Investigation of progression mechanism of potential Small-Scale internal erosion and its effect on soil mechanical properties

潜在的な小規模内部侵食の進行メカニズムと土の力学特性に与える影響の解明

Investigation of progression mechanism of potential Small-Scale internal erosion and its effect on soil mechanical properties

Degree Grantors Yamaguchi University

In recent years, many reports have been made of damage to reservoir embankments due to frequent heavy rains and earthquakes. “Internal erosion” is the phenomenon in which soil particles move through the pore spaces of embankments and flow out of them because of long-term seepage in the soil structures of reservoirs and river embankments. Internal erosion is classified according to the scale and form of the erosion. “Suffusion” is the phenomenon in which only the fine-grained soil particles move through the pore spaces formed by the coarse-grained soil particles. Suffusion is known to cause an increase in hydraulic conductivity and a decrease in soil bearing capacity. However, the mechanisms and causes of the occurrence and progression of this phenomenon, as well as its effect on soil structures, have not been fully elucidated. Therefore, the purpose of this study was to clarify the effect of suffusion on the strength properties of reservoir embankments as well as the mechanism of the progression of suffusion, focusing on the particle size of the eroded soil particles.
In Chapter 1, the background of the study and the classification of internal erosion were provided.
In Chapter 2, the mechanism of suffusion and its effect on geomaterials, which have been clarified in previous studies, were described.
In Chapter 3, the effect of suffusion on the strength properties was firstly investigated by means of soil samples used for the reservoir embankments which were determined to be at risk of suffusion based on an internal stability evaluation. A triaxial compression testing system was established to simulate the suffusion inside each specimen. In the testing system’s apparatus, only the fine particles were able to be discharged from the specimen through a sieve by supplying water. The results showed that changes in the peak and residual strength occurred independent of changes in the void ratio due to suffusion, and that the strength properties differed depending on the presence or absence of suffusion, even when the void ratio was the same as that before shearing. However, since the amount of erosion was small (less than 0.7% of the total), it is considered that the change in strength was influenced by the fine soil particles that moved through the specimen but remained in the specimen without finally flowing out of it.
In Chapter 4, in order to define the mechanism of small-scale internal erosion, a method was devised to examine the temporal changes in the size of the discharged soil particles, focusing on the relationship between the concentration and the turbidity of the drainage. The above method was also applied to the drainage obtained by seepage with suffusion in Chapter 3. As a result, the erosion rates were found to correlate with the particle size of the discharged particles.
In Chapter 5, a one-dimensional water-passing experiment with suffusion was conducted to investigate in detail the relationship among the flow rate, soil particle discharge, and turbidity of the drainage. The particle size composition of the discharged soil, which varied with the progress of suffusion, was examined using the method described in Chapter 4. The results showed that, under all conditions, the soil particles with smaller diameters were selected from among those that were able to pass through the wire mesh at the outlet boundary and then discharged. It was also found that the particle size composition of the discharged soil changed with time as the suffusion progressed, and that this trend varied greatly depending on the saturation degree of each specimen before the start of permeation. After the completion of the experiment, the particle size composition of the soil samples showed that not only the distribution of the erodible component in the height direction, but also the particle size composition of the erodible component itself became heterogeneous, suggesting that soil particles with smaller particle sizes moved longer distances through the pores.
Furthermore, the effect of overburden pressure on the behavior of suffusion under constant hydraulic gradient conditions was explored, and it was revealed that higher overburden pressure resulted in less frequent suffusion when the hydraulic gradient was large. The particle size composition of the discharged soil was examined, and no difference was found among the various levels of overburden pressure.
In addition, the progression of suffusion was investigated under a fluctuating hydraulic gradient. The suffusion continued even after 30 cycles of hydraulic gradient variation. The particle size composition of the discharged soil particles showed that the small soil particles, that flow easily, had already been discharged when the specimen was first subjected to permeation force, and that the soil particles discharged by the change in hydraulic gradient were relatively large in diameter.
The particle size of the soil particles discharged by suffusion were seen to depend on the conditions surrounding the occurrence of suffusion and the time of the progression. In studies on suffusion, the particle size composition of the soil discharged by suffusion is a significant factor that changes over time and can be used to evaluate the degree of the progression of suffusion.

Creators 石丸 太一

Award ため池堤体の細粒分流出を再現した実験システムの構築と流出進行メカニズムの解明 21J14582

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