1993 Fiscal Year Final Research Report Summary
Research on Internal Erosion in Fill Dams for Agriculture due to Hydraulic Fracturing
| Project/Area Number |
03452270
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
農業土木
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| Research Institution | Gifu University |
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Principal Investigator |
NAKANO Ryoki Gifu University, Agriculture Professor, 農学部, 教授 (40135182)
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| Co-Investigator(Kenkyū-buntansha) |
NISHIMURA Shinichi Gifu University, Agriculture Assistant, 農学部, 助手 (90228221)
SHIMIZU Hideyoshi Gifu University, Agriculture Lecture, 農学部, 講師 (90144005)
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| Project Period (FY) |
1991 – 1993
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| Keywords | Earth-fill dam / Hightening of a fill-dam / Leakage / Hydraulic fracturing / Volcanic ash loam / Deformation modulus in tension and compression / Limit tensile strain / FEM analysis of a fill dam |
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| Research Abstract |
Oyachi dam, an earth-fill dam recently constructed on soft foundation(N value < 5) composed of volcanic ash loam at Tsunan-cho in Niigata Prefecture, suffered a considerable amount of leakage through its foundation when the reservoir water level was raised to EL653.5m. As it was suspected that this was caused by hydraulic fracturing, a series of in-situ tests were carried out by injecting dyed water and glycerine into test sections at various depths in auger holes.
1) It was found that hydraulic fracturing is invariably induced in any case by tensile cracks nucleated around a test hole when injection pressure is increased to some value. It was also found that once hydraulic fracture is induced and cracks are formed, hydraulic fracture is always observed in the following tests at a much lower injection pressure, which is nearly equal to initial horizontal earth pressure.
2) Using fundamental mechanical parameters such as deformation modulus E_u, Poisson's ratio nu, limit tensile and compr
… More
essive strains and strengths obtained by Laboratory tests on undisturbed foundation soil, FEM analyzes were carried out to obtain stresses and strains induced in the soft foundation by the embankment dam. It was found that a considerable region of the foundation is in the failure state indicating the occurrence of many failure cracks due to extension or compression just after construction. It was also found that the horizontal stresses in the foundation become less than the reservoir water pressure when the reservoir water level is raised to EL.653.5m.
3) Based on the evidences mentioned above, it was concluded that the severe leakage through the foundation is caused by hydraulic fracturing due to the reopening of the cracks, suggesting that covering the reservoir slope of the foundation soil with disturbed clayy loam is the feasible countermeasures to stop the leakage. This countermeasures was actually undertaken before second test-impounding in 1993 in complete success, reducing the leakage practically to nil. Less
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