Effects of Fines Content and Stress History on Surface Erosion of Cohesive Soils

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.

Effect of Directional Stress History on Anisotropy of Initial Stiffness of Cohesive Soils Measured by Bender Element Tests

Seepage-induced internal erosion is observed in both artificially engineered fill structures and natural soil deposits. Fine content is of great significance for the fabric of soil. This paper aimed to determine the critical fine contents of natural soil deposits beyond which the internal stability of the mix was distinctly altered and illustrate the internal erodibility from the viewpoint of fabric. To this end, a fixed-wall permeameter capable of accurately detecting the critical hydraulic gradient of erosion initiation and collecting the cumulative eroded soil mass at a constant inflow rate was developed. Silty clay particles and sandy gravel particles extracted from original soil were employed to reconstitute specimens with fine contents ranging from 0 to 100%. Porosity measurement, seepage testing, and direct shear testing were conducted on remolded samples. Companion control specimens were tested under different flow rates to verify the applicability of the experimental device. The results indicate that according to critical fine content, the fabric of the soil samples with different fine contents could be split into coarse-particle-supported structure (CPSS), fine-particle-supported structure (FPSS), and transitional coarse–fine-particle-supported structure (TCFP). The newly developed experimental device provides a feasible methodology to investigate the internal erodibility of natural soil deposits. Different fabrics correspond to disparate shear strengths and distinct erosion characteristics, including the critical hydraulic gradient of erosion initiation, cumulative eroded soil mass, and average hydraulic conductivity. For coarse-particle-supported structure specimens, the coarse particles predominantly govern the mechanical and hydromechanical properties. An increase in fine content within pores formed by coarse particles could increase shear strength and reduce susceptibility to internal erosion. The mechanical and hydromechanical properties of FPSS specimens were basically controlled by fine particles. Coarse particles suspended in a fine matrix could somewhat increase the soil’s shear strength and reduce internal erodibility. TCFP specimens were most vulnerable to internal erosion in terms of minimum critical hydraulic shear stress and maximum cumulative eroded soil mass. It is essential to expand the scope of research to cover the transitional coarse–fine-grain-supported structure instead of remaining limited to the coarse-grain-supported structure.

The effect of stress history on the critical shear stress of bedload transport in gravel-bed streams

Removal of soil around a bridge foundation due to scour results in a reduction of the lateral and vertical foundation capacity due to the loss of soil support. The common approach in modeling the scour phenomenon of removal of soil springs without modifying the parameters of the remaining soil fails to consider the change of stress state of the remaining soil and the formation of scour hole geometry around the pile foundation. In practice, both of these factors impact the mechanical properties of the remaining soil and the resulting expected structural response of the pile under loadings. This paper proposed a methodology to comprehensively evaluate the combined effects of stress history and scour hole dimensions on piles under scour conditions in uniform soil. It enabled the examination of the lateral and axial behaviors of a loaded pile subject to scour and is applicable for both cohesive and cohesionless soils. The methodology was validated with results from field tests for no-scour scenarios and verified with existing numerical models for scour scenarios. Quantification of the soil effects was investigated through lateral pile deflection and load-settlement curves for lateral and axial behaviors, respectively. Load-settlement curves demonstrated that including the effect of stress history results in increases of up to 34.1% and 61.1% in estimated pile settlement for sand and clay, respectively, leading to potential unconservative designs if soil effects are not properly included in the analysis.

Several studies have concluded that the effect of matric suction on the shear strength of unsaturated soil is very important. However, little attention is given to the stress history due to unsaturation and drainage condition in the arrangement of these experimental results. This paper attempts to show the relationship between shear properties and matric suction under a specified stress history and drainage condition. In this study, an unsaturated specimen is set up by dehydrating the normally consolidated soil with controlled matric suction to a specified stress history. Triaxial apparatus and hollow cylindrical torsional shear apparatus are employed to examine the effects of stress history and drainage condition, respectively. Through systematic experiments, the effects of stress history and drainage condition on the stress-strain relationship are clarified. The increase in shear strength by matric suction which is measured at the point of maximum shear stress is observed to be independent of stress history and drainage condition. However, in strain softening behavior it was found that the ultimate strength is independent of matric suction and traces a unique line which is parallel to the critical state line on the pnet-q plane.

The influence of fines content on ground collapse due to internal erosion of sand-fines mixtures around defective pipes

Internal erosion is one of the most important factors that cause earth structures that retain water, such as embankment dams, to collapse. Concentrated leak erosion, one of the forms of internal erosion, occurs in cracked fine-grained soils and pressurized flow conditions. To evaluate the concentrated leak erosion risk of cracks/voids, it is necessary to ascertain the erosion resistance of these materials. The erosion rate and critical shear stresses determine internal erosion resistance in concentrated leak erosion. This study determined soil’s concentrated leak erosion resistance using test equipment that allowed the flow to pass through a hole with stress-free (no loading), anisotropic-compression stress, anisotropic-expansion stress, and isotropic stress conditions. The stresses that developed in the samples’ hole wall where erosion occurred were determined with numerical modeling as pre-experimental stress conditions. The experiments were performed under a single hydraulic head on four selected cohesive soils with different erosion sensitivity. Time-dependent flow rates obtained from the test system can be used to determine hydraulic parameters, such as energy grade lines, with the help of basic theorems of pipe hydraulics in theoretical hydraulic models. Moreover, the erosion rates were quantitatively determined using the continuity equation, while critical shear stresses were qualitatively compared for concentrated leak erosion developed by the dispersion mechanism. As a result of the experiments, stress conditions influence the concentrated leak erosion resistance in the soil samples with dispersive erosion. Moreover, the shear strength in the Mohr–Coulomb hypothesis can explain the erosion resistance in these soils under stress conditions depending on the sand/clay ratio.

Critical assessment of jet erosion test methodologies for cohesive soil and sediment

Considerable research has been conducted on the erosion of cohesive soils, yet predictive models remain rudimentary at best and few design data are available. This research summarizes the current knowledge on cohesive soil erosion. Cohesive soil erosion is a complex phenomenon, determined not only by soil properties and flow hydraulics, but also by the chemical interaction between the soil pore water and the eroding fluid. While noncohesive soils erode as individual grains, cohesive soils erode as aggregates; thus, interped bonding is also important. The erosion resistance of cohesive soils is further affected by changes in the amount and physical state of soil pore water: significant increases soil erodibility have been correlated to freeze-thaw cycling. Considered a soil property, soil erodibility expresses the rate at which a soil will erode, once erosion starts. Typically, the erosion rate of cohesive soils is predicted using a model relating soil erodibility to a measure of the hydraulic forces on the soil. The most common expression is known as the excess shear stress equation, which states the erosion rate is proportional to the difference between the applied boundary shear stress and the soil critical shear stress. Originally used for noncohesive soils, the critical shear stress is defined as the hydraulic stress at which a soil will erode. For cohesive soils, critical shear stress is difficult to predict accurately; there is no precise definition of critical shear stress as there is rarely a defining the point at which erosion starts. Several researchers have developed empirical relationships between the critical shear stress of cohesive soils and soil properties, but the prediction of fluvial entrainment rates based on soil physical properties has had limited success. This lack of adequate methods to predict soil erodibility and critical shear stress for cohesive soils has led to the development of several field test methods using an impinging jet. Ongoing research is comparing these test methodologies.

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

An experimental study was carried out to investigate the effects of the mean net stress and suction history on the initial shear stiffness, G0, of a compacted clayey silt. Isotropic tests were performed using two suction-controlled devices, a triaxial cell and a resonant column torsional shear (RCTS) cell, so as to investigate the volumetric behaviour of this material. As for saturated soils, one can expect to find a strong correlation among stress history, volumetric state, and G0. Initial shear stiffness was measured almost continuously along various isotropic stress paths, including compressions and drying–wetting single stages or cycles, by using the RCTS cell. The collected data demonstrate a strong dependency of G0 on mean net stress (p – ua) and suction (ua – uw). Cycles of suction, in particular increasing suction beyond the past maximum value, induce significant accumulation of irreversible strains and increase of stiffness, confirming that G0 is not univocally related to the stress state (p – ua, ua – uw).Key words: unsaturated, compacted, small strain, stiffness, volumetric behaviour, stress history.

Dispersive soils cause great damage to hydrophilic earth structures and unprotected slope surfaces because of their high erosion sensitivity. In fine-grained soils, erosion resistance decreases with increased dispersibility. The dispersion mechanism of clays is controlled by the clay mineralogy and by the physicochemical repulsive forces in the clay–water system. The erosion resistance of dispersive soils can be determined by surface and internal erosion tests. In this study, the internal erosion resistance of soils was determined by using new test equipment that allowed the flow to pass through a hole when the dispersion mechanism in fine-grained soils was activated. The experiments for this study were performed under a single hydraulic head on two different natural dispersive soils with similar clay mineralogy. In this experimental system, both uniform and fully developed flow conditions were achieved. Time-dependent flow rates obtained from the experimental system can be used to determine hydraulic parameters, such as energy grade line, at very low error rates with the help of basic theorems of pipe hydraulics in theoretical hydraulic models, which were formed using a physical hydraulic model. Moreover, the erosion rates were quantitatively determined by using the continuity equation, and critical shear stresses were qualitatively compared for internal erosion developed by the dispersion mechanism. The sand/clay ratio determined the erosion resistance and behavior of the dispersive soils.

The aim of this paper is to study the impact of certain soil treatments on the internal erosion characteristics of treated compacted silt. The experiments measured the internal erosion using the hole erosion test (HET). This study aims to describe the effects of clay, lime and cement soil treatments on the internal erosion, specifically with regard to the amount of treatment used and the curing time. A new enhanced HET was developed to apply a high inlet pressure up to 650 kPa and thus generate a hydraulic shear stress up to 10,000 Pa. The internal erosion resistance was quantified by the coefficient of soil erosion and the critical shear stress. The results demonstrated that clay treatment could reduce the coefficient of soil erosion depending on the nature and percentage of clay added to the soil. The results also showed that lime and cement treatment primarily increased the critical shear stress of the tested silt. This increase was higher with cement treatment and was dependent on the amount o...

Microscopic mechanism of the combined effects of confining pressure and fines content on suffusion in gap-graded underfilled soils