SMALL DAMS

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Chapter IV Adjusting the characteristics of materials available on the site by screening, adding bentonite, drying or wetting, may be envisaged. But changing the moisture content of a material with a high clay content is both difficult and costly. It requires careful and constant inspections, which can be a problem for small dams. DAM FOUNDATION TREATMENT The dam foundation must always be stripped to a depth of at least 0.50 metre to remove topsoil. The mechanical characteristics of loose materials in the foundation (alluvium, colluvium, eluvium) are often sufficient to support a fill dam less than about 10 metres high. An in-depth study must be undertaken for fill dam heights over about 20 metres. When there is a problem with the stability of the foundation (see stability analysis below, p. 82), the solution consists either in deeper excavation or a considerable widening of the fill dam's base. It is rare to install a drainage system to accelerate consolidation of the loose foundation. A filter may have to be placed at the fill dam/foundation interface because of the nature of the materials. The rules governing such practice are given under Filters and drains below, (p. 77). Settlement of a loose foundation due to the weight of the fill dam is evaluated by compressibility tests. After construction, it should generally not exceed 5% of the total thickness of the compressible layers. Watertightening and drainage systems must be installed to achieve an acceptable leakage flow and avoid any risk of piping (retrogressive internal erosion) and uplift on the downstream side. 71 FOUNDATION WATERTIGHTNESS The three following cases can be considered for type 1 and type 2 dams (homogeneous and zoned): !"foundation consisting of relatively impermeable materials: it is recommended that a cut-off trench be built of compacted clay materials in order to deal with any surface cracking or heterogeneous zones. The dimensions of such a trench should be: - minimum width at the base 3 metres (width of the machinery); - side slopes of the order of 1/1; - several metres depth with a minimum of 2 metres below natural ground level (see photo 1 p. I). !"foundation with permeable layers that are not revealed beyond a depth of several metres: the trench must cut through those layers and be anchored in a watertight layer.
F ill dams 72 If the latter is unaltered rock, after it has been cleaned and possibly its surface has been smoothed, a first layer of wet (optimum moisture content (OMC) + 2 or 3) clay a few decimetres thick is placed to guarantee good contact; it may be necessary to set a filter between the downstream face of the trench and the permeable foundation materials. !" permeable foundation to a significant depth: grouting can be used both for a loose foundation and for a more or less cracked rock mass, with the grout adapted to the material being treated (bentonite-cement grout, specially designed grouts); the cut-off will usually involve three lines of staggered drillholes; as grouting cannot be effective at the surface, either the first few metres of grouting are relayed by the cutoff trench, or treatment is started at a certain height in the fill. Another watertightening technique is the diaphragm wall made of self-hardening bentonite cement grout or plastic-concrete; this solution is quite conventional for loose ground but can also be used in rock foundations with the more costly technique of the hydrodrill. It can cause problems if major displacements occur, for example the fill may be punctured in the event of severe settlement of a soft foundation (with the parallel risk of excessive lateral friction); if it is located at the upstream base of the fill, the diaphragm may suffer considerable shear at its top. As concerns type 3 dams (with an artificial watertightening element), the connection between the watertightening structure in the fill and watertightening in the foundation is a difficult point, except in cases where a diaphragm wall provides all watertightening from the dam crest (see under that heading hereafter, p. 82). When the reservoir cannot be watertightened by a cut-off at the dam, the solution consists in sealing the reservoir basin totally or partially with a geomembrane (see that heading below, p. 81) or with a blanket of compacted clay materials (at least two layers about 0.20 metre thick each), with the latter protected from any risk of drying out. Such techniques always result in a high price per cubic metre of water stored. As concerns the support for these systems, it is necessary to: !"meet filter conditions (see Filters and drains below, p.77) for an upstream blanket; !" eliminate any rough areas that might puncture the geomembrane; !" avoid any risk of uplift, in particular due to gases under the geomembrane. FOUNDATION DRAINAGE For drainage of flows from the foundation, the most satisfactory solution consists in placing a drainage blanket at the base of the downstream shoulder, at the fillfoundation contact, leading to the vertical or sloped drain in the central part of the fill (see Fill drainage system below, p. 78). This blanket, which may be compartmented in order to determine the behaviour of each different zone, should be placed for any large structure (H 2 V > 700). For smaller dams ( H 2 V < 700), where geological conditions permit, the drainage blanket may be reduced by placement of draining strips (in particular in the areas judged to be the most vulnerable in the river banks).
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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
Page 19 and 20: C hoice of site and type of dam AVA
Page 21 and 22: C hoice of site and type of dam CON
Page 23 and 24: 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: F ill dams For fill, the wetter the
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 and 82: F ill dams It is therefore necessar
Page 83 and 84: F ill dams The selected profile sho
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
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Chapter V Arch dams Arch dams trans
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Chapter V Masonry dams are no longe
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Chapter V Finally, the slope of the
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Chapter V FOUNDATION TREATMENT Hydr
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Chapter V and account will be taken
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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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