A Stress-controlled Erosion Apparatus for Studying Internal Erosion in Soils

Transcription

1 Geotechnical Testing Journal, Vol. 34, No. 6 Paper ID GTJ Available online: at D. S. Chang 1 and L. M. Zhang 2 A Stress-controlled Erosion Apparatus for Studying Internal Erosion in Soils ABSTRACT Suffusion in soil involves selective erosion of fine particles within the matrix of coarse soil particles under seepage flow. Such loss of fine particles affects the hydraulic and mechanical behavior of the soil. In this study, a stress-controlled erosion apparatus was developed to investigate the initiation and development of suffusion under complex stress states and to study the effect of suffusion on soil stress-strain behavior. The apparatus allows independent control of hydraulic gradient and stress state. The hydraulic gradient is controlled using a water-head control method. The eroded soil and the outflow rate are measured using a soil collection system and a water collection system, respectively. The measurements can be used to study the erosion rate and variations in soil permeability during the erosion process. A series of erosion tests was conducted on a gap-graded soil under the same confining stress but different deviatoric stresses. The results show that the maximum erosion rate, the variations in soil permeability, and the total deformation of the soil specimen increase with the increase of deviatoric stress. After the loss of a significant amount of fine particles in the soil, the stress-strain behavior of the test soil changes from dilative behavior to contractive behavior. KEYWORDS: internal erosion, stress state, seepage, permeability, shear strength, hydraulic gradient Introduction Internal erosion can be initiated by concentrated leak erosion, backward erosion, soil contact erosion, or suffusion (Fell and Fry 2007). This paper focuses on suffusion. Suffusion involves selective erosion of fine particles within the matrix of coarse soil particles under seepage flow. For a soil susceptible to suffusion, once the fine particles are removed, the microstructure of the soil will change accordingly. This could induce a reduction of the soil shear strength and a mutation of hydraulic conditions in the soil (Schuler 1995). Some embankment dam failures and distresses are associated with suffusion (e.g., Fell et al. 2003; Zhang and Chen 2006; Xu and Zhang 2009; Zhang et al. 2009). Previous laboratory tests were conducted primarily to quantify the potential of soil suffusion under self-weight conditions (e.g., Kenney and Lau 1985; Honjo et al. 1996; Wan and Fell 2008). The conventional rigid-walled permeameter was developed to perform tests under constant downward hydraulic conditions. The occurrence of suffusion was identified from changes in the sizes of the grains in different layers of the soil specimen after tests. Fannin and Moffat (2006) modified the conventional permeameter by introducing local hydraulic gradient measurement and the average hydraulic gradient was applied in multiple-steps in their tests. To find the critical hydraulic gradient that triggers suffusion of cohesionless soils under self-weight conditions, Skempton and Manuscript received March 30, 2011; accepted for publication June 11, 2011; published online July Ph.D. Candidate, Depart. of Civil and Environmental Engineering, The Hong Kong Univ. of Science and Technology, Clear Water Bay, Hong Kong, cechangd@ust.hk 2 Professor, Depart. of Civil and Environmental Engineering, The Hong Kong Univ. of Science and Technology, Clear Water Bay, Hong Kong, cezhangl@ust.hk Brogan (1994) developed an upward-flow permeameter with local water head measurement. The critical hydraulic gradient was defined when there was a sudden increase in outflow rate. The soils within embankment dams or levees are usually under complex stress states, especially in embankment dams with core walls (Maranha das Neves 1991). The influence of stress state on soil erosion has been highlighted recently (e.g., Reddi et al. 2000; Tomlinson and Vaid 2000; Moffat and Fannin 2011). Moffat and Fannin (2006) developed a large rigid-walled permeameter to investigate the onset of internal instability of cohesionless soils under the K 0 stress state. Bendahmane et al. (2008), byperforming internal erosion tests under isotropic stress conditions, stated that the maximum erosion rate in sandy clay doubled when the confining stress decreased from 150 to 100 kpa. Richards and Reddy (2008, 2010) developed a true triaxial system to investigate the piping potential of both cohesive and cohesionless soils. The piping potential was found to be influenced more by the major principal stress than by the minor principal stress. Shwiyhat and Xiao (2010) studied changes in soil permeability and soil volume during the internal erosion process under a constant hydraulic gradient using a triaxial apparatus and concluded that suffusion could cause soil settlement and a reduction of soil permeability. Most of the previous studies were set to quantify the potential of internal stability. As far as the authors are aware, few attempts have been made to investigate both the initiation and development of internal erosion subjected to multi-stage seepage flow and complex stress states. The performance of embankment dams could be affected by the loss of fine particles. The investigation on soil stress-strain behavior after loss of certain fine particles is so far mainly from the numerical standpoint. Scholtes et al. (2010) and Wood et al. (2010), by conducting numerical analysis, pointed out that the soil shear strength may decrease once some fine particles are removed. Yet no laboratory experimental investigation on the effect of soil Copyright VC Copyright by 2011 ASTM by ASTM Int'l (all International, rights reserved); 100 Thu Barr Jan Harbor 12 06:09:24 Drive, PO EST Box 2012 C700, West Conshohocken, PA

2 2 GEOTECHNICAL TESTING JOURNAL FIG. 1 Schematic of the testing apparatus. erosion on soil stress-strain behavior has been reported in the literature. The main objective of this research is to develop a stress-controlled erosion testing apparatus, which can be used to systematically study the initiation and development of internal erosion subjected to multi-stage seepage flow and complex stress states, and the stress-strain behavior of soil subjected to internal erosion. First, the main components of the testing apparatus are introduced. Then the step-by-step testing procedures are described. Finally, results of a series of tests on a gap-graded soil under different stress states are presented. Testing Apparatus The general layout of the testing apparatus is shown in Fig. 1. It is composed of a triaxial system, a pressurized water supply system, a soil collection system, and a water collection system. The detailed description of each component is presented in the following four sections. Triaxial System A computer-controlled triaxial testing apparatus was modified to allow the independent control of hydraulic gradient and stress state for investigating the initiation and development of soil internal erosion, and the stress-strain behavior of soil subjected to internal erosion. The precisions of the vertical load and effective stress are 5 N and 0.5 kpa, respectively. Figure 2(a) is a photograph of the triaxial apparatus. A hollow base pedestal is designed for the eroded soil and seepage water to pass through and a perforated plate (plate II) with a thickness of 10 mm (diameter 95 mm) is attached to support the specimen (Figs. 1 and 3). The base pedestal is detachable to allow easy assembly of the specimen. A rigid base mesh or a geotextile disk is then placed on the perforated plate (Fig. 1). Its opening size ranges from to 5 mm in accordance with the particle sizes of the specimen to be tested. The selection of the base mesh follows Fannin and Moffat (2006). Namely, the mesh size is determined by the coarse fraction of a gap-graded soil. The mesh size adopted in this research is 1.18 mm. The mesh holds the coarse fraction of the soil and allows the fine fraction to erode by seepage flow. The eroded soil passes through the pedestal and is collected in the soil collection system. The conventional permeameter with rigid walls may involve leakage through the interface between the wall and the specimen (Kenney and Lau 1985); hence a flexible membrane is adopted in the current research to minimize probable interface leakage. Since the permeability of the test soil is generally high (i.e., the maximum coefficient of permeability is on the order of 10 4 m=s), the commonly used porous stone in the conventional triaxial test cannot be adopted otherwise a significant head loss will be induced. Therefore, another perforated plate (plate I) with a hole diameter of 1 mm is put between the top cap and the soil specimen to diffuse the seepage flow on the specimen uniformly and lessen the head loss as shown in Fig. 1. Moreover, a layer of steel mesh with opening size of mm is fixed at the bottom of the perforated plate (plate I in Fig. 1) to prevent clogging of the plate by the coarse particles. To further ensure that the water inflow is sufficiently uniform, a piece of highly permeable filter is set between the top cap and the perforated plate. The plate and the top cap are made of light PVC sheet and light transparent acrylic, respectively, to reduce the self-weight effect on the final stress of the specimen, especially when the specimen is under low stresses. Furthermore, the transparent top cap can be used to identify whether the water level has risen to the top face of the specimen or not during the saturation step.

3 CHANG AND ZHANG ON A STRESS-CONTROLLED EROSION APPARATUS 3 FIG. 2 Components of the testing apparatus: (a) triaxial system; (b) pressurized water supply system; (c) soil collection system; and (d) water collection system. The final stress state can be approached by setting a stress path through the control system automatically. During the erosion process, the soil structure changes and the soil specimen deforms simultaneously. A linear variable differential transformer (LVDT) with a precision of 0.02 mm is employed to measure the total vertical displacement of the specimen. The lateral deformation during the erosion process can be measured by using a photographic method (White 2002). A digital camera with a resolution of FIG. 3 Details of the hollow base pedestal and two base plates.

4 4 GEOTECHNICAL TESTING JOURNAL pixels was mounted 1500 mm in front of the triaxial system. The viewing area of the camera was mm 2. The average image scale was around 80 lm=pixel. Pressurized Water Supply System A supply of de-aired water is provided as inflow into the soil specimen. To accommodate high permeability soils, three transparent acrylic water tanks 200 mm in diameter, 400 mm in height are used to provide sufficient water as shown in Fig. 2(b). The hydraulic head, at the tip of the copper tube, is air-pressure controlled. A pressure gauge and a pressure regulator are connected to a pressurized air supply; hence the pressurized air can be regulated first. The precision of the pressure gauge is 0.5 kpa; thus the hydraulic gradient can be changed at a precision of 0.05 for a soil specimen 100 mm in height. During the test, the inlet water head can be either increased gradually or kept constant. Soil Collection System The soil collection system includes three main components: a transparent subsidence funnel, a three-way connector, and a detachable container as shown in Figs. 1 and 2(c). One purpose of the transparent subsidence funnel is to help identify the initiation of internal erosion; another is to avoid soil clogging in the base pedestal. Moreover, the eroded soil can quickly settle down in the detachable container by using the subsidence funnel. For cohesionless soils, a T-fitting is used to separate the outflow water and eroded soil by putting a steel mesh screen at the inlet of the horizontal drainage hose as shown in Fig. 2(c). The size of the mesh screen is in accordance with the minimum particle size of the test soil. For cohesive soil, a turbidimeter can be connected to the outflow hose to measure the rate of erosion of the fine particles. In this research, only cohesionless soils were tested, and the mesh size was mm. The container is detachable using quick fitting to collect the eroded soil at certain time interval. To prevent the soil specimen from desaturating during the removal of the container, the container and the quick fitting are both submerged during the entire testing process as shown in Figs. 1 and 2(c). Water Collection System Figure 2(d) shows a photograph of the water collection system. It consists of a downstream reservoir, an acrylic cylindrical container, and an electronic balance. To maintain a constant water head at the bottom of the soil specimen, a downstream cylindrical acrylic reservoir 80 mm in diameter, 500 mm in height is used as shown in Figs. 1 and 2(d). Before testing, the reservoir is fully filled with de-aired water. A constant water head can be achieved by allowing spilling of the outflow into the container. The elevation of the water surface is slightly higher than that of the bottom of the soil specimen to prevent desaturating of the specimen. To measure the variations in soil permeability during the erosion process, the outflow is collected by the container and weighted using the electronic balance at certain time interval (i.e., 2 min in this research). The permeability of the soil specimen can be obtained based on the measured discharge flow rates and the controlled hydraulic gradients during the whole testing process. Soil Specimen Preparation Soil Type FIG. 4 Grain size distribution of the test soil. The soil for this study was obtained by mixing two soils: commercial washed Leighton Buzzard sand (fraction E), and completely decomposed granite (CDG). The CDG was extracted from a construction site on Beacon Hill, Hong Kong. The grain-size distribution (GSD) of the test soil is shown in Fig. 4. The soil is described as SP according to the Unified Soil Classification System (ASTM 2000). First, the CDG was separated into 8 portions by wet sieving according to its particle sizes: mm, mm, mm, mm, mm, mm, mm, and mm. Then the test soil was mixed by weighting the oven-dried particles of each portion following the GSD in Fig. 4. Finally, the Leighton Buzzard sand was weighted and carefully mixed with the well-mixed CDG. The physical properties of the test soil are summarized in Table 1. Specimen Preparation Each triaxial specimen, 100 mm in diameter and 100 mm in high, was prepared by moist tamping to prevent soil segregation. Physical Property TABLE 1 Physical properties of the test soil. Value Standard Compaction Tests Maximum Dry Density, q dmax (kg=m 3 ) 1895 Optimum Water Content, w opt (%) 6.5 Grain Size Distribution Percentage of Gravel (%) 2 Percentage of Sand (%) 98 Mean Particle Size, d 50 (mm) 1.62 Coefficient of Uniformity, C u 16.7 (H=F) min 0.75 (d 15c =d 85f ) gap 10.2 Gap Ratio, G r 7.9 Specific Gravity, G s 2.61

5 CHANG AND ZHANG ON A STRESS-CONTROLLED EROSION APPARATUS 5 Initially the soil was oven-dried at 105 C for 24 h and then cooled inside a desiccator. De-aired water was added and mixed thoroughly with the dried soil to the optimum water content of 6.5 %. Thereafter, the well-mixed wet soil sample was kept inside a zipped plastic bag for moisture equalization for about 24 h in a humidity and temperature controlled room. The soil sample was statically compacted to the desired dry density at a rate of 1.25 mm=min (Fig. 5). A steel mould, mm in inner diameter and 120 mm in height, was used. To prevent any disturbance to the specimen or to avoid collapse of the cohesionless soil specimen during the installation, a layer of membrane was wrapped inside the mould by applying a small vacuum during compaction as shown in Fig. 5. The soil was compacted in eight layers, each layer being 12.5 mm thick. To prevent excessive densification of the underlying layers during the compaction of the succeeding layers, which may have a significant effect on internal erosion, the under-compaction procedure proposed by Ladd (1978) was adopted. The solid plate was used during the compaction in order to support the specimen and level its bottom face, while it was replaced by a perforated plate after specimen preparation as shown in Fig. 3. Moreover, the specimen was wrapped by another layer of membrane after compaction to avoid punching of the inner membrane during compaction. Testing Procedures To investigate the effect of stress state on the initiation and development of internal erosion in cohesionless soils and to study the stress-strain behavior of the soils experiencing internal erosion, a series of tests was conducted in five steps: specimen saturation, consolidation at controlled stress state, internal erosion testing, shear strength testing, and post-test particle size distribution analysis. The details of each step are presented in the following sections. Specimen Saturation After the soil specimen was installed into the triaxial system, a low confining stress (i.e., 10 kpa) was applied to prevent collapse of the specimen and any leakage between the specimen and the membrane during the saturation step. As the soil specimen is connected to the downstream reservoir and the downstream reservoir is open to atmosphere, back pressure saturation is not suitable. A differential water-head method was adopted in this research. First, carbon dioxide was slowly injected into the specimen from the bottom of the specimen to displace the air within the specimen. After about 2 h, the carbon dioxide was replaced by de-aired water. The de-aired water was slowly introduced into the specimen by increasing the water level. The inflow rate was sufficiently small (i.e., 15 g=h), regulated using valve II [Fig. 2(c)], to prevent soil segregation at the bottom of the specimen and to achieve a high degree of saturation. Once the water level reached the top of the specimen, a low vacuum (i.e., 0.5 kpa) was applied at the top of the specimen to remove the remaining air bubbles. The entire saturation process requires approximately 60 h. After all the visible air bubbles were removed from the specimen and the inflow rate was equal to the outflow rate, the whole triaxial system was connected to an automatic control system (Li et al. 1988) and B-value checking was performed to measure the degree of saturation. The B-value was about 0.85 for most of the tests. Specimen Consolidation at Controlled Stress State After the saturation step, consolidation of the specimen was conducted by increasing the confining stress gradually (i.e., 1 kpa=min) to prevent segregation of the soil at the bottom of the specimen. For the tests under the isotropic stress condition, internal erosion testing was performed after the confining stress was incrementally increased to the proposed value. For the tests under FIG. 5 Specimen preparation using static compaction.

6 6 GEOTECHNICAL TESTING JOURNAL the anisotropic stress condition, after the specimen had been isotropically consolidated to the proposed state, the vertical stress was gradually increased (i.e., 1 kpa=min) to the proposed value and finally internal erosion testing was conducted. There was some soil loss during the specimen saturation and consolidation steps; thus the soil collection system was installed to collect the eroded soil during these two steps. Internal Erosion Testing After the proposed stress state had been applied to the specimen, the pressurized water supply system and the water collection system were connected to the triaxial system as shown in Fig. 1. The hydraulic gradient, i, was increased in stages to the final value (i.e., 0.15 per 10 min for i 1.0, 0.25 per 10 min for 1.0 < i < 2.0, and 0.50 per 10 min for i 2.0). A typical increasing process of the hydraulic gradient is shown in Fig. 6. The selection of the rates was based on the following considerations: [1] the hydraulic gradient that initiates the internal erosion of a gap-graded soil is usually low (Skempton and Brogan 1994), thus the hydraulic gradient is increased at a low rate (i.e., 0.15 per 10 min) when the hydraulic gradient is less than 1.0 to capture the initiation value more accurately; [2] the hydraulic gradient that induces a significant loss of fine particles could be much higher than the initiation gradient according to Moffat and Fannin (2011), so the hydraulic gradient is increased at a higher rate (i.e., 0.50 per 10 min) when the hydraulic gradient is larger than 2.0 to shorten the testing period; and [3] the erosion process is influenced by the rate of hydraulic gradient increase (Tomlinson and Vaid 2000), hence a medium rate (i.e., 0.25 per 10 min) serving as a smooth transition is chosen when the hydraulic gradient is between 1.0 and 2.0 to lessen the effect of sudden change of hydraulic gradient from 0.15 to 0.5. During each 10 min, the hydraulic gradient was increased successively within the first 2 min, and then it was kept constant during the following 8 min. Once the soil erosion had been initiated, the hydraulic gradient was kept constant till no further soil loss was observed (i.e., < 25.0 g=m 2 per 10 min, namely, 0.20 g per 10 min in this research, which is 0.05% of the fine fraction of the test soil). Then the hydraulic gradient was increased to another level. The maximum applied hydraulic gradient was about 8.0. FIG. 6 Illustration of increasing hydraulic gradient during internal erosion testing. The eroded soil was collected by the soil collection system every 10 min. The extracted soil was oven-dried overnight to study the erosion rate. First, valve I [Fig. 2(c)] was closed to prevent change of seepage condition within the specimen. Then, the container with eroded soil was removed, and meanwhile another container fully filled with de-aired water was connected to the funnel. Finally, the valve was opened to collect the eroded soil. This process could be finished within 10 sec. The weight of the outflow was recorded every 2 min to investigate the variations in soil permeability during the entire erosion process. With the loss of fine particles, the specimen deformed simultaneously. The vertical deformation of the soil specimen was recorded every 10 min using a LVDT. The radial deformation was captured by using a photographic method. The duration of the most internal erosion tests was about 7 8 h. Once there was no further soil loss when the hydraulic gradient was increased to the proposed maximum value (i.e., 8.0 in this paper), the internal erosion test was terminated, and the hydraulic head was decreased gradually till there was no head difference between the inflow and the outflow. The specimen was allowed to reach equilibrium eventually. When water flows though the pipes, energy loss can happen due to the friction of the pipes. Based on the measured outflow velocities, the calculated head loss within the pipes is within 2 % of the applied hydraulic head, which can be neglected. Drained Shearing Test To investigate the stress-strain behavior of the soil experiencing internal erosion, a drained shearing test was carried out at the end of every erosion test at an axial strain rate of 0.05 %=min. The rate was determined based on the suggestion by Head (1994). Valve II [Fig. 2(c)] was closed first, and then the inflow hose was connected to the total volume change measuring device. The shearing test was started at the stress state applied during the internal erosion test. The confining stress was kept constant, and the vertical stress was increased gradually. The maximum axial strain was around 30 %. During the whole shearing process, the soil collection system was also assembled to measure the soil loss due to shearing. Post-test Grain Size Distribution Analysis After the shearing test, the GSDs in different layers of the specimen were measured to study the effect of internal erosion on the soil mechanical properties in terms of grain-size distribution. The specimen was equally divided into three layers. The soil in each layer was oven-dried at 105 C for 24 h and then the GSD of each layer was measured using a sieving method. The soil in each layer was sieved into five portions according to its particles sizes: < 0.15 mm, mm, mm, mm, and mm. Experimental Results A series of tests was performed on the test soil (Fig. 4) under multi-stage seepage flow and complex stress states using the developed stress-controlled erosion apparatus. The purposes are to investigate the influence of complex stress states on the initiation and development of internal erosion (i.e., erosion rate,

7 CHANG AND ZHANG ON A STRESS-CONTROLLED EROSION APPARATUS 7 TABLE 2 Summary of the internal erosion tests. Final Stress State Specimen Identity Initial Void Ratio Void Ratio at Final Stress State Relative Compaction (%) Confining Pressure (kpa) Effective Mean Stress (kpa) Deviatoric Stress (kpa) I I-WIE II II-R III deformation, permeability), and to study the stress-strain behavior of the soil experiencing internal erosion. The details of the testing program are summarized in Table 2. All the test specimens are under the same relative compaction of 93 %. According to the geometric criteria proposed by Kenney and Lau (1985), Fannin and Moffat (2006), andchang and Zhang (2011), the soil is assessed to be internally unstable. Therefore, the fine particles could erode once the seepage flow reaches the critical conditions. FIG. 7 (a) Cumulative eroded soil and (b) variations in coefficient of permeability during the internal erosion tests under different stress states. The weight of the segregated particles during the saturation step and the consolidation step is within 2 % of the fine fraction. General Feature of Internal Erosion under Complex Stress States Take specimen II (mean stress p ¼ 83 kpa, deviatoric stress q ¼ 100 kpa) as an example. When the hydraulic gradient increases to 1.20, the fine particles start to erode as shown in Fig. 7(a). Once the weight of the eroded soil in 10 min is less than 0.2 g, the hydraulic gradient is increased to another level. Sudden increases in erosion rate, hydraulic conductivity, and deformation of the specimen are observed at a hydraulic gradient of 3.15 as shown in Figs. 7 and 8. The erosion rate increases nearly 5 times. The average coefficient of permeability varies from to m=s, and the vertical strain and average lateral strain increase nearly 4.5 and 1.9 %, respectively. With the increase of hydraulic gradient, the fine particles start to erode when the applied hydraulic gradient is larger than a critical value. The initiation hydraulic gradient, i start, is defined as the hydraulic gradient that initiates internal erosion. The specimen can reach a new equilibrium state easily under the applied stresses and the relatively low hydraulic gradient. When the applied hydraulic gradient is further increased to another critical value, referred to as skeleton deformation hydraulic gradient, i sd, sudden increases in erosion rate, soil permeability, and deformations in both axial and radial directions are observed. After the loss of significant fine particles, the soil specimen reaches a new equilibrium state. Then, after the hydraulic gradient is increased to the maximum value (i.e., i ¼ 8.0) following the multi-stage procedure as shown in Fig. 6, additional 13.0 g of fine particles are eroded in the 210 min. The soil permeability decreases during this period because many eroded fine particles are captured in the bottom layer during the mutation process, which induces clogging of the constrictions among the coarse particles in the bottom layer. The decreased diameters of the effective pore throats result in a reduction of permeability. Figure 9(b) shows the evidence that the fine fraction of the soil in the top layer is nearly 6.0 % less than that in the bottom layer. The tests conducted by Fannin and Moffat (2006) also exhibited the same trend that the loss of fine particles in the top layer is more than that in the bottom layer. The incremental deformation of the soil specimen is generally limited within this period as shown in Fig. 7, which shows that the soil reaches a new equilibrium state.

8 8 GEOTECHNICAL TESTING JOURNAL Erosion Rate, Permeability, and Deformation under Different Stress States Specimens I, II, and III were tested under the same confining stress, but different deviatoric stresses. Figure 7(a) shows the cumulative weights of the eroded soil in the three tests. It is found that both the average erosion rate within the testing period and the maximum erosion rate (within 10 min) increase with increasing deviatoric stress. The maximum erosion rate for specimen III is about 3.16 g=m 2 s, which is 2.8 times of that for specimen I. The average erosion rates for specimens I, II, and III are 0.18, 0.27, 0.32 g=m 2 s, respectively. As the applied stress ratio increases, the primary structure formed by the coarse particles becomes more unstable. Combined with the effect of the loss of the fine particles that provide lateral support for the primary soil skeleton structure, the erosion rate in the largest stress ratio case (specimen III) is much larger than that in the isotropic stress case (specimen I). Moreover, due to the loss of fine particles, the permeability increases significantly in all three tests as shown in Fig. 7(b). The average coefficient of permeability increases from to m=s for specimen I, from to m=s for specimen II, and from to m =s for specimen III, which is coincidental with the amount of loss of fine particles. The more the fine particles are lost, the larger the permeability is increased. Finally, the permeability reaches a relatively constant value. Figure 8 shows the axial and radial strains of the soil specimens caused by soil erosion. The total axial strains of specimens I, II, and III are 0.14, 5.1, and 6.5 %, respectively, and the total lateral strains are 0.07, 2.4, and 3.4 %, respectively. Here the negative FIG. 8 Deformation of the specimen during internal erosion testing under different stress states: (a) axial strain and (b) radial strain. FIG. 9 Grain size distributions in different layers of the test soil under different stress states: (a) p 0 ¼ 50 kpa, q¼ 0 kpa; (b) p ¼ 83 kpa, q¼ 100 kpa; and (c) p 0 ¼ 100 kpa, q¼ 150 kpa (final hydraulic gradient ¼ 8.0).

9 CHANG AND ZHANG ON A STRESS-CONTROLLED EROSION APPARATUS 9 values mean the sample diameter increases. A higher stress ratio is associated with larger axial and radial strains during the erosion process. Moreover, the deformation is small when the applied hydraulic gradient is smaller than the skeleton deformation gradient. This supports the hypothesis provided by Skempton and Brogan (1994) that the fine particles sustaining low lateral force within the macro pores formed by the coarse particles are easier to be eroded, and the primary structure formed mainly by the coarse particles remains stable. After the specimen experiences a significant amount of loss of fine particles (i.e., g for specimen I, g for specimen II, and g for specimen III) and a mutation in erosion rate, permeability, and deformation, it reaches a new equilibrium state, and the deformation increment of the specimen becomes negligible after the new equilibrium state has been reached. Therefore, the erosion resistance of the deformed structures becomes higher. Drained Shear Strength To quantify the effect of internal erosion on the shear strength of the soil, drained shear tests were carried out starting at the stress states applied during the internal erosion tests. Another test specimen (specimen I-WIE) was used to test the drained stress-strain behavior of the original soil without erosion as shown in Table 2. The specimen was prepared and tested following the same testing procedures for specimen I except without internal erosion testing and soil loss before hand. Figure 10 presents the drained triaxial test results for specimens I, I-WIE, II, and III. The original soil shows a slightly strain softening and dilative response. However, once a significant amount of fine particles is eroded, the soil exhibits a strain-hardening and contractive response. This is mainly because the void ratio increases when the fine particles are washed out. The mechanical behavior of soil depends on its void ratio. The phenomena that a dilative stress-strain behavior changes to be a contractive one when a certain amount of fine particles of a soil is deleted is also observed by Scholtes et al. (2010) and Wood et al. (2010) from numerical simulations. The shear strength is also significantly affected by internal erosion. The peak stress decreases from 219 kpa for the specimen without erosion to 184 kpa for specimen I, 191 kpa for specimen II, and 208 kpa for specimen III. The observation suggests that modification of the mechanical properties of embankment soils is necessary to predict the long-term stability of embankment dams or levees that suffer internal erosion. FIG. 10 Stress-strain relationships without and with internal erosion under different initial stress conditions: (a) deviatoric stress versus axial strain and (b) volumetric strain versus axial strain (final hydraulic gradient ¼ 8.0). FIG. 11 Repeatability tests for the test soil under the same stress state (p ¼ 83 kpa, q ¼ 100 kpa): (a) cumulative eroded soil and (b) axial strain.

10 10 GEOTECHNICAL TESTING JOURNAL Changes in Grain-size Distribution Figure 9 shows the variations in the grain-size distributions in different layers of the soil specimen. A general trend is that more fine particles in the top layer are eroded than those in the bottom layer. The differences range from 3.7 to 7.1 %. During the mutation process in terms of microstructure, the fine particles lost in the top layer under seepage force and self-weight can be captured in the bottom layer. Moffat and Fannin (2006, 2011) observed that the local hydraulic gradient in the top layer is higher than that in the bottom layer under downward seepage flow; this could also induce more fine particles being eroded in the top layer. Test Repeatability To investigate the repeatability of the test results and to validate the testing apparatus, specimen II-R was tested under the same initial void ratio and stress state as those of specimen II. Figure 11(a) shows the cumulative weights of the eroded soil in the two tests. The maximum erosion rate and average erosion rate are 3.26 and 0.23 g=m 2 s for specimens II, and 2.75 and 0.26 g=m 2 s for specimen II-R, respectively. The differences are approximately 16 and 12 %, respectively. The two critical hydraulic gradients (i.e., i start and i sd ) for the repeatability test (specimen II-R) are 1.45 and 3.45, respectively, which are very close to the values of specimen II. Figure 11(b) shows the axial strains of the two specimens during erosion. The total axial strain of specimen II-R is 4.8 %, which is comparable to 5.1 % of specimen II. The difference in the measured quantities (e.g., weight of eroded soil and axial strain) could be due to the non-uniformity of the two specimens. Summary and Conclusions A stress-controlled erosion apparatus was developed to investigate the initiation and development of internal erosion subjected to multi-stage seepage flow and complex stress states and to study the stress-strain behavior of the soil experiencing internal erosion. The apparatus allows independent control of hydraulic gradient and stress state. The hydraulic gradient is controlled at a precision of 0.05 using a water-head control method. The vertical and lateral deformations are measured using a LVDT and a photographic method with a precision of 0.01 and 0.08 mm, respectively. The eroded soil and the outflow rate are measured using a soil collection system and a water collection system. The measurements can be used to study the erosion rate, variations in soil permeability, and soil deformation during the erosion process. A series of tests was carried out under multi-stage seepage flow and complex stress states using the developed apparatus. Within the limit of the applied hydraulic gradient, there exist two critical hydraulic gradients: at initiation of internal erosion and at deformation of the soil skeleton. When the applied hydraulic gradient reaches the critical value that causes the soil skeleton to deform, there is a mutation of the specimen in terms of soil microstructure. Under the same confining stress but different deviatoric stresses, the maximum erosion rate and the total weight of the eroded soil show an increasing trend with the increase of deviatoric stress. Moreover, the erosion-induced deformation of the specimen also increases with the increase of deviatoric stress. Finally, after the loss of a significant amount of fine particles in the soil, the original dilative stress-strain behavior changes to be contractive one and the peak stress decreases. The post-test grain size distributions in different layers of the specimen are different. More fine particles in the top layer are eroded than those in the bottom layer. Acknowledgments The research was substantially supported by the Ministry of Science and Technology (2009BAK56B05) and Research Grants Council of the Hong Kong SAR (Grant No ). References ASTM D2487, 2000, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), Annual Book of ASTM Standards, Vol , ASTM International, West Conshohocken, PA, pp Bendahmane, F., Marot, D., and Alexis, A., 2008, Experimental Parametric Study of Suffusion and Backward Erosion, J. Geotech. Geoenviron. Eng., Vol. 134, No. 4, pp Chang, D. S. and Zhang, L. M., 2011, Internal Stability Criteria for Soils, Rock and Soil Mechanics, Vol. 32, No. S1, pp (in Chinese). Fannin, R. J. and Moffat, R., 2006, Observations on Internal Stability of Cohesionless Soils, Géotechnique, Vol. 56, No. 7, pp Fell, R. and Fry, J. 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