SMALL DAMS

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Chapter IV Determination of forces !" permanent force: dead weight of the fill; !" variable force: pore pressure or pressure of reservoir water according to head on the spillway; !" accidental force: earthquakes. Combinations of forces In designing small and medium-size dams, three combinations of forces are usually considered: !"dead weight of the fill and pore pressures at the end of construction (a practically permanent combination); !"dead weight of the fill and pore pressure field induced by rapid reservoir emptying (frequent combination); !" dead weight of the fill and pore pressure field induced by the reservoir at full supply level (a practically permanent combination); Where appropriate, earthquake forces (accidental combination), should also be considered. STABILITY CALCULATIONS If there are no fine materials in the foundation and in the fill, the designer is confronted with one of two cases: !" the materials may be highly permeable and stability will depend on their internal friction angle ; !" the materials may be semi-permeable and account must also be taken of the flow pattern and therefore pore pressures during rapid reservoir drawdown. In the rest of this paragraph, we consider only cases where there are fine materials (excluding mud and peat), either in the foundation or in the fill, or in both. Fine materials generally result in using two types of calculations for slope stability: !"a short term calculation corresponding to stability at the end of construction, before consolidation, using characteristics determined by unconsolidated undrained triaxial tests interpreted with consideration of total stresses; !"a long-term calculation after consolidation and after a rapid drawdown operation 1 for the upstream slope, considering effective stresses using the characteristics determined by the consolidated undrained (or drained test in some cases) triaxial test. Calculation methods for circular failure modes such as the FELLENIUS or BISHOP methods (the FELLENIUS method being the more pessimistic) are suitable in routine cases. Methods for non-circular failure modes such as the SPENCER method should be used for some zoned dams and when the foundation is partially (in one layer) or totally made up of weak materials. 83 1. The calculation in "rapid drawdown conditions" is done assuming that emptying is instananeous, which is not very far from reality.
F ill dams The selected profile should guarantee the stability of both dam slopes, in the short and the long term, with a satisfactory factor of safety, usually between 1.3 and 2. 84 Long-term stability (practically permanent load combination with the reservoir full or frequent drawdown combination) When the reservoir is emptied rapidly, it can generally be considered that the saturation line is horizontal (normal water level) in the watertight zone of the fill (upstream slope or central zone) all the way to the chimney drain. This approximation has no significant impact on the results of the stability calculation, which is also relatively unaffected by variations in the unit weight of the materials. The factor of safety F mainly depends on the values of c’ and ϕ’ 1 and therefore on how well they represent the material. When several triaxial tests are done on a single type of material from a borrow area, its c’ and ϕ’ will be determined from the straight line of regression of all the failure circles represented by (’ 1 + ’ 3 )/2 and (’ 1 - ’ 3 )/2 (see figure 5). First a and are calculated by adjusting the co-ordinate points [(’ 1 + ’ 3 )/2, (’ 1 - ’ 3 )/2] for all of the triaxial tests considered. Then c’ and ϕ’ are deduced as indicated in figure 5. A new approach is proposed for long-term stability calculation, in three stages 2 : ! a first stage consisting in studying, in a conventional manner, the stability of the slopes using the mechanical characteristics c' and ϕ' of the foundation and the fill (factor of safety F close to 1.5). But this calculation alone is not enough, as a single value for F can be insufficient in some cases and quite sufficient in others; ! a proposed second stage consisting in evaluating the influence of each mechanical characteristic on the factor of safety F with successive decreases in each c' by 10 kPa (with the boundary being c' = 0) and each ϕ' by 5°. Depending on the slope considered (upstream or downstream), the relative weight of each mechanical characteristic on F can be assessed; decreasing just one of them, by 10 kPa or 5°, results in F being decreased only slightly, by several hundredths, or more significantly, by several tenths, according to the greater or lesser influence of fill height and soft foundation thickness. This may mean that additional tests are considered necessary or that greater prudence is required in the choice of certain values. It can be noted in general that F is more sensitive to a 10 kPa decrease than to a 5° decrease for dams around 10 metres high and less sensitive when the fill height is around 30 metres; !"a third stage consisting in calculing F with a decrease in all mechanical characteristics by 10 kPa and 5°. The cross-section is considered satisfactory if F equal to or slightly higher than 1 for the upstream slope and 1.2 dor the downstream slope, which requires greater safety; if F is significantly higher than these boundary values the slopes may be steeper, while if F is lower than these values should be gentler. Comment: these values of 10 kPa and 5° seem suitable for the following reasons: 1. c' (cohesion) and ' (internal friction angle) are intergrain characteristics of the soil, obtained via the consolidated triaxial test interpreted in effective stress conditions. 2. See Bibliography, reference 7.
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FRENCH COMMITTEE ON LARGE DAMS COMI
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PHOTOGRAPH REFERENCES Cover: CACG,
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M E M B E R S O F T H E R E A D I N
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TABLE OF CONTENTS Foreword - What n
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Table of contents FILL DAM DESIGN..
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Table of contents Chap. VII - Life
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W hat need for guidelines for small
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W hat need for guidelines for small
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C hoice of site and type of dam TOP
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C hoice of site and type of dam AVA
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C hoice of site and type of dam CON
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P reliminary determination of desig
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Page 31 and 32: P reliminary determination of desig
Page 37 and 38: G eological and geotechnical studie
Page 67 and 68: F ill dams GEOTECHNICAL STUDIES 68
Page 69 and 70: F ill dams For fill, the wetter the
Page 71 and 72: F ill dams 72 If the latter is unal
Page 73 and 74: F ill dams As concerns freeboard R,
Page 75 and 76: F ill dams Wave height h (m) Thickn
Page 77 and 78: F ill dams In the case of a widely
Page 79 and 80: F ill dams Fig. 3 - Sloped granular
Page 81: F ill dams It is therefore necessar
Page 85 and 86: F ill dams 86 Case of compressible
Page 87 and 88: F ill dams In present practice, the
Page 89 and 90: F ill dams MONITORING SYSTEM 1 The
Page 91 and 92: F ill dams The most common measurem
Page 93 and 94: F ill dams 94 When H 2 V > 30, the
Page 95 and 96: F ill dams DESIGN OF THE FREE OVERF
Page 97 and 98: F ill dams 98 In the case of very w
Page 99 and 100: F ill dams TENDERING AND DAM CONSTR
Page 101 and 102: F ill dams In the case of a rock fo
Page 103 and 104: F ill dams However, when the materi
Page 105 and 106: F ill dams All of the inspections m
Page 107 and 108: F ill dams SPECIFIC FEATURES OF VER
Page 109 and 110: F ill dams !"engineering costs. The
Page 111 and 112: ▲ 1 - Compacting clay in the cut-
Page 113 and 114: ▲ 7 - Scarification after passage
Page 115 and 116: ▲ 12 - Rip-rap on a dam built for
Page 117 and 118: ▼ 20 - Overspill part of a side i
Page 119 and 120: C H A P T E R V Small concrete dams
Page 121 and 122: Chapter V Arch dams Arch dams trans
Page 123 and 124: Chapter V Masonry dams are no longe
Page 125 and 126: Chapter V Finally, the slope of the
Page 127 and 128: Chapter V FOUNDATION TREATMENT Hydr
Page 129 and 130: Chapter V and account will be taken
Page 131 and 132: Chapter V Thrust of ice This force
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Chapter V The commonly accepted cri
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Chapter V MONITORING SYSTEMS The ge
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Chapter V The typical cross-section
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Chapter V LOUBERRIA DAM Louberria d
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Chapter V Some of these barrages, a
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Chapter V Fig. 8 - Dam with a tilti
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Chapter V BIBLIOGRAPHY 1 - Ministè
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M anagement of water quality EUTROP
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M anagement of water quality !"the
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M anagement of water quality Proces
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M anagement of water quality 148 Al
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M anagement of water quality While
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M anagement of water quality It con
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M anagement of water quality It sho
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M anagement of water quality These
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4. Grégoire (A.), 1987- Caractéri
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L ife of the dam SPECIAL FEATURES O
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L ife of the dam GENERAL SURVEILLAN
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L ife of the dam ORGANISATION OF SU
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L ife of the dam During such period
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L ife of the dam MAINTENANCE 168 Th
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L ife of the dam BIBLIOGRAPHY 1 - C
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C onclusion FLOOD DATA It is fairly
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SMALL DAMS

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