law of bonding strength. The micro mechanism of tunnel erosion of three areas is discussed from the aspects of particle erosion rate, cement evolution and micro mechanical field. The results show that the more the fine particle content is, the greater the cementation is, the stronger the erosion resistance is, the less developed the erosion rate of the particles is, and the dissolution failure rate of the cement is usually earlier than the erosion rate of the particles, and the variation range of the anisotropy of the contact force is also the smallest. However, under high water pressure, their erosion rates are almost synchronous and reach the slow erosion stage quickly. The anisotropic parameters of the contact force of the three samples virtually decreased gradually in the process of erosion, and finally tended to be stable.

Numerical simulation of early stages of scour around a submarine pipeline using a two-phase flow model

Current-induced scour around a submarine pipeline during onset and tunnel erosion are investigated in both clear-water and live-bed regimes using an Eulerian–Eulerian two-phase model for sediment transport. The Hybrid Fictitious Domain-Immersed Boundary (HFD-IB) method is adopted to simulate the pipeline structure. The model is validated against two published experimental data: (1) flow past a pipeline near a rigid bed and (2) scour around a pipeline over a sand-bed, showing favorable results for the flow and scour patterns around the pipe. The driving mechanisms of the onset and tunnel erosion are investigated by examining the detailed fluid–sediment interactions around the pipe in both regimes. The driving mechanisms for the onset and tunnel erosion are found similar in both regimes, except their time and magnitude scales. The onset is triggered by the piping mechanism induced by the upstream–downstream pressure gradient beneath the pipe. Meanwhile, the jet flow is the driving mechanism for the tunnel erosion. The simulated live-bed condition shows a higher upstream–downstream pressure gradient beneath the pipe, faster piping mechanism, more intense jet flow velocity and sediment flux compared to the clear-water condition. The model well capture these underlying mechanisms, suggesting its promising capability as a predictive tool for scouring.

Unresolved CFD and DEM Coupled Simulations on Scour around a Subsea Pipeline

In this paper, numerical studies were carried out on scour around a subsea pipeline. A coupled solver between computational fluid dynamics (CFD) and discrete element method (DEM) was selected to simulate fluid flow and particle interactions. To select and validate the numerical model parameters in the solver, angles of repose and incipient motion were simulated. From the validation studies, the selected coefficient of rolling friction with spherical particles could predict the behavior of non-spherical particles. The fluid flow around the subsea pipeline was simulated, and the motion of individual soil particles was tracked. Particle motions were generated by the drag force, due to a high velocity. Three scour development process, such as onset of scour, tunnel erosion, and lee-wake erosion, were studied and discussed. The scour depth evolution showed good agreement with the experimental data. It was confirmed that the selected solver, with numerical model parameters, predicted the scour process around a subsea pipeline well.

Analysis of the slope failure mechanism a under tunnel erosion environment in the south-eastern Loess Plateau in China

As a unique geological hazard, the occurrence of loess tunnels is closely related to slope failures. However, almost no research on the relationship between loess slope failures and tunnel erosions has been presented, which seriously restricts the deep understanding of the genesis mechanism and evolution process of loess slope failures. This paper focuses on revealing the failure mechanism of loess slopes in tunnel environments via methods including field investigation, exploration, field water injection tests, tracking tests and numerical simulations. The results show that the whole process of permeability coefficient of the soil layer in the test site is divided into three stages: the rapidly decreasing stage, the slowly decreasing stage and then the stable stage; the slope failure process is divided into five stages based on different failure characteristics: the soaking and softening stage, the erosion connecting stage, the slope local failure stage, the slump failure stage and the resting stage. Seepage has a great influence on loess slope failures in the tunnel environment, and it is as an important inducement of failure, while tunnels themselves play an aggravating catalytic role. The soil water interaction is shown to be an important internal cause of slope failure both by field tests and numerical simulations. The existence of tunnels is not conducive to the overall stability of the slope as tunnels expand the influence range of the soil-water interactions. This study re-evaluates the failure mechanism of loess slopes from a novel perspective based on the tunnel erosion environment.

Coupled Flow-Seepage-Elastoplastic Modeling for Competition Mechanism between Lateral Instability and Tunnel Erosion of a Submarine Pipeline

The instability of a partially embedded pipeline under ocean currents involves complex fluid–pipe–soil interactions, which may induce two typical instability modes; i.e., the lateral instability of the pipe and the tunnel erosion of the underlying soil. In previous studies, such two instability modes were widely investigated, but separately. To reveal the competition mechanism between the lateral instability and the tunnel erosion, a coupled flow-seepage-elastoplastic modeling approach was proposed that could realize the synchronous simulation of the pipe hydrodynamics, the seepage flow, and elastoplastic behavior of the seabed soil beneath the pipe. The coupling algorithm was provided for flow-seepage-elastoplastic simulations. The proposed model was verified through experimental and numerical results. Based on the instability criteria for the lateral instability and tunnel erosion, the two instability modes and their corresponding critical flow velocities could be determined. The instability envelope for the flow–pipe–soil interaction was established eventually, and could be described by three parameters; i.e., the critical flow velocity (Ucr), the embedment-to-diameter ratio (e/D), and the non-dimensional submerged weight of the pipe (G). There existed a transition line on the envelope when switching from one instability mode to the other. If the flow velocity of ocean currents gets beyond the instability envelope, either tunnel erosion or lateral instability could be triggered. With increasing e/D or concurrently decreasing G, the lateral instability was more prone to being triggered than the tunnel erosion. The present analyses may provide a physical insight into the dual-mode competition mechanism for the current-induced instability of submarine pipelines.

Application of computational modeling for large wood dynamics with collisions on moveable channel beds

This study aims to analyze model reproducibility on large wood collisions on moveable channel beds. Flume experiments were conducted to observe the responses of moveable bed change with the behavior of large wood, including wood collisions. In the experiments, we employed time intervals of 5 and 9 s in wood supply, which modified the collision patterns. Subsequently, we conducted simulations based on the experimental conditions. Herein, to analyze the wood collision effect in the simulations, we employed the dashpot-spring model in a large wood dynamics model. Subsequently, we compared the simulations with and without wood collision and discussed the reproducibility of the model. The results of the simulations and experiments revealed that the wood pieces exhibited diverse behaviors, including floating, sliding, deposition, plane rotation, rolling, collision, remobilization, jam formation, and causing local bed changes. In particular, the results indicate that model reproducibility is dependent on wood collision because this can actively alter wood deposition patterns. We also described the piping and tunnel erosion processes linked to wood collision and moveable beds and suggested the corresponding, suitable computational treatment. This study presents an experimental and modeling methodology on wood collision and the interaction between bed change and large wood. Thus, useful information for solving numerical difficulties linked to large wood dynamics, including wood collisions, is provided.

Geomorphic landscape design integrated with progressive mine restoration in clay quarries of Catalonia

ABSTRACT Geomorphic-based mine restoration of clay quarries in Tortosa (Catalonia) was co-funded by the European Union’s LIFE programme. The landform design was made with GeoFluv-Natural Regrade. Their building was performed with existing machinery pool and operators. The main constraint was the impossibility of setback regrading of pre-existing-benched highwalls. Progressive geomorphic mine restoration neither reduced mineral production nor changed the operations. The approach has resulted in higher landscape functionality and integration. Monitoring showed localised erosion due to poorly planned discharge of road runoff and sporadic tunnel erosion. Sediment movement at the designed drainage network is similar to the local fluvial dynamics.

Erosion characteristics of loess tunnels on the Loess Plateau: A field investigation and experimental study

AbstractLoess tunnels are a common geo‐hazard in the Loess Plateau and not only cause considerable soil and water loss, but also aggravate and even induce the occurrence and development of other disasters such as ground fissures, mudflows, collapses, and landslides. To date, research on the hydrological characteristics and erosion behaviour of loess tunnel systems has focused on field investigation data and limited river basin observation data, whereas field test information and data are very scarce. In this study, field surveys, observations, field scouring experiments, and laboratory‐based sediment percentage tests were conducted to analyse the erosion characteristics, spatial distribution, and hydrological characteristics of a large‐scale loess tunnel system in the hilly Loess Plateau southeast region of northern China. The results showed that the loess tunnel erosion exhibited periodicity. Tunnel erosion in each period shows a similar erosion process, that is, thin‐layer water flow erosion and lateral expansion, tunnel wall collapse and deposit due to the loss of support from the lateral erosion, and erosion and transport of deposits by water flow. Waterfall erosion, lateral erosion, headward erosion, and the resulting collapses were the main forms of tunnel erosion. Besides this, the base level of erosion significantly affects the erosion characteristics of the loess tunnel. The hydrological characteristics during field scouring experiments exhibited three different stages: a lag effect, attributed to the temporary loss of water velocity in the first stage; small water flow fluctuations in the second stage; and an increase in total seepage loss with increased water injection flow in the third stage. The erosion rate was positively correlated with the flow quantity. The results of this study not only provide valuable reference data for research on the mechanism and velocity of erosion events in loess, but also provide a theoretical basis for the prevention of loess tunnel disasters in engineering construction. © 2020 John Wiley & Sons, Ltd.

Two-phase flow simulation of scour beneath a vibrating pipeline during the tunnel erosion stage

A new numerical model is developed to simulate and investigate scour beneath a vibrating pipe during the tunnel erosion stage. This study is motivated by the fact that existing numerical models are not able to properly simulate scour under a vibrating pipeline, and the underlying physical mechanisms are not well understood due to the complex fluid-structure-sediment interaction. The present model incorporates the hybrid fictitious domain-immersed boundary method into a recently developed rheology-based two-phase model. The present model is validated against published experiment results of flow beneath a vibrating pipeline near a rigid boundary and scour beneath a fixed pipe. The flow velocity at the gap and the scour profile beneath the pipe are generally well produced by the model. Subsequently, the proposed model is applied to simulate scour under a vibrating pipe with different vibration amplitudes and frequencies. Among other things, it is found that maximum pipe acceleration has a dominant effect on the underlying physics that induce scour, irrespective of the combination of the vibration amplitude and frequency. An explanation for this finding is proposed based on various quantitative simulated results.

Experimental study on the physical modeling of loess tunnel-erosion rate

Loess tunnel erosion is a distinctive soil erosion pattern that commonly occurs in collapsible loess areas. The resulting soil erosion is a serious and complex problem. At present, most of the research methods on loess tunnel erosion are based on field investigation, and the knowledge obtained is, therefore, mostly qualitative. Erosion characteristics and erosion rates of loess tunnels are rarely revealed quantitatively by laboratory physical modeling or field investigation. In this paper, we report the results of a small model test based on three types of archetypal geological structures that are recognized by field investigation, i.e., vertical fractures, horizontal fractures, and round holes, These results reveal the effect of different initial flow cross sections on the erosion characteristics of loess tunnels. We found that, where water crosses an area of loess, waterfalls dominate initial erosion. We also studied the mud erosion rate to quantitatively reveal tunnel-erosion rates and their variation with time and found that tunnel erosion occurs in three stages: a rapid-erosion stage, an erosion slow-down stage, and a slow uniform-erosion stage. Using a model test to undertake quantitative studies into the relationship between the erosion rate of a loess tunnel and both initial dry density and initial water content, we demonstrated a significant linear negative correlation between these parameters. Through physical simulations combined with field investigation and analysis of survey results, we found that the erosion of loess tunnels occurs with a certain periodicity and that the tunnel-erosion base level plays a decisive role in the erosional features and final morphology of loess tunnels. By increasing our understanding of the internal mechanisms of loess tunnel erosion, our work will contribute to the prevention of cave erosion and reduction in the risks of associated disasters in the Loess Plateau of the Yellow River in China and in other loess-containing areas around the world. Our studies also provide a theoretical basis for carrying out soil and water conservation work in similar areas.

Marl qualitative effects on the distribution of Quaternary deposits

Views Marls with badland, waterway, torrent, surface, rill, gully, rock and tunnel erosion, as the most sensitive lithology against erosion and weathering, have a major role in the sedimentation of watersheds. Marls of Iran based on minerals solvability are divided into two types: Evaporative and non-evaporative which have Special features and have been distributed in Tehran province. Formations and units of geology are important in sediments production and distribution marl units. In Tehran province, from the geological point of view, there is a combination of Alborz and Central Iran sedimentary units that based on maps geology information, on a scale of 1: 100000 and 1:250000 such as Tehran, Semnan, Amol, Ghazvin-Rasht, Marzanabad (Chalus) the geology formations and units exist often. There isn’t any gap in geology maps. Often marl geology formation and units such as Karaj, Ziarat, upper red and under red formation, are Cenozoic age and has been distributed in Tehran province North. Based on studies and this research geology marl Formations and units have been effective about percent 10 on sediment production and quaternary distribution in Tehran province. https://doi.org/10.14331/ijfps.2018.330119

Analysis physical and chemical properties of soil and morphometric impacts on gully erosion

It is widely known that the development of gullies causes the loss of a great amount of soil and can be considered as one of the principal causes of geo-environmental degradation. The study area of the present research is located in the eastern shores of Urmia Lake. This study sought to evaluate the rate of growth and effective factors on the area’s gully dynamic over a year. To achieve this goal, levels of Electrical Conductivity (EC), Sodium Adsorption Ratio (SAR), pH, types of soil, and morphometric analysis were used for categorizing and analyzing effective factors on the creation of gully erosion. In addition, as the climate index, the coefficient of moisture in the area’s formation for analyzing the area’s gully erosion potential was also employed as relevant indicators. The results based on climate indices show that the area’s gully erosion potential is high. Analyzing the taken samples’ structures indicated that the constructing components of geological formations provided highly appropriate conditions for vertical and horizontal leveling of gullies because of the weak building of aggregates in a way that the results of the morphometric monitoring of the gullies represent the area’s high growth speed and dynamicity. High accumulation of soluble salts especially high values of SAR and EC have led to dissolution erosion that provides the conditions for creating tunnel erosion. The results of the research showed gully erosion formation in the study area and its expansion due to two factors, climate factors together with the chemical and physical characteristics of ‌an area’s soils. The results of the present research are of great importance for gully erosion modeling tasks in the study area and can be used as the base of future researches for identifying the relevant criteria and indicators in gully erosions.

Influence of micro–structure on erosion resistance of plasma sprayed 7YSZ thermal barrier coating under gas turbine operating conditions

This paper presents the results of an experimental investigation of the effect of topcoat micro-structure on the erosion resistance of air plasma sprayed 7wt% YSZ thermal barrier coating (TBC) at high temperature. Two sets of air plasma sprayed YSZ TBCs of differing micro-structures (porosities of 12.9 ± 0.5% and 19.5 ± 1.2%) were tested in a high temperature erosion tunnel to simulate modern gas turbine engine's operating conditions. The tests were conducted over a range of temperatures between 537°C and 980°C, gas velocities between 122m/s and 305m/s, and impingement angles between 20° and 90°. The two types of erodent powders used in the erosion tests were nominal 27µm white fused Alumina and nominal 15µm A3 test dust. The results demonstrate reduced erosion resistance associated with increased porosity in the topcoat layer.

Numerical simulation of scour around a submarine pipeline using computational fluid dynamics and discrete element method

• Numerical simulation of scour around a pipeline using a coupled CFD-DEM model. • Simulated scour evolution is in good agreement with published experimental results. • Detailed information, e.g., particle position, velocity, and force, is obtained to improve the understanding of scour mechanism. • Particle motion and particle–particle interactions are found to be most intense during tunnel erosion stage. • Both pressure gradient and drag forces are important at onset and early stages of scour, but drag is dominant at the last stage. Scour under a submarine pipeline can lead to structural failure; hence, a good understanding of the scour mechanism is paramount. Various numerical methods have been proposed to simulate scour, such as potential flow theory and single-phase and two-phase turbulent models. However, these numerical methods have limitations such as their reliance on calibrated empirical parameters and inability to provide detailed information. This paper investigates the use of a coupled computational fluid dynamics-discrete element method (CFD-DEM) model to simulate scour around a pipeline. The novelty of this work is to use CFD-DEM to extract detailed information, leading to new findings that enhance the current understanding of the underlying mechanisms of the scour process. The simulated scour evolution and bed profile are found to be in good agreement with published experimental results. Detailed results include the contours of the fluid velocity and fluid pressure, particle motion and velocity, fluid forces on the particles, and inter-particle forces. The sediment transport rate is calculated using the velocity of each single particle. The quantitative analysis of the bed load layer is also presented. The numerical results reveal three scour stages: onset of scour, tunnel erosion, and lee-wake erosion. Particle velocity and force distributions show that during the tunnel erosion stage, the particle motion and particle–particle interactive forces are particularly intense, suggesting that single-phase models, which are unable to account for inter-particle interactions, may be inadequate. The fluid pressure contours show a distinct pressure gradient. The pressure gradient force is calculated and found to be comparable with the drag force for the onset of scour and the tunnel erosion. However, for the lee-wake erosion, the drag force is shown to be the dominant mechanism for particle movements.