Dongsheng Chang

Critical Hydraulic Gradients of Internal Erosion under Complex Stress States

AbstractInternal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow, which affects the hydraulic and mechanical behavior of the soil. In this research, extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths: isotropic, drained triaxial compression, and triaxial extension stress paths. These tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients. The entire erosion process can be divided into four phases: stable, initiation, development, and failure. Accordingly, three critical gradients termed as initiation, skeleton-deformation, and failure hydraulic gradients, can be defined. These critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton, the buckling of the strong force chains formed by the coarse particles, and the soil failure, re...

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

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.

Mechanical consequences of internal soil erosion

Internal erosion sometimes occurs in dams and slopes. Under seepage, fine particles in soil may migrate through the pores formed by the coarse matrix. The mechanical properties of the soil may then change due to the loss of fine particles. Extensive laboratory tests are conducted in this research to study the shear strength of a dense, internally unstable soil subjected to the loss of fine particles. The soil exhibited strain-softening behaviour prior to internal erosion. Once a significant amount of fine particles was lost, the strain-softening behaviour of the soil would turn into strain hardening. The peak shear strength of the soil decreased slightly, but the shear strength at the critical state increased slightly in response to the loss of fine particles. The peak friction angle decreased by 1.1–5.9° after a loss of 2.5–6.8% of fine particles. Finally, based on the findings of the study, a probable mechanism of the formation of sinkholes and cavities in slopes and slope failures associated with internal erosion problems is presented.

Stress-Strain Behavior of Granular Soils Subjected to Internal Erosion

AbstractInternal erosion is a major cause of dam or dike failures and incidents. In this study, laboratory tests were conducted to investigate the deformation of two gap-graded soils during internal erosion and the stress-strain behavior of the soils that experienced loss of fine particles due to internal erosion. An erosion-controlled experimental method was adopted to achieve a designated loss of fine particles during internal erosion by adding a predefined amount of salt into the soil sample during sample preparation and dissolving the salt in water under a controlled stress condition. Both the radial and axial deformations during erosion were measured using a photographic method. Subsequently, drained triaxial compression tests were performed to study the stress-strain behavior of the soils that had lost different amounts of fine particles. The peak friction angle and critical friction angle of the soil decreased with the loss of fine particles. After a significant loss of fine particles, the stress-s...

Changes in Soil Deformation and Shear Strength by Internal Erosion

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.

Closure to “Behavior of Coarse Widely Graded Soils under Low Confining Pressures” by H. F. Zhao, L. M. Zhang, and D. S. Chang

Yang Xiao, S.M.ASCE; and Jun Liao Ph.D. Candidate, College of Civil and Transportation Engineering, Hohai Univ., Nanjing 210098, China. E-mail: hhuxyanson@163.com Senior Engineer, State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu Univ. of Technology, Chengdu, Sichuan, 610051, China; and College of Environment and Civil Engineering, Chengdu Univ. of Technology, Chengdu, Sichuan, 610051, China (corresponding author). E-mail: liaojun3308@163.com