Study of mechanical characteristics and strength deterioration of wide-graded sand under internal erosion

Experimental study on the failure of a dike foundation caused by backward erosion piping under gradual and sudden hydraulic loads

To investigate the mechanism of road collapse induced by structural defects in underground drainage/sewerage pipelines in water-rich sands, laboratory physical model tests were conducted to reproduce the macroscopic development of surface subsidence. A computational fluid dynamics-discrete element method (CFD-DEM) model was then established and validated against the tests to assess its reliability. Using the validated model, we examined the effects of defect size and groundwater level on the progression of groundwater-ingress-driven internal erosion and tracked the evolution of vertical stress and intergranular contacts around the pipe. Results show that internal erosion proceeds through three stages—initial erosion, slow settlement, and collapse—culminating in an inverted-cone collapse pit. After leakage onset, the vertical stress in the surrounding soil exhibits a short-lived surge followed by a decline on both sides above the pipe. The number of intergranular contacts decreases markedly; erosion propagates preferentially in the horizontal direction, where the reduction in contacts is most pronounced. Within the explored range, higher groundwater levels and larger defects accelerate surface settlement and yield deeper and wider collapse pits. Meanwhile, soil anisotropy strengthens with increasing groundwater level but peaks and then slightly relaxes as defect size grows. These qualitative findings improve understanding of the leakage-induced failure mechanism of buried pipelines and offer references for discussions on monitoring, early warning, and risk awareness of road collapses.

ABSTRACT The coupled fluid–particle modeling and study of microscopic failure mechanisms for engineering projects with complex boundary conditions present significant challenges. A three‐dimensional numerical model for foundation pits considering fluid–particle coupling was successfully established by partitioning the fluid domain for excavation into several subdomains to individually construct computational grids and then assembling them into the actual fluid domain. The macroscopic and microscopic characteristics of seepage failure in excavations under two scenarios with and without displacement of retaining structure were studied, and the critical hydraulic gradient of seepage failure and the earth pressures on the inside and outside of the retaining structure were compared with the theoretical values. The results show that whether or not the retaining structure was allowed to displace, the seepage failure in foundation pits could be divided into four stages: internal erosion stage, upward erosion stage, reverse erosion stage, and shear failure stage. As the hydraulic gradient increased, when the displacement of retaining structure was allowed, the ground surface settlement outside the pit would first occur, followed by the uplift at the pit base, whereas the reverse would occur when the retaining structure was not allowed to displace. This study not only proposes an innovative method for establishing a fluid–particle coupled model for foundation pits but also reveals the microscopic mechanism of seepage failure in foundation pits, providing a reference for the establishment of three‐dimensional fluid–particle models for other projects and the prevention and control of seepage instability in foundation pits.

Modes and ecological damage effects of internal erosion of the redbeds.

Abstract Understanding the hypergravity effect on seepage and internal erosion is the essential precondition for dam hydraulic disaster modeling using geotechnical centrifuges. Soil column testing is useful to bridge this knowledge gap, but previous attempts did not provide adequate functionality in centrifuge environments. This study develops a centrifuge-available apparatus for seepage and internal erosion soil column tests (CASIE). CASIE ensures a consistent and stable circulating water supply with no less than 34,000 ml/min at 80 g via double-bowl upstream and downstream water tanks and a vertical, multistage centrifugal pump. The hydraulic gradient can be controlled by adjusting the elevation of the upstream water tank using a servo lifting system with a vertical displacement range of 1.2 m and a maximum vertical speed of 155 mm/min. A rigid-wall permeameter is developed for multiple applications in soil column tests for seepage and internal erosion. The flowrate through the specimen can be measured using four parallel-installed oval gear flowmeters with a large measurement range of 10–10,000 ml/min. To validate the capabilities of CASIE, two suffusion (one form of internal erosion) tests were conducted at 1 g and 30 g. The results reveal that the scaling factor for the critical hydraulic gradient of 30 g to 1 g is 1/10. It is much less than the predicted value of 1, indicating that suffusion failure is more readily triggered in the hypergravity environment.

Erosion and particle transport analysis in ribbed pipe bends based on CFD-DEM coupling

This study evaluates the retrofit design of the Semat weir on the Kali Gawe in Jepara Regency. The retrofit aims to adjust the weir’s hydraulic capacity to accommodate estimated flood discharges while ensuring the structure’s stability under applied loads. In the agricultural context, adequate water availability for irrigation directly affects crop yields; conversely, the rainy season often increases river flow and flood risk. Irrigation structures such as weirs are therefore required to raise river water levels to divert flow into irrigation channels and to regulate water distribution. Flood discharge estimates were derived from precipitation data and watershed (drainage basin) characteristics. Flood hydrograph planning is a critical design step for the weir. Log-Pearson Type III analysis was used to determine probable precipitation values for several recurrence intervals. Those design precipitation values were then converted into design flood discharges using synthetic unit hydrograph methods, specifically the Snyder, Nakayasu, and Gamma HSS approaches. Employing the Gamma synthetic unit hydrograph for the 50-year return period (Q50) produced a design flood discharge of 2,536.52 m³/s for that recurrence interval. Structural stability analyses of the redesigned weir indicate safety factors well above customary thresholds: overturning resistance factor = 11.6 (required ≥ 1.5), sliding resistance factor = 4.80 (required ≥ 2.0), and piping (internal erosion) factor = infinite (required ≥ 4). All evaluated stability parameters therefore satisfy standard safety criteria.

A fully-resolved micromechanical simulation of piping erosion during a suction bucket installation

Backward erosion piping in numerical models: A literature review

ABSTRACT Internal erosion in gap‐graded soils poses significant risks to water‐retaining structures such as earth dams. However, its underlying mechanisms at the particle scale remain poorly understood. This study couples the discrete element method (DEM) with computational fluid dynamics (CFD) to simulate internal erosion in gap‐graded soil assemblies and employs data‐driven techniques to detect early‐stage erosion. Particle‐scale parameters, such as contact forces, particle velocity and fluid velocity, are extracted from the transient CFD‐DEM simulations. These features are transformed into multi‐dimensional voxel‐based tensors representing the particle–fluid interactions, which are used to train deep learning models. Autoencoder models with 3D convolutional neural network (CNN) layers as encoder and decoder are developed to investigate the micro‐scale patterns within the particle‐fluid assembly. Through sequential training techniques, the temporal evolution of anomalies is captured, enabling identification of the initiation point of internal erosion. The results reveal how microscale behaviours, such as particle motion, contact forces and contact number, contribute to macroscale erosion processes. The outcome of this research can inspire further research into AI‐based early detection techniques for internal erosion in earth dams.

Internal erosion is a significant issue caused by water flow within soils, resulting in structural collapse of hydraulic structures, particularly in coarse soils located near rivers. These soils typically exhibit granulometric instability due to low clay content, resulting in poor hydraulic and mechanical properties. To mitigate this problem, cement treatment is applied as an alternative to soil removal, reducing transportation and storage costs. The hole erosion test (HET) and Crumbs tests, shearing behaviour through consolidated undrained (CU) triaxial, and microstructure analyses regarding scanning electron microscopy (SEM), mercury intrusion porosimeter (MIP) and thermogravimetric analysis (TGA) were conducted for untreated and treated coarse soil specimens with varying cement contents (1%, 2%, and 3%) and curing durations (1, 7, and 28 days). The findings indicate a reduction in the loss of eroded particles and overall stability of treated soils, along with an improvement in mechanical properties. SEM observations reveal the development of hydration gel after treatment, which enhances cohesion within the soil matrix, corroborated by TGA analyses. MIP reveals the formation of a new class of pores, accompanied by a reduction in dry density. This study demonstrates that low cement addition can transform locally unsuitable soils into durable construction materials, reducing environmental impact and supporting sustainable development.

ABSTRACTThis study relies on the high ablation resistance strategy of char layer expansion ceramization to strengthen the char layer and improve ablation performance. The use of phase change material, that is, paraffin wax, was effective in simultaneously improving the ablation resistance and thermal insulation performance of liquid methyl vinyl silicone rubber (LMVSR) composites, which was attributed to the low density, enhanced char layer, and improved heat‐absorption capacity. The introduction of paraffin wax lowered the ablation temperature and increased the specific heat capacity of LVMSR‐based composites, thereby enhancing heat dissipation and reducing thermal erosion. In addition, paraffin wax was able to promote the ceramization reaction. The mass erosion rate, pyrolysis rate, and graphitization degree of the char layer were significantly improved when the content of paraffin wax was increased from 5 to 15 phr. The composites with 15 phr paraffin wax demonstrated the best performance and the highest modification effect. The paraffin modification was effective in reducing the mass ablation rate, especially at 10 phr, which showed a decrease of 34.43% when compared with that of the L‐P0 counterpart. The maximum back‐face temperature was reduced by 29.63% upon adding 15 phr paraffin wax when compared with L‐P0, exhibiting excellent thermal insulation performance. The above results showed that the ablative resistance, internal erosion resistance, and thermal insulation properties of silicone rubber‐based composites were significantly improved by using the strategy of expansion ceramization and enhanced heat absorption/dissipation by adding phase change materials, which shows potential application in preparing flexible thermal ablative material for thermal protection purposes.

Abstract The study investigates the interaction between the grout curtain and bedrock foundation of the Nosice Dam, which is located in a geologically heterogeneous environment on the Váh river in Slovakia. By applying numerical modeling (the Finite Element Method) and analyzing water pressure test data, the research aims to assess the effectiveness of a grout curtain in controlling seepage. An extensive parametric study examines the impact of different rock permeability values on the reduction of seepage, which aids in the optimization of the depth of a grout curtain. This approach underscores the importance of advanced numerical modeling for dam safety and seepage control. The results could contribute to the knowledge of dam seepage control measures and could provide valuable insights about similar hydraulic structures. They could potentially mitigate risks, such as internal erosion and foundation instability associated with uncontrolled seepage.

A major hazard to embankment dams is internal erosion and piping arising from concentrated leaks through transverse cracks in the core near the crest due to differential settlement or desiccation. Erosion is controlled by the wall stresses resulting from flow in the crack, the critical shear stress, and rate of erosion properties of the soil. Many older embankment dams have filters or transition zones downstream of the core to arrest erosion, but the filter or transition is coarser than required to satisfy modern no-erosion filter design criteria, so erosion potentially initiates and progresses. We previously presented an analysis of flow through such transverse cracks without a filter, applicable to homogeneous embankments. This present analysis models flow in embankment cracks with a vertical filter downstream. Capillary and adhesion processes are neglected. Multiphase flow computations are avoided by assuming that the filter is saturated and then demonstrating that pressure along the separating streamline between the crack discharge and the saturated zone is ∼0. The consequent water levels and hydraulic shear stresses within the cracks are then determined. Plausible methods for adapting the assumed geometry and flow conditions to other commonly-occurring arrangements are also presented.

To accommodate the extreme thermodynamic effects and erosion damage in throttling equipment for ultra-high-pressure natural gas wells (175 MPa), a coupled multiphase flow erosion numerical model for nozzles was established. This model incorporates a real gas compressibility factor correction and is based on the renormalized k-ε RNG (Renormalization Group k-epsilon model, a turbulence model that simulates the effects of vortices and rotation in the mean flow by modifying turbulent viscosity) turbulence model and the Discrete Phase Model (DPM, a multiphase flow model based on the Eulerian–Lagrangian framework). The study revealed that the nozzle flow characteristics follow an equal-percentage nonlinear regulation pattern. Choked flow occurs at the throttling orifice throat due to supersonic velocity (Ma ≈ 3.5), resulting in a mass flow rate governed solely by the upstream total pressure. The Joule–Thomson effect induces a drastic temperature drop of 273 K. The outlet temperature drops below the critical temperature for methane hydrate phase transition, thereby presenting a substantial risk of hydrate formation and ice blockage in the downstream outlet segment. Erosion analysis indicates that particles accumulate in the 180° backside region of the cage sleeve under the influence of secondary flow. At a 30% opening, micro-jet impact causes the maximum erosion rate to surge to 3.47 kg/(m2·s), while a minimum erosion rate is observed at a 50% opening. Across all opening levels, the maximum erosion rate consistently concentrates on the oblique section of the plunger front. Results demonstrate that removing the front chamfer of the plunger effectively improves the internal erosion profile. These findings provide a theoretical basis for the reliability design and risk prevention of surface equipment in deep ultra-high-pressure gas wells.

ABSTRACTThe subsoils of river dikes are often composed of highly permeable and low‐density river sediments. Thus, erosion signatures (leaks, sand boils, sinkholes) can appear in the protected floodplain during floods, highlighting the development of hydromorphodynamic phenomena below the surface, which may harm the safety of the dike system. A multiscale methodology is deployed to understand and analyze the influence of floodplain architecture in terms of geological formations on the appearance of local erosion signatures. Particular attention is paid to the morphology of paleovalleys and paleochannels in order to image the subsurface in terms of substrate types and interfaces using geophysical methods. This information makes it possible to propose internal erosion scenarios. Application to a study area in the South of France (the Agly dike system) leads to new results. The classical backward erosion piping scheme is not relevant to explain the observed sand boils, as they are mainly caused by the suffusion‐type internal erosion process. Suffusion and contact erosion appear to be the origin of sinkholes. The distribution of these signatures appears to be directly related to the shape and dimensions of the paleovalley and paleochannels, as well as to the presence of a low‐permeability topsoil.

The internal erosion effect causes fine particles in the soil to move through seepage, and the loss of these fine particles leads to changes in porosity, which in turn affects the soil's hydraulic properties and mechanical performance, posing a threat to the safety of dam and levee engineering. To understand the formation and development of internal erosion under reverse seepage, a simulation test device for internal erosion was designed, and experiments were conducted on three granite residual soil samples with identical soil properties under different water flow speeds (25L/H, 50L/H, and 100L/H). By comparing and analyzing the wetting front, the amount of internal erosion, and the water content, the influence of water flow speed on reverse seepage internal erosion was studied. The results show that under reverse internal erosion, as the water flow speed increases, the internal erosion rate accelerates, as evidenced by the faster advancement of the wetting front and the increase in cumulative internal erosion. As internal erosion develops, the fine particle accumulation curve enters a stable phase. After the soil's water content reaches its peak, it slightly decreases and then remains relatively stable. Fluctuations in the soil water content occur due to the formation of preferential internal erosion channels or the redeposition of fine particles. The soil particle movement, fine particle loss, and redeposition caused by internal erosion create an internal erosion channel that narrows from the inlet to the outlet.

Numerical investigation of threshold effects in internal erosion of granular soils using coupled DEM-DFM

Numerical modelling of rainfall-induced internal erosion process within vegetated deposited slopes