Unstable Soils Research Articles

Synergistic effects of rock fragment cover and polyacrylamide application on erosion of saline-sodic soils

Many erodible and unstable bare soil slopes were exposed under rainfall during the tidal flat reclaimed to be farmland in the coastal provinces of China. There is an urgent need for erosion control methods that are not only efficient and effective long term but also economical and environmentally friendly for slopes with saline-sodic soils. A combination of rock fragment cover (RC) and polyacrylamide (PAM) might be a potential choice. We investigated the synergistic effect of RC and PAM amendment rates on infiltration, runoff, and erosion processes for slopes of 15° with saline-sodic soils that were subjected to two 60-min simulated rainstorms with an intensity of 60 mm/ h under laboratory conditions. Nine treatments with three RC (0%, 15%, and 30%) and three PAM applied rates (0, 0.4, and 1.0 g/kg) in the upper soil layer were all exposed to two sequential rains of deionized water, and these two rainfall events (E1 and E2) were 4 h apart. The results showed that the embedded rock fragment cover decreased the infiltration rate (IR) and increased the runoff rate, which increased sediment yield rate (Rs) in rainfall E1. In E2, RC had a significant (P < 0.05) negative impact on the sediments yield rate by blocking the runoff in the rill. The PAM amendment significantly (P < 0.05) increased the IRs for both rainfall erosion processes, but the IRs were reduced with the PAM application rates increasing from 0.4 to 1.0 g/kg. PAM significantly (P < 0.05) decreased the soil loss in E1 and E2. The positive effect of RC on mean Rs in E1 was restrained as the PAM application rates increasing to 1.0 g/kg. An application rate of 0.4 g/kg was recommended for improving infiltration and controlling erosion because the increased infiltrated water could dissolve soil salt and transport it to drainage ditch though the underground pipes. The optimized combination formula had a PAM application rate of 0.4 g/kg and RC of 15% after taking the two sequential rainfall events into account.

A multi-disciplinary macrofossil study of late glacial to early Holocene sediments from Søndre Kobberdam, Hareskovene, Denmark

During the early part of the Allerød period, from c. 13 600 to 13 330 years BP, unstable soils with a tundra-like open, treeless vegetation with Betula nana and Dryas octopetala were found around Søndre Kobberdam in Hareskovene. Open Betula pubescens woodland was not established until the middle Allerød about 13 330 years BP. During the Younger Dryas, Betula nana and Dryas octopetala spread again, and Betula pubescens almost disappeared. From the onset of the Holocene warming an open tundra landscape characterised the area. About 11 300 years BP Betula pubescens started to recolonise the region and Populus tremula and Pinus sylvetris arrived at c. 11 000 years BP, replacing the open landscape by woodland. Along the margin of the lake Carex paniculata, Carex riparia and Cladium mariscus were growing. The lake fauna included a rich and diverse fauna of molluscs that thrived in the carbonate-rich waters. We did not find any evidence for the local presence of Pinus sylvestris during the late glacial.

A semi-empirical dilatancy model for ballast fouled with plastic fines

ABSTRACTIn the era of high speed trains, it is very important to ensure the stability of rail tracks under adverse conditions including the fouling of ballast. Fouling of ballast from unstable and saturated soft subgrade soil is one of the major reasons for track deterioration. The reported results of a number of large-scale laboratory experiments on the shear behaviour of ballast and fouled ballast are analysed, herein. It was observed that fines have a significant influence on the shear behaviour of ballast. Shear strength increases and dilatancy decreases with the addition of fines. In this paper, a semi-empirical mathematical model has been proposed to capture the dilatancy of ballast fouled with fines during shearing. The empirical constants a, b and c proposed in the model are a function of the fines content Void Contamination Index (VCI). The results of the model have been compared with the laboratory experiments and are found to be in good agreement.

Combined Geometric Hydraulic Criteria Approach for Piping and Internal Erosion in Cohesionless Soils

Abstract Piping is an erosion mechanism that plays a significant role in river barrages, such as dikes and dams, that are founded on poorly graded cohesionless soils like, for instance, fine or medium sands. Likewise, in gap-graded or widely graded cohesionless soil, finer grains can pass through the pore matrix of coarse soil because of seepage. This phenomenon, which is called internal instability or suffusion, can occur in granular filter in or under dams, dikes, barrages, or any other water-retaining structure. For economic reasons, unstable soils are generally used in practice if the hydraulic force or the hydrodynamic energy is smaller than the critical hydraulic force or the critical hydrodynamic energy. Therefore, an approach of combined geometric-hydraulic criterion is expected to yield better results. Based on this premise, laboratory tests of piping and internal erosion are carried out using five cohesionless soils. An analysis of the results considering the instability index shows that the critical hydraulic gradient required to initiate the piping and internal erosion depends on the curve of the grain size distribution, the initial relative density of the soil, the seepage direction, and the initial stress condition in the soil. Based on these laboratory results and the results reported by several other authors, a combined geometric-hydraulic criterion with respect to piping and internal erosion in cohesionless soils is developed and proposed.

Soil aggregate stability under different rain conditions for three vegetation types on the Loess Plateau (China)

Soil aggregate stability is essential for moderating the soil quality and preventing soil erosion. Vegetation restoration may effectively increase the stability of soil aggregates via soil organic matter. This study was designed to investigate the effects of vegetation types with long-term revegetation on the soil aggregate characteristics. Three vegetation type zones (grass land, forest-grass land and forest land) were selected in the Yanhe Watershed (northwest China) as the subjects. Soil aggregate stability was determined by the method of Le Bissonnais, including three disruptive tests: fast wetting (FW), slow wetting (SW) and mechanical breakdown (WS). The results showed that the mean weighted diameter (MWD) significantly differed from the tests and vegetation types. In the 0–10 cm soil layer, MWD ranged from 2.65 to 3.26 mm for the SW test, which corresponded to very stable soil aggregate; they ranged from 0.53 to 1.08 mm for the WS test, and from 0.57 to 1.96 mm for the FW test, both of which corresponded to very unstable soil aggregates. In the 10–20 cm soil layer, MWD ranged from 2.75 to 3.33 mm for the SW test, 0.39 to 0.83 mm for the WS test, and 0.44 to 1.37 mm for the FW test. The MWDs under the three tests were the lowest for the grass land at both soil layers, and the MWDs for the WS and FW tests were significantly lower than the MWD for the SW test. In all three tests, MWDs showed the same order: forest land > forest-grass land > grass land. MWD indicated that forest land had much stronger ability to resist soil erosion no matter the rain conditions. The correlations between soil organic matter content and MWD for the FW and WS tests were significant (P < 0.05). These results demonstrated that vegetation types had significant effects on the soil aggregates under the different rain conditions, and the soil organic matter and clay contents were significantly related to the soil aggregate stability. These results will guide the practice of reducing soil erosion for the different conditions and different vegetation types.

Using soil organic matter fractions as indicators of soil physical quality

AbstractThe objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM), rather than SOM per se, as indicators of soil physical quality (SPQ) based on their effect on aggregate stability (AS). Chemically extracted humic and fulvic acids (HA and FA) were used as chemical fractions, and heavy and light fractions (HF and LF) obtained by density separation as physical fractions. The analyses were conducted on medium‐textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se. In both geographical regions, soils under cropland showed the smallest content of SOC, HA and carbon concentration in LF and HF, and the largest HF/LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA/SOC (r = −0.83**), HA/FA (r = 0.58**), carbon concentration of LF (r = 0.69**) and HF (r = 0.70**) and HF/LF ratio (r = 0.80**), cropland showed lowest AS. These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.

The Erosion of expansive soils through a zoning of Taza (Morocco)

Like other cities in the region of Fez-Meknes, Taza undergoes the phenomena of erosion of soils facing the builder in general and the town planner in particular. This type of aggression of the land is sometimes amplified by the swelling-withdrawal of soils. In effect the marl clay layers are considered as unstable soils because of their volumetric changes depending on the duration and intensity of extreme climate changes (weather, droughts, ventilation.). This phenomenon, which is part directly or indirectly so-called natural disasters, can lead to significant costly damage and particularly intense (soil erosion, landslides, obstruction of the course of the water, hydraulic works and sanitation networks, cracks of the constructions and their annexes’, deformations of pavements, etc.). The knowledge and mastery of this phenomenon are essential to prevent, limit or remedy the consequences and damage related to their impacts. We focus as well our contribution on the geotechnical characteristics of the soils of the city of Taza with highlighting of the hazards in susceptibility to erosion of soils in particular through their potential of swelling-removing and this for the benefit geotechnical zoning of Taza.

A performance-based method for the design of drainage trenches used to stabilize slopes

Drainage trenches are widely used as a measure for slope stabilization. Their effect is generally quantified in terms of a safety factor against a possible collapse mechanism of the slope. A more rational and economical design approach consists in admitting the occurrence in some time intervals of limit conditions in the slope, and evaluating the performance of the slope in terms of accumulated permanent displacements of the unstable soil mass. The resulting displacements should avoid the occurrence of significant damage and preserve the serviceability of existing structures. In this context, a new method is proposed in the present paper for the design of drainage trenches used to control the mobility of translational landslides periodically activated by rainfall. This method is based on some analytical equations that relate rain to the groundwater regime as modified by the trenches, and a simple sliding block model to evaluate the landslide displacements. The proposed method also accounts for the delay of the hydraulic response at different depths. The above-mentioned equations are validated by several comparisons with numerical results obtained using a finite element code or published in the literature. Finally, the method is applied to some case studies to show the effectiveness of the drainage trenches to control the landslide mobility owing to rainfall.

Establishing and validating a root water uptake model under the effects of superabsorbent polymers

AbstractIn‐depth understanding of the patterns of root water uptake (RWU) is the key to improving the water‐use efficiency of plants. Superabsorbent polymers (SAPs) can repeatedly absorb and then release water to plants when needed, which substantially increases the water‐use efficiency of plants in arid and semiarid farming. However, it also creates a challenge in modeling RWU due to unstable soil hydraulic parameters and the unclear response of root growth to SAP application. A controlled soil column experiment was conducted to investigate the response of two important root indexes, that is, root length density (RLD) and root nitrogen mass density (RND), to SAP application, as well as their relationships with RWU. The results showed that the use of the SAP caused a steady increase in RND, and it also substantially increased the RLD in the early stage, but the rate of increase gradually slowed over time. The effect of unstable soil hydraulic parameters on RWU was analyzed in a numerical experiment, which indicated that the time‐dependent effect of SAP on soil hydraulic parameters should be considered to avoid errors in calculating RWU. A specific method for calculating the RWU source/sink term in the Richards equation was proposed and ultimately used as the theoretical value of RWU to verify the RWU models based on RLD and RND to ensure the best one was selected. The results showed that RND was better able to be used for development of the RWU model to simulate the patterns of RWU under SAP application.

Stabilisation of Dispersive Soil Using Fly Ash, Bottom Ash and Hydrated Lime

Dispersive soils are structurally unstable soil and it is highly flexible to erosion and piping. Using this type of soil for infrastructure programs like hydraulically structures, earth dams, road embankments and railways can lead to problems. Therefore, these dispersive soils need to be stabilized. Therefore, the research attempts to seek the feasibility of stabilization of dispersive soil using fly-ash, bottom-ash and hydrated lime. Fly ash and bottom ash are two of the waste materials generated from the thermal power plant located at Norrochcholai. At present, storage and disposal of these two by-products are a critical issue. In this research identifying dispersive soils using pinhole test and crumb test from the location of Kudaoya reservoir has been carried out. Each test was conducted using three replicates. The engineering properties of dispersive soil have been determined using Atterberg limit test, standard proctor compaction test and permeability test. The optimum mixture of fly ash, bottom ash and hydrated lime to remove the depressiveness of the soil was carried out using the pinhole test. Thereafter, the engineering properties (dry density, permeability) were also assessed for optimum stabilized mix proportions of stabilizing agents. The dispersive soils in Kuda Oya area was found to be moderately dispersive as per the pinhole test results. Two optimum proportion mixtures were identified as 2% of hydrated lime+98% of dispersive soil and 2% of bottom ash+98 % of dispersive soil to remove the dispersiveness effectively based on the results of pinhole test and cost analysis. When the dry densities of these two mixtures were compared it was found that 2% of hydrated lime+98% of dispersive soil were higher than that of 2% of bottom ash+98% of dispersive soil. Therefore, 2% of hydrated lime+98% of dispersive soil was selected as the optimum solution. The maximum dry density of this sample (2% of hydrated lime+ 98% of dispersive soil) was found to be 1.6 mg/m3 which was higher than the dry density (1.54 mg/m3) of virgin dispersive soil sample. The plasticity index of the stabilized dispersive soil mixture (31%) was less than the virgin soil (35%). However, the coefficient of permeability of the optimum selected mixture 2.56× 10-8 cm/s was less than that that of the virgin soil (1.41×10-7 cm/s). Therefore, based on these results, the selected optimum proportion (2% of hydrated lime with 98% of soil) can be used for stabilisation of dispersive soils.Keywords: Dispersive soils, Erosion and piping, Stabilising agents, Engineering properties

The decline of soil due to the pile of highway project Medan-Kualanamu (STA 35 + 901) with the finite element method

Consolidation is the process of discharge of water from the ground through the pore cavity. Consolidation occurs in soft soil or unstable soil that allows an improvement in order to make the soil more stable. The method of using Prefabricated Vertical Drain (PVD) is one way to improve unstable soils. PVD works like a sand column that can drain water vertically. This study aims to determine the decrease, pore water pressure and soil consolidation rate with Prefabricated Vertical Drain (PVD) and without PVD analytically and using finite element method that affect the duration of soil decline to reach 90% consolidation or in other words soil does not decline anymore. Based on the analytical calculation, the decrease obtained is equal to 0.47 m meanwhile the result of calculation using finite element method is 0.45 m. The consolidation rate obtained from analytical calculation is 19 days with PVD and 115 days without PVD. The consolidation rate obtained from finite element method is 63 days with PVD and 110 days without PVD. And the pore water pressure is 0.92 KN/m2.

TECHNICS AND STAGES OF WORK ORGANIZATION IN THE TRENCHLESS LAYING OF PIPELINES USING MICROTUNNELLING TECHNOLOGIES

The authors describe technics and stages of work organization in the construction of pipeline crossings through natural and artificial obstacles and construction of underground engineering communications of urban purpose using microtunnelling technologies. The essence of this technology is the penetration in the soil carried out by a tunnelling machine (microscope), the translational motion of which is provided by a powerful jack station, installed in the pit at a depth that corresponds to the depth of laying of the pipeline. The depth of laying of the pipeline should be in stable soils not less than two diameters, and in unstable soils – not less than three diameters from the surface of the earth to the upper creature pipeline. With the help of hydraulic jacks, the microchip is driven into the ground by a length corresponding to the length of the pipe sections used, after which the next pipe is installed on the jack station and the process is repeated. The types of microtunnel tunnelling complexes depending on the system of soil removal and bottomhole load are presented. The technological process of preparation and regeneration of the bentonite solution injected over the shell of the tunnelling machine and the forcing pipe sections to ensure the stability of the bottomhole and reduce the frictional forces between the pipeline and the soil and supplied into the bottomhole area for transportation of the developed soil from the bottomhole is considered. The technique of calculating the required punching force of pipe sections on a rectilinear section to be created by a jack station is proposed. The schemes of arrangement and principles of operation of the three modules (electronic laser system ELS, electronic laser system with hydrostatic water levelling ELS-HWL and gyro navigation system with a pointer to the north GNS-P) of the developed by Herrenknecht AG company for standardization of precise navigation in microtunnelling regardless of the length of the tunnel, its diameter and direction universal navigation system UNS are considered. The main advantages of microtunnelling are precisely in the direction of the plan and the profile (deviation from the design axis within 10-30 mm), fast (average rolling speed is 10-15 m / day) and with the minimum amount of subsidence of the daily surface (does not exceed 10 mm) construction of pipelines in combination with the possibility of laying both a rectilinear and complex curvilinear trajectory, at large depths (up to 100 m) and at considerable distances (up to 1000 m), in the whole range of engineering-technological and hydrogeological conditions.

Improved bearing resistance of soil foundations of buildings with injectable polyurethane composites

The article deals with the analysis of SPT®polyurethane materials and maintenance technologies based on them, and an analytical review of literature on the topic. Solidification with SPT®materials and technologies makes it possible to compact unstable foundation soils of structures and buildings. The solidification is implemened through a guided increase of polymeric material injected into a foundation. As a result, polymeric root-like reinforcing bodies are formed in the foundation, and the water is extruded out of the soil, thus considerably increasing the bearing capacity of the foundation. In order to assess the actual efficiency of the material the authors conducted the full-scale research into the general overhaul technology of the pipe culvert, the static and the dynamic tests on the models of solidified soil was conducted and compared with the results of the tests on the models of non-solidified soil. The authors investigated into the life ratio of SPT®material and the soil solidified with it. By comparing characteristics of SPT®materials and technologies with those of conventional materials, the basic analogies and differences were established. And the possibility to use them for construction, refurbishment and maintenance of rail facilities was determined.

Progressive Internal Erosion in a Gap-Graded Internally Unstable Soil: Mechanical and Geometrical Effects

This paper investigates the posterosion geomechanical behavior of internally unstable granular material due to removal of fines caused by erosive forces of water flow. Posterosion undrained behavior of a gap-graded internally unstable soil was investigated for a range of erosion durations and inflow velocities using a triaxial-erosion apparatus. Test results indicated that the undrained behavior of the original specimen changed from a strain hardening behavior to a flow-type behavior with limited deformation after internal erosion. The initial peak strength improved and the flow potential decreased during the initial stage of erosion. This observed increase in initial peak strength is believed to be the result of a better interlocking between the coarse particles posterosion. In contrast, the slip-down movement of the particles due to an increase in the posterosion void ratio postponed the dilation tendency. Test results also suggested that even erosion of a small percentage of fine particles improved the mechanical frictional behavior of the soil. However, there was a threshold value for the loss of fine particles at which this positive effect deteriorated. This might have been due to formation of local metastable structures and/or overcoming contractive behavior after loss of the semiactive fines and a considerable increase in the global void ratio. Shear strength results, rate of erosion, and local vertical strains together suggest that the intergranular void ratio is a powerful index in evaluating the posterosion mechanical behavior of internally unstable soils.

Geophysical methods for monitoring soil stabilization processes

Soil stabilization involves methods used to turn unconsolidated and unstable soil into a stiffer, consolidated medium that could support engineered structures, alter permeability, change subsurface flow, or immobilize contamination through mineral precipitation. Among the variety of available methods carbonate precipitation is a very promising one, especially when it is being induced through common soil borne microbes (MICP – microbial induced carbonate precipitation). Such microbial mediated precipitation has the added benefit of not harming the environment as other methods can be environmentally detrimental. Carbonate precipitation, typically in the form of calcite, is a naturally occurring process that can be manipulated to deliver the expected soil strengthening results or permeability changes. This study investigates the ability of spectral induced polarization and shear-wave velocity for monitoring calcite driven soil strengthening processes. The results support the use of these geophysical methods as soil strengthening characterization and long term monitoring tools, which is a requirement for viable soil stabilization projects. Both tested methods are sensitive to calcite precipitation, with SIP offering additional information related to long term stability of precipitated carbonate. Carbonate precipitation has been confirmed with direct methods, such as direct sampling and scanning electron microscopy (SEM). This study advances our understanding of soil strengthening processes and permeability alterations, and is a crucial step for the use of geophysical methods as monitoring tools in microbial induced soil alterations through carbonate precipitation.

Visualisation of seepage-induced suffusion and suffosion within internally erodible granular media

A rigid-walled ‘transparent soil’ permeameter has been developed to study visually the mechanisms occurring during seepage-induced internal erosion in susceptible granular media under upward flow. The experiments use borosilicate glass particles in place of soil, and an optically matched oil mixed with fluorescent dye in place of water. The technique known as plane laser-induced fluorescence (PLIF) enables a two-dimensional plane of particles and fluid to be viewed inside the permeameter, away from the walls. Results of tests have provided close agreement with those of other researchers on soil of comparable particle size grading. Unstable materials showed migration of fine grains under mean hydraulic gradients as low as i = 0·25, whereas stable materials eventually failed by heave at hydraulic gradients close to unity. Internally unstable soils where the loads were predominantly supported by the coarser fraction exhibited suffusion (fines migration without disruption of the load-bearing system); those supported by both coarse and fine particles exhibited suffosion (i.e. volume change during fines migration). Quantitative image analysis conducted on one unstable sample showed areas of open void space migrating through the sample at low hydraulic gradients near critical, as defined by Skempton and Brogan in 1994. This occurred before the externally measured local hydraulic gradients began to diverge significantly from the mean. The testing technique developed shows that optically matched glass and oil behave mechanically similarly to soil and water, and that the PLIF technique coupled with image analysis can provide an additional insight into the mechanisms of internal erosion.

Riesgo geológico que afectará al poblado Tapacocha (Recuay ‐ Ancash)

Las construcciones civiles edificadas sobre suelos inestables son vulnerables a los procesos naturales como la geodinámica interna, externa y el cambio climático. El poblado de Tapacocha que está asentado sobre suelo deslizado es sensible a los procesos naturales. El terremoto de mayo de 1970 ocasionó fracturamiento de la planicie, destrucción de sus construcciones; las intensas lluvias de los años 1942 y 1997 ocasionaron huaycos que destruyeron algunas de sus edificaciones y terrenos de cultivo. La geodinámica interna modeló el basamento con un intenso plegamiento y fracturamiento de las rocas sedimentarias; la geodinámica externa por geoformas modeladas por los movimientos rotacionales, deslizamientos, erosión diferencial. En la actualidad el efecto del cambio climático se manifiestan por las inusuales torrenciales lluvias que erosionan periódicamente y ponen en peligro las edificaciones civiles y la agricultura. Con el presente estudio se propone alertar a las autoridades y sus pobladores para prevenir y mitigar los efectos naturales destructivos.

Flume-scale experiments on suffusion at bottom of cutoff wall in sandy gravel alluvium

This paper presents a series of flume-scale experiments to investigate suffusion at the bottom of a cutoff wall in an internally unstable sandy gravel alluvium. The initiation, progression, and potential failure of suffusion and the interactive effects of geomechanical and hydraulic conditions with the evolution of suffusion were investigated in this particular application. Temporal and spatial development of pore pressure, earth pressure, and settlement demonstrated suffusion was a multi-phase (involving pore water, fine and coarse fractions) and multi-field (involving seepage, seepage-induced fine-fraction variation, and stress–deformation) coupling phenomenon. Suffusion initiated at the downstream side of the tip of the cutoff wall and then generally progressed backward to the upstream side. The monitored earth pressure provided an evidence of the heterogeneous stress distribution in internally unstable soil. Two linear empirical formulas for average hydraulic gradients at the initiation of suffusion and at blowout were derived based on the flume-scale model experiments.

Quick assessment on susceptibility to suffusion of continuously graded soils by curvature of particle size distribution

As a major type of soil internal erosion, suffusion is causative for many geotechnical problems of earth structures and foundations. Although full laboratory tests are laborious, it is inevitable for susceptible soils. However, a good quick assessment can define those susceptible soils by eliminating quickly very stable and unstable soils from consideration. Traditional assessment methods often correlate and combine some predefined fixed particle sizes representing coarse and fine fractions of soils. This paper presents a novel, simple assessment method for identifying susceptibility to suffusion based on the general curvature of particle size distribution of soils. The further down a particle size distribution departs from its mean line, the more likely it behaves suffusive. The new assessment method has been tested against experimental data, and the results show some good correlation.

A Theoretical Model to Predict the Critical Hydraulic Gradient for Soil Particle Movement under Two-Dimensional Seepage Flow

The soil particle movement under seepage flow is one of the predominant mechanisms responsible for incidents and failures of dams and streambanks. However, little attention has been paid to the critical hydraulic gradient under two-dimensional (2-D) seepage flow. In this study, a theoretical model was established under 2-D seepage flow to predict the critical hydraulic gradients for soil particle movement. In this model, the sediment particle rolling theory was used, while taking into account the relative exposure degree of the soil grains and the seepage direction. The model was validated through qualitative analysis and comparison with previous data, and showed considerable superiority over Terzaghi's model. In addition, the effect of the soil internal instability, implying that the critical hydraulic gradient of unstable soil is lower than that of stable soil, was discussed. Various parameters of the model were also analyzed. The results showed that the seepage direction angle was positively related to the critical gradient, whereas the void and the mean diameter of the soil were negatively related to it. Finally, the model proposes a calculation method for the particle movement initiation probability, which is regarded as a key parameter in the sediment transport model.