Small Dams and Weirs in Earth and Gabion Materials Agl/Misc/32/2001
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AGL/MISC/32/2001 SMALL DAMS AND WEIRS IN EARTH AND GABION MATERIALS AGL/MISC/32/2001 SMALL DAMS AND WEIRS IN EARTH AND GABION MATERIALS FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Land and Water Development Division Rome, 2001 Foreword Beginning in the 16th century, engineers in Europe used wicker baskets - Italian gabbioni - filled with soil to fortify military emplacements and reinforce river banks. Today, the same simple technology - now known as “gabions” - is used as the building blocks of low-cost, long- lasting hydraulic structures in developing and developed countries. These days galvanized steel wire-mesh has replaced Wicker Basket - Gabbioni wicker, and stones are used instead of soil, but the underlying strength of gabions is unchanged. The intrinsic flexibility of a gabion structure enables it to bend rather than break, thereby preventing loss of structural efficiency. Since gabions are bound together, they will withstand a degree of tension that would severely test a dry stone construction and be dangerous with plain concrete and masonry. Gabion baskets can also be manufactured - literally, by Modern gabion hand - at village-level. This is a double advantage: it lowers the purchase cost of the baskets and creates a small rural industry, using local labour. This is in line with trends towards increasing use of labour intensive techniques in modern development projects. Gabions are now recognised by most engineers throughout the world as being a standard construction material. FAO has long experience with gabion construction in various countries of the developing world, including Botswana, China, Egypt, Eritrea, Ethiopia, Guinea Bissau, Haiti, Niger, Malawi, Nigeria, Viet Nam, etc., where water development and irrigation projects have all made use of either imported or locally-made gabion baskets. The structures most frequently consist of earth retaining and anti-erosion structures such as retaining walls for earth excavations and embankments, sea, river and canal banks lining , small dams, spillways, weirs, groynes, etc. The design and construction of gabion structures have not always been up to standard and, as a consequence, have caused partial or total failure of the works due to lack of (or excessive subsidence of) foundations and, most often, due to the progressive infiltration and leakage of water along the interface between the gabions and adjacent earth materials and foundations. In fact, as with any civil engineering structure, unless the same care is given and attention paid to the proper design and construction of a gabion structure the result can just as well end up in failure. However, it is worthwhile noting that the inherent flexibility and ability to survive distortion without fracture is such that a badly designed gabion structure is more likely to survive than would a similar monolithic structure. The present publication is Damages a set of practical guidelines and norms for field project engineers for the design and building of small hydraulic structures using earth and gabions. This publication would be useful in designing small earth dams with a gabion spillway, intake weirs for gravity irrigation schemes, groynes, river bed training works and for protection against hydraulic erosion. Gabion construction Acknowledgements This publication is the result of a collective effort: field engineers having considerable experience have contributed, often under difficult conditions, to the design and the construction of hydraulic structures made in earth and gabion materials in FAO-executed projects in a number of countries with a large array of cultural and climatic differences. This vast experience includes mainly successful results. However, a few drawbacks have been experienced as being the result of harsh meteorological and hydrological conditions which may have been underestimated. Gabions have moved, have had unacceptable distortion and warping, some layers of gabions have been outstripped and rolled over several meters. However, these negative experiences have also had their positive side as they have contributed to an increasing experience in building safe structures under often a tight budget. Design and construction of new projects should then be carried out with a much greater factor of safety. We are grateful to the authors of this bulletin, John Charman, the late Lubtcho Kostov, Luciano Minetti, Joop Stoutesdijk and Dario Tricoli who have prepared the main report and the Annexes containing case studies. The coordination was provided by Olivier Berney. Special thanks are due to Simone Morini for his valuable assistance in the preparation and formatting of the text. John Charman further contributed in editing the final document. Small dams and weirs in earth and gabion materials vii Contents FOREWORD iii ACKNOWLEDGEMENTS iv INTRODUCTION 1 The investigation approach 1 The design approach 1 The construction approach 2 1. GENERAL CONSIDERATIONS 3 Types of small dams 3 Earthfill dams 4 Gabion weirs 4 Earthfill dikes 5 Main components of a small dam project 5 Earth embankment 6 Spillway 7 Outlet structures 8 Investigation surveys and design 8 Collecting existing data 8 Preliminary site survey 8 Feasibility studies 8 Detailed design 9 Technology of gabions 9 2. HYDROLOGICAL STUDIES 13 Catchment 14 Basin morphology 15 Shape 15 Relief 15 Drainage pattern 16 Soils 16 Rainfall 17 Hydrological methods 18 Erosion and sediment transportation 21 Evaporation 22 viii Contents 3. GEOTECHNICAL INVESTIGATIONS 23 Purpose of geotechnical investigations 23 Scope of ground investigation 23 Character and variability of the ground 23 Nature of the project 24 Methods of ground investigation 26 Exploratory holes 26 Methods for taking samples 30 In-situ tests in boreholes 35 Laboratory testing 40 4. DESCRIPTION AND CLASSIFICATION OF SOILS AND ROCKS 43 General 43 Visual methods for the uscs classification of soils 43 Equipment and general procedure 43 Selection and preparation of the sample 46 Coarse-grained soils 46 Fine-grained soils 47 Description of soils 49 Subsidiary components 50 Consistency or relative density 50 Geotechnical properties of soil groups 50 Compacted soil properties 51 Permeability 51 Compressibility 51 Dispersivity 54 Description of rocks 55 Rock name 55 Rock texture and fabric 56 Weathering classification 57 Strength 58 Discontinuity classification 58 5. DESIGN OF GABION SPILLWAYS 59 Mitigating changes in hydraulic energy 59 Types of spillway 59 Discharge channel 61 Dimensioning 61 Protection of the sides and bottom 66 Spillway weir structure 67 Shape of weirs 67 Stilling basin 68 Hydraulic design 68 Stability analysis 79 Resistance of the foundation soil 81 Small dams and weirs in earth and gabion materials ix 6. EMBANKMENT DESIGN 83 Construction materials 83 Impervious material for upstream blanket 83 Aggregates for concrete 83 Aggregates for filters and drains 84 Rock for rip-rap, gabions and rock-fill 84 Suitable earth-fill material 84 Water loss and piping 85 Mechanism 85 Downstream drainage 85 Cut-off trench 86 Impermeable upstream blanket 87 Geotextiles as filter layers and slope protection 88 Materials 88 Use in slope protection 89 Use as separators 91 Slope stability 93 Nature of the problem 93 Stability computations 93 7. CONSTRUCTION AND QUALITY CONTROL OF EARTH AND GABION MATERIALS 101 Project planning 101 Resources required 101 Work plan 102 Work site organization 103 Earthworks 103 Site preparation 103 Embankment construction 106 Rock-fill embankments 115 Filters and protections 116 Other control checks 118 Assembly and erection of gabions structures 120 Site preparation 120 Gabion construction 121 8. CONSTRUCTION AND CONTROL OF CONCRETE 129 Aggregates 129 Grading 129 Particle shape and texture 131 Contamination 131 Durability 131 Expansion and shrinkage 131 Relative density 132 Water 132 Quality of water 132 Water content 132 x Contents Mix design 133 Quality control 135 The slump test 135 Compressive strength tests on test cubes 136 Site practice 136 Storage 136 Batching 137 Mixing 137 Curing 137 9. MAINTENANCE FOR SMALL DAMS 139 Local people’s participation 139 Management 139 Maintenance organization 140 Maintenance of earth structures 140 Vegetation growth 140 Rip-rap maintenance 140 Small repairs 141 Erosion 141 Repair of cracks 141 Maintenance of gabion structures 142 BIBLIOGRAPHY 145 ANNEX 1. CASE STUDY: HARA IRRIGATION SCHEME: RECONSTRUCTION OF HEADWORKS-GABION WEIR 147 ANNEX 2. TORRENT CONTROL WEIRS IN HAITI 161 ANNEX 3. HAND-MADE GABION MANUFACTURE 169 Small dams and weirs in earth and gabion materials xi List of figures 1.1 Typical cross-section of an earth embankment dam 6 1.2 Zoned earthfill dam 7 1.3 Examples of spillway locations 7 1.4 Woven wire mesh 11 2.1 Watershed representation 14 2.2 Steps in defining the watershed 14 2.3 Watershed shape 15 2.4 Hypsometric curve 16 2.5 Examples of watershed drainage patterns 16 2.6 Discharge hydrograph 19 3.1 Example of a trial pit log 27 3.2 A light cable percussion boring rig 28 3.3 Examples of cable tools used in percussion boring 29 3.4 Examples of undisturbed sampling and standard penetration test equipment 30 3.5 Example of drilling bits and core barrels for use in rotary core drilling 31 3.6 Example of a borehole log 32 3.7 Block sampling from a trial pit 34 3.8 Falling and rising head tests in boreholes 38 3.9 Packer test 39 4.1 USCS soil classification chart 44 4.2 Data form for visual classification of soil samples 45 4.3 Terms in the soil description for angularity of coarse particles 51 4.4 Typical