SMALL DAMS PETITS BARRAGES

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7.3.4.1 Seepage Barriers and Cutoff Systems This section provides a brief overview of commonly used seepage control systems. Seepage cutoffs and barriers are intended to reduce the amount of seepage and minimize downstream pressures and exit gradients. Depending on the site and foundation conditions, seepage cutoff systems may provide either: Positive cutoff that completely penetrates pervious zones and ties into impervious soil layers or bedrock in the dam foundation and abutments, or − Partial cutoff, which is sometimes used for deep pervious foundations to lengthen the seepage path and reduce exit gradients to acceptable levels. There are many different types of cutoff systems that are used for dam rehabilitation. The primary objective is to provide a low permeability element within the dam and/or its foundation, either as the primary seepage barrier or to supplement existing barriers. Cutoffs may be classified according to their stiffness, the type of materials used, or construction methods used. The terms cutoff “wall” or “diaphragm” are often used to distinguish seepage barriers that are thin compared to the surrounding embankment. Barriers employed for dam rehabilitation may be constructed through the dam and foundation from the existing dam crest (diaphragm or wall). Alternatively barriers may be constructed using a liner system on the upstream face of the dam in combination with vertical or horizontal seepage barriers extending from the upstream toe. Some of the seepage barriers that are commonly used for dam rehabilitation are listed in Table 7.1 .Most of those solutions require lowering of the reservoir level significantly, in order to reduce hydraulic gradients during rehabilitation and the risk of wash-out of cement and bentonite. In case of construction of rehabilitation works on the upstream face of the dam, the reservoir must be emptied. Table 7.1 – Common Seepage cut-off measures identified for Alluvium and Earthfill Foundations of small dams Type of cutoff Method Open trench is Materials used Remarks References and Core trench: excavated and, if stable, filling with one of the materials is done Earthfill Soil-bentonite Concrete-bentonite Depth limited to some metres Slurry trench: Slurry wall, concrete wall, tremmy Open trench by extended backhoe, kept open and stabilized with bentonite slurry Concrete by tremmy pipe. Primary and secondary panels are inserted once sufficient strength has been identified Figure 7.4 and Figure 7.9 and 7.10 Sheet piles curtain Inserted by vibratory Steel Curtain not taken to 126
Tube-a-manchettes injection system for grouting Grout curtain and hydraulic pile drivers Drilling with special packer rod tube and grouting through rubber sleeve Drilling holes, primary, then secondary and tertiary holes. Grouting of holes with cement grout. Cement Cement rock foundation i.e. a non-positive cutoff. Only for rock foundation Figure 7.5 Only for rock foundation Figure 7.6 and Figure 7.12 and 7.13 Fig. 7.12: Vertical rock joints shown with angled grout hole orientation (Penn Forrest Dam Rehabilitation, USA) Fig. 7.13 – Grout holes are drilled at optimized angles for rock joints patterns (image from www.advancedconstructiontechniques.com/). 127
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SMALL DAMS Design, Surveillance and
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FOREWORD The ICOLD - International
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ACKNOWLEDGEMENT The Ad Hoc Committe
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FOREWORD ACKNOWLEDGEMENT SUMMARY TA
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5.5.5 Compaction in confined areas
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7.2.1 Introduction 7.2.2 Spillway c
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1. SMALL DAMS DEFINITION AND CLASSI
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2 Fig. 1.2 - Relationship H . V wit
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1.3 REFERENCES [1] French Committe
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2.3 ZONED EARTHFILL DAMS In arid pa
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sloping core guarantees water impou
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Fig. 2.11 - Hlinky Concrete Gravity
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Fig. 2.13 - Photograph of Neusberg
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The gabion dam as the gabion retain
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Some inflatable dams are operated b
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3.1 INTRODUCTION 3. SAFETY OF SMALL
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Condition Effect on safety 13. Dama
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Fig. 3.2 - Massive dam break overto
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and suffusion or internal instabili
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downstream part of the embankment m
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small dam impounding a lot of water
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Whenever the Supervising Authority
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emergency management organizations
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c) The decommissioning plan should
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5.1 INTRODUCTION 5. FEATURES OF THE
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5.2 DESIGN FLOODS Inadequate flood
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downstream slope will show seepage
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Fig. 5.2 - Design Freeboard 5.4.3 F
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2 U f Fe S 1400 D Where Uf is the d
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Maximum compaction is obtained at c
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characteristic must be determined.
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5.6.1 Foundation Watertightness The
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� A continuous vertical chimney d
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Thickness of pervious layer Thick D
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t 5.7.1.4 Practical Considerations
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It is generally accepted that cohes
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� Simple Bishop (1955) � Spence
Page 75 and 76: � The elements for surface draina
Page 77 and 78: The supporting layer for the rip-ra
Page 79 and 80: For very small reservoirs (fetch of
Page 81 and 82: The junction of the downstream slop
Page 83 and 84: Fig. 5.14 - Hongshiyan homogenous d
Page 85 and 86: material has been left in place or
Page 87 and 88: The homogeneous dam is recommended
Page 89 and 90: [3] OL and OH soils are not recomme
Page 91 and 92: part (core) lies in the zone (Fig.
Page 93 and 94: 6. GUIDELINES ON SURVEILLANCE OF SM
Page 95 and 96: cases the owner may retain the serv
Page 97 and 98: Surface displacement on an embankme
Page 99 and 100: Fig. 6.1 - Leakage measurement usin
Page 101 and 102: Water Level Sounder - Details of th
Page 103 and 104: pressure) Left abutment 2 5 Weir ga
Page 105 and 106: Fig. 6.9 - Recommended standpipe pi
Page 107 and 108: the others weir gauges need to be i
Page 109 and 110: � Earthquakes sensible at the dam
Page 111 and 112: APPENDIX INSPECTION CHECKLIST FOR S
Page 113 and 114: Exposed reinforcement � The condu
Page 115 and 116: These spillways can be lined or unl
Page 117 and 118: flowing water. Flowing water with h
Page 119 and 120: Fig. 7.3 - Installation of the arti
Page 121 and 122: Fig. 7.7 - Dam overtopping protecti
Page 123 and 124: complete construction was in 2001,
Page 125: In some cases, the downstream face
Page 129 and 130: 7.3.5 Slope Protection Measures 7.3
Page 131 and 132: Pipe Rehabilitation SLIPLINING MODI
Page 133 and 134: Conditions Pros Cons Comments types
Page 135 and 136: Embankment dams that have properly
Page 137 and 138: and minimize erosion. Design of sui
Page 139 and 140: [20] SMEC- “Draft Guidelines for
Page 141 and 142: present visual information. It must
Page 143 and 144: Fig. 8.1 - Three small dams in casc
Page 145 and 146: Social Disruption low (rural area)
Page 147 and 148: Fig. 8.4 - Flood water level after
Page 149: 8.9 REFERENCES [1] BURREC (1994) -
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SMALL DAMS PETITS BARRAGES

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