Your Dam Your Responsibility (PDF~1.2MB)

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5 Construction of a <strong>Dam</strong> Mixing topsoil with the bank material to save costs or because of convenience is asking for trouble! The organic matter in the topsoil will decay in due course, causing leakage paths to develop later, and may even lead to difficulty in obtaining adequate impermeability in the embankment in the short term. A core (cut-off) trench should be excavated along the centreline of the bank to provide good protection against under-bank leakage. The cut-off trench should extend the full length of the bank including up the abutments. It must be wide enough to allow the construction equipment to achieve the required standard of compaction; its depth will depend on site soil conditions. In most cases, it is not difficult to find suitable foundation materials relatively close to the surface, if unsuitable upper soils (including topsoil) are first removed. The founding material must be sufficiently stiff and impermeable, and must extend to sufficient depth to allow the storage behind the dam to be retained without significant leakage, but also to avoid any appreciable settlement of the constructed embankment. In some locations, upper soils below topsoil are soft, weak or contain gravel or other inclusions (such as calcareous materials) which can form a leakage path. This is one of the reasons for recommending the inclusion of a cut-off trench below the embankment, no matter how low, so that the content of the next layers below the embankment can be checked out before construction of the embankment begins. A large number of dams are still being constructed using a method whereby the topsoil is mixed into, or left under the clay material that forms the bank. These dams are more likely to leak or fail. This is because the organic matter in soil layers rots and allows seepage to occur. It is also because topsoil is likely to be more permeable than clay in the first place. 5.4 Embankment Materials and Construction 5.4.1 Selection of Materials Most dam walls (whether on or off waterways) are constructed of earth materials. If your dam is constructed from other materials, then much of the following sections will not apply to you, but expert advice will be required in any case. While construction of dams has to be practical, and is limited by available materials within economical distance, selection of appropriate materials is vital for dam safety and performance. This applies not only to the materials used in the embankment, but also to the materials on which it is founded, as noted above. The embankment must be capable of securely retaining water. This is generally done by (a) ensuring that the materials for the embankment contain sufficient clay, and (b) ensuring that the materials are adequately compacted. Piping failure due to excess rock, gravel, silt and no clay being used. 5 23
24 5 5 Construction of a <strong>Dam</strong> Typical embankment slip resulting from construction or material defects. In some cases where materials with sufficient clay content are scarce, the outer portions of the embankment can be constructed from less clayey material (as long as it has sufficient strength) while a central core is built with high clay content. However, construction of this kind (called ‘zoned construction’) requires attention to detail from an engineer, and in any case is generally confined to dams of more than a few metres in height. This is not only for economy, but also because of practical reasons, which limit the access of compaction equipment for lower dams. In no case is it recommended that a core of non-earth material (such as concrete) be attempted without engineering design and supervision. 5.4.2 Placement of Embankment Materials Embankment material placement should be done in horizontal layers of uniform thickness. Good compaction requires that each new layer is bonded onto the previous layer. To achieve the best results the material must be placed with sufficient moisture to make it pliable just before becoming crumbly and not so wet that it stains the hands or flows under compaction. The Optimum Moisture Content for compaction is normally determined by laboratory testing. For certain soils (especially dispersive clayey soils), it is critically important that the placement not be below the Standard Optimum Moisture Content as determined by laboratory tests. Note that such soils occur in many parts of Victoria. 5.4.3 Compaction of Embankment Materials The materials in the embankment must not only contain sufficient clay, they must also be adequately compacted, with each compacted layer bonded to the one underneath. Inadequate compaction can result from: • the use of material which is too wet or too dry (the acceptable margins are quite small). Note that it is generally easier to compact and handle material which is a little below its Standard Optimum Moisture Content, and that is commonly done with embankments for other purposes such as roads, but the consequence of doing so is a marked increase in the leakage potential of the finished product, hence it is not appropriate for dams; or • The use of tracked plant rather than compaction rollers designed for compacting and kneading the soil layers into each other is asking for trouble. Even one layer of inadequately compacted material in a bank can result in seepage leading to failure. It is very tempting, and very common, to use tracked plant for small dams because it is available and is much less costly, but again, this PHOTO COURTESY OF J WHITEWOOD Embankment failure PHOTOS COURTESY OF B LEWIS
Page 1 and 2: Irrigation and Commercial Farm Dams
Page 3 and 4: ii Published by the Victorian Gover
Page 5 and 6: iv Contents 6 Safety Surveillance .
Page 8 and 9: 1 Introduction It makes good busine
Page 10 and 11: 2 Risks Posed by Dams 2.1 Owner’s
Page 12 and 13: 2 Risks Posed by Dams The most comm
Page 14 and 15: 2 Risks Posed by Dams 2 Note: The r
Page 16 and 17: 3 Licensing Requirements for Farm D
Page 18 and 19: 3 Licensing Requirements for Farm D
Page 20 and 21: 4 Planning to Build a Dam It is imp
Page 22 and 23: 4 Planning to Build a Dam 4.3 Types
Page 24 and 25: 4 Planning to Build a Dam 4 2. Turk
Page 26 and 27: 4 Planning to Build a Dam becomes a
Page 28 and 29: 4 Planning to Build a Dam 4.6.2 Ext
Page 32 and 33: 5 Construction of a Dam is inviting
Page 34 and 35: 6 Safety Surveillance 6.1 Purpose o
Page 36 and 37: 6 Safety Surveillance QUICK VISUAL
Page 38 and 39: 6 Safety Surveillance 6 Warning: If
Page 40 and 41: 7 Operation and Maintenance of Dams
Page 48 and 49: 8 Dam Failure Emergency Section 8 i
Page 50 and 51: 8 Dam Failure Emergency Action 4. I
Page 52 and 53: 9 Decommissioning of Dams 9 The sit
Page 54 and 55: 10 Glossary of Terms Cut-off An imp
Page 56 and 57: 10 Glossary of Terms Monitoring Rec
Page 58 and 59: Publications directly relevant to f
Page 60 and 61: Appendix A A • probe - a ten-mill
Page 62 and 63: Appendix A Inspection Procedures Ge
Page 64 and 65: Appendix A A and the resistance to
Page 66 and 67: Appendix B 1. Seepage Problem Possi
Page 68 and 69: Appendix B Problem Possible Causes
Page 72 and 73: Appendix B 2. Cracking, Deformation
Page 76 and 77: Appendix B 3. Miscellaneous Problem
Page 78 and 79: Appendix B utlet Works and Concrete
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Appendix C The depth of the core tr
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Appendix C will be needed. In stabl
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Appendix C The width of the dischar
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Appendix C Other Pipe Details At th
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D Figure C-4: Design guide for a fa
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