Effects of pre-shearing stress ratio on the mechanical behaviours of gap-graded soils subjected to internal erosion

Internal erosion in granular soils with different microstructures under cyclically increased hydraulic gradients

Internal erosion processes in soils play an important role on the instability analyses of hillslopes and embankment dams. Field observations support the assumption that the internal fine particles may migrate among the channels formed by coarser particles under the high hydraulic gradient condition, where the enrichment of fine particles has great potential on the increase of local pore-water pressure due to their low permeability. Although a number of traditional seepage experiments in laboratory have provided data showing the effect of soil properties on the macroscopic permeability, however, much remains unknown particularly for microscopic erosion processes. Therefore, in the current study, a series of one-dimensional soil seepage tests were firstly conducted by controlling the coarse to fine particle size ratio, and then the X-ray tomography tests were carried out at beamline BL13W1 at the Shanghai Synchrotron Radiation Facility (SSRF) to obtain the particle distributions and three-dimensional pore structures. By coupling discrete element method (DEM) with Darcy’s law, the internal particle erosion processes were back-analyzed. The results reveal that the preferential erosion can occur in the top and bottom regions of the soil specimen, and the migrated fine particles can be supplied when the pore size is large enough along the seepage path.

The purpose of this paper is to assess the difference between surface and internal erosion processes using results from flow pump tests. Samples of 70% Ottawa sand + 30% kaolinite mixture were used with distilled water and NaCl solutions as permeants. Two kinds of tests were conducted, a surface erosion test where the permeant was pumped through a cylindrical hole of 7-mm diameter and an internal erosion test where the permeant was pumped through intact compacted samples in compaction permeameters. A simple capillary tube model was used to estimate the critical shear stresses needed to cause erosion in surface erosion experiments. It was found that although surface erosion critical shear stresses were exceeded in the intact soil samples, particle clogging in the pores and redeposition of eroded particles prevented mobilization of particles into the effluent stream. Erosion rates estimated using surface erosion parameters were significantly greater than those observed in internal erosion experiments. The results suggest that the fate of eroded particles, including particle redeposition and pore clogging, may govern the internal erosion process far more than the surface erodibility of the soil.

The loss of fine particles from the skeleton formed by coarse particles due to seepage action significantly affects the grading, void ratio, and mechanical properties of soil. This results in several issues of engineering hazards. In order to analyze the effect of internal erosion on the mechanical properties of gap-graded soils from macro and micro perspectives, triaxial consolidation and drainage shear tests were simulated in this paper using the particle flow discrete element software PFC3D. A linear contact model was employed to simulate internal erosion by randomly removing fine particles. The results showed that the void ratio of the specimens increased with the erosion degree. The variation in void ratios of the specimens with the erosion degree before loading was greater than those after loading. The peak deviatoric stresses of the specimens decreased with the increase of the erosion degrees. The larger the erosion degree, the more the maximum volumetric strain and the resistance capacity to deformation was also reduced. The average particle coordination number (Z) of the specimens generally tended to decrease as the erosion degree increased. When the average effective stress was not large, the critical state line gradually increased with the erosion degree, while the void ratio was also found to correlate with the erosion degree under the critical state of the specimens with zero average effective stress.

Suffusion is a particular internal erosion process that can lead to important disorders in water retaining structures such as embankment dams and levees. It causes modifications in the soil micro-structure and may modify the mechanical behaviour of the soil leading to deformations at the macroscopic scale. Therefore, the aim of this study is to investigate the consequences of internal erosion on the mechanical properties of the soil. We present such an investigation through numerical and experimental approaches. For the experimental approach, a newly developed suffusion test apparatus is used while for the numerical approach, a model is established based on the discrete element method (DEM) with a one-way fluid-solid coupling.

Internal erosion can trigger severe engineering disasters, such as the failure of embankment dams and uneven settlement of buildings and sinkholes. This paper comprehensively reviewed the mechanisms of soil internal erosion studied by numerical simulation, which can facilitate uncovering the internal erosion mechanism by tracing the movement of particles. The initiation and development of internal erosion are jointly influenced by the geometric, mechanical, and hydraulic conditions, which determine the pore channels and force chains in soil. The geometric conditions are fundamental to erosion resistance, whereas the mechanical conditions can significantly change the soil erosion resistance, and the hydraulic conditions determine whether erosion occurs. The erosion process can be divided into particle detachment, transport, and clogging. The first is primarily affected by force chains, whereas the latter two are mostly affected by the pore channels. The stability of the soil is mainly determined by force chains and pore channels, whereas the hydraulic conditions act as external disturbances. The erosion process is accompanied by contact failure, force chain bending, kinetic energy burst of particles, and other processes due to multi-factor coupling.

Keywords: Ephemeral gully erosion Erodibility Internal erosion Landslides Pipeflow Soil pipes. Abstract. Many field observations have led to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved, and inconsistent use of terms have exacerbated the problem. One such piping process that has been the focus in numerous field observations, but with very limited mechanistic experimental work, is flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes results in internal erosion, stabilizes hillslopes by acting as drains, destabilizes hillslopes via pore-pressure buildups, and results in gully formation or reformation of filled-in ephemeral gullies. The objectives of this article are to review discrepancies in terminology in order to represent the piping processes better, to highlight past experimental work on the specific processes of soil pipeflow and internal erosion, and to assess the state-of-the-art modeling of pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies forming due to mass failures caused by flow into discontinuous soil pipes, and the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion. In some of these studies, the soil pipes were simulated with perforated tubes placed in the soil, while in others the soil pipes were formed from the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes to calculate critical shear stress and erodibility properties of soils. The most common numerical models for pipeflow have been based on Richards’ equation, with the soil pipe treated as a highly conductive porous medium instead of a void. Incorporating internal erosion into such models has proven complicated due to enlargement of the pipe with time, turbulent flow, and episodic clogging of soil pipes. These studies and modeling approaches are described, and gaps in our understanding of pipeflow and internal erosion processes and our ability to model these processes are identified, along with recommendations for future research.

Change in mechanical behaviour of gap-graded soil subjected to internal erosion observed in triaxial compression and torsional shear

HighlightsThis study provides data from internal erosion tests on four intermediate-scale homogeneous embankment dams.Soil properties influence the breach formation process and breach timing.Results showed that observed erosion rates of the internal flow path varied by several orders of magnitude.Quality control of embankment construction can greatly influence breach development.Abstract. Internal erosion and embankment overtopping are the two most common causes of embankment dam and levee failures and incidents. Internal erosion is the removal of soil material by the flow of water through a continuous defect, cavity, or crack within a compacted fill and/or its foundation. Internal erosion initiates from vulnerabilities within the embankment. The embankment soil material plays a key role in both the erosion process and rate of failure, but characterizing soil properties and how they relate to the rate of failure can be challenging. Soil properties such as texture, density, strength, moisture content, and erodibility can vary greatly; thus, it is important to study the effects of these properties on the breach formation process and breach timing. The USDA Agricultural Research Service performed internal erosion breach experiments on four intermediate-scale homogeneous earthen embankments constructed of soils ranging from a silty sand to a lean clay material. The embankments were constructed to a height of 1.3 m, a top width of 1.8 m, and upstream and downstream slopes of 3(H):1(V). The embankment materials were characterized by water content, density, texture, strength, and erodibility. Erodibility was measured using a jet erosion test (JET) apparatus. A 40 mm diameter, continuous steel pipe was placed through each embankment during construction and removed to form an open-ended void through the embankment connected to the upstream reservoir. The removal of the pipe initiated internal erosion. The objectives of the experiments were to observe the development of the internal erosion process over time and to examine the influence of soil properties on the erosion rate, breach timing, geometry of the breach opening, and breach outflow. The rate of erosion and failure observed in these tests varied by several orders of magnitude, with the silty sand embankment eroding most rapidly and the lean clay embankment with a mean moisture content of 18% dry basis at standard compaction eroding the slowest. These observations were indicative of the soil textures. Although the two lean clay embankments were constructed of similar soils, the difference in erosion rates speak to the importance of quality control (e.g., compaction moisture content) during construction. Soil properties including soil texture, erodibility, and compaction moisture content are key predictors of erosion rate and observed failure. Keywords: Breach, Dam failure, Dams, Embankments, Erodibility, Internal erosion, Levees, Overtopping.

Internal erosion involves the migration of soil particles under seepage flow and is a primary cause of dam failure and distress. With the loss of soil particles, the remaining soil grains will rearrange, resulting in soil deformations and significant settlement. In this research, a photographic method was developed to measure the soil deformations caused by internal erosion on a modified triaxial apparatus. Photos were taken by a high-resolution camera at a certain time interval. By comparing image data between any two snapshots, the process of soil deformation can be obtained. In the experiments, the soil specimen experienced marked deformations during internal erosion. Heterogeneous soil deformations also developed across the specimen because of the nonuniformity of the soil pores and the nonuniform migration of fine particles. Both the global and local soil deformations during the whole internal erosion process were obtained using the photographic method. The maximum lateral deformation occurred at the middle part of the soil specimen, whereas the soil deformations along the vertical direction were relatively uniform.

Investigation on the failure mechanism of the internal erosion in inverse grading sand based on DDA and Darcy’s law

Internal erosion is a major cause for failures and incidents in slopes, embankment dams, landslide dams and dikes. After the loss of some fine particles, the microstructure and mechanical behaviour of the soil change. In this study, a series of tests was conducted on a gap-graded soil using salt to replace part of soil particles to investigate soil deformations and shear strength changes caused by the loss of a predefined amount of fine particles. The dissolution of predefined amounts of salt in the soil specimen during saturation process successfully simulated different degrees of erosion. Drained triaxial compression tests were performed on the samples already subject to internal erosion to study the changes in the mechanical behaviour of the soil. After loss of a significant amount of fine particles, the void ratio became larger, the critical state line rose substantially, the soil behaviour became less dilative, and the shear strength decreased significantly.

Internal erosion can cause geotechnical structures to fail due to the loss of fine particles, which is induced by internal erosion that can weaken the mechanical properties of soil. Few studies have focused on the effect of internal erosion on the mechanical properties of soil. Accordingly, the present paper studies the evolution laws of the drained mechanical properties of soil due to internal erosion by conducting a series of triaxial consolidated drained shear tests and using the fine particle elimination method. The main conclusions of the study are as follows. Under the same confining pressure, the strain-softening phenomenon of the stress–strain curve will decrease with an increasing loss of fine particles, and a soil sample with a lower loss rate of fine particles will have a more dilative response. The loss of fine particles decreases the drained failure strength and drained internal friction angle. Mathematical relationships between the drained failure strength and the loss rate of fine particles and between the drained internal friction angle and the loss rate of fine particles have been established. A modified Duncan–Chang model that considers the loss of fine particles induced by internal erosion has been established.

Internal erosion (IE) often occurs in poor graded sand, one of the traditional treatments is reducing the permeability by grouting. Today, nano-silica becomes a choice of grouting materials as its low viscosity and good penetration capacity. According to present literature, the effect of decreasing loss mass during IE after the improvement of nano-silica was rarely studied. One of the important reasons is that, mass loss during IE was previously focused and was weighed after filtering the effluent by electronic balance, more accurate weighing method should be studied after adding nano-silica which cannot be precisely weighed by electronic balance. In this paper, a new grading method is conducted to monitoring the particle size distribution in the effluent and illustrate the process of IE. Erosion time and permeability are also recorded and analyzed as comparison. The experimental results show that the grading method can monitor precisely the mass loss and the composition of the effluent, grading range of 1-1000 can be adopted to monitor the coagulation of silica gel particles (1-50 , average diameter 11±5 ) and fine sand particles (50-100 , average diameter 65±7 ), grading range of 1-1000 can be adopted to monitor the smaller coagulation of silica gel particles (concentrated in the range of 1-250 nm). Through grading method, the IE of nano-silica improved sand can be divided into three stages: Removal and release of unbonded nano-silica particles and unbonded fine particles; Movement and discharge of bonded particles; Expansion of pores and instability of the whole sample skeleton.

Many field observations have led to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved and inconsistent use of terms have exasperated the problem. One such piping process that has experienced a lot of field observations but very limited mechanistic experimental work involves flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes: result in internal erosion, stabilize hillslopes by acting as drains, result in hillslope instability by causing pressure buildups, result in ephemeral gully formation or reformation of filled-in gullies. The objective of this paper was to review discrepancies in terminology to better explain the piping processes and highlight the experimental work done to date on the specific processes of soil pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies reforming due to mass failures caused by flow into discontinuous soil pipes, and the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion. In some of these studies the soil pipes were simulated with perforated tubes placed in the soil, while in other studies the soil pipes were formed out of the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes in order to calculate critical shear stress and erodibility properties of soils. Numerical models have been applied to describe flow through soil pipes but incorporation of internal erosion into such models has proven complicated due to enlargement of the pipe with time as well as temporary clogging of soil pipes. These studies and modeling approaches will be described and a discussion will ensue that considers the gaps in our understanding of pipe flow and internal erosion processes and our ability to model these processes.