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Strength characteristics of fine-grained soils at dyke slope surfaces
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| Title: | Strength characteristics of fine-grained soils at dyke slope surfaces |
| Other Titles: | 堤防表層における細粒土の強度特性 |
| Authors: | Panta, Anand Browse this author |
| Issue Date: | 25-Sep-2018 |
| Publisher: | Hokkaido University |
| Abstract: | Shallow slope instability poses challenging problem to the maintenance of many natural and engineered slopes, as triggered by heavy or incessant rainfall events, and the frequency of such events are predicted to increase due to the global climate change. For the shallow slope stability analysis of such earth structures, accurate characterization of soil strength at low effective stresses (5-20kPa), corresponding to soil failure at shallow depths of about 1-2m depth, is required. The accuracy of characterization of soil strength at the low effective stress, particularly in the fine-grained soils, is strongly dependent on capturing the characteristics of strength envelope with appropriate strength parameters, and how this is affected by soil structure and then potentially altered in field environment. In the first part of this study, the undrained strength behaviours of three fine-grained soils in natural (intact), reconstituted (intrinsic) and compacted states were investigated in laboratory to explore the characteristics of strength at low stresses. The studied fine-grained soils include two natural plastic clays, heavily overconsolidated Izumi clay and soft normally consolidated Atsuma clay, and a clay-sand mixed soil sampled from a river dyke construction site in Maizuru. The study focused on characterizing the state bounding surfaces of fine-grained soils by performing a series of constant-volume direct shear tests supplemented by hollow cylinder simple shear tests on Izumi clay, Atsuma clay and Maizuru clay-sand mixed soil in intact, reconstituted (intrinsic) and compacted states. Comparisons of non-linear bounding surfaces expressed by a power-law strength criterion for different states of clays suggested that the normalized strength and the degree of its non-linearity at intact states were higher than at the corresponding intrinsic states, probably due to inter-particle bonding as confirmed by the hollow cylinder simple shear tests. Whereas, the normalized compacted strength was significantly lower than the equivalent intrinsic strength, which was explained by meso-scale discontinuities in the compacted specimens by X-ray micro CT-Images. This difference was not significant in the clay-sand mixed soil. From these observations, it should be noted that the meso-structure in compacted clays is significantly different than that in more homogenous intact and reconstituted clays, and the conventional normalization based on the equivalent pressure concept does not capture it. In the second part, the study focused on characterizing the temporal and spatial variations in the near-surface strength of river dyke by taking a particular dyke site as example; Maizuru dyke, supplemented by limited data from Higashinosato dyke. Portable static/dynamic cone penetrometer tests were performed on a monthly basis in the year 2016/2017, and the results were interpreted with laboratory-compacted model ground penetration tests and consolidated-undrained triaxial tests. The surface strength of Maizuru river dyke increased gradually with depth showing small spatial and temporal variations in strength, and the strength was consistently found higher at a depth of about 0.4-0.5m from the surface than the Higashinosato river dyke and laboratory-compacted model ground, which were young and showed uniform strength along the depth. This may indicate that the Maizuru surface was subject to crusting (overconsolidation of clays as a result of desiccation) after a few cycles of seasons. The strength of soils from the studied dyke sites was characterized by following the approach discussed in the first part of this study, but with triaxial apparatus. Synthesis of the laboratory-measured strength parameters and the field-measured pore water pressure led to estimated field strength of the shallow layers. This surface strength and static portable cone penetrometer resistance exhibited a correlation consistent with lab-established one for 0.4-0.5m depth, but that for depth shallower than this was significantly different. A further study is necessary to endorse extending a simple, single-line correlation between the strength and cone penetrometer resistance to very shallow depth of compacted earth structure. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第13346号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 准教授 西村 聡, 教授 石川 達也, 教授 渡部 要一, 准教授 磯部 公一 |
| Degree Affiliation: | 工学院（環境フィールド工学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/71822 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （工学）
課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
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