The designer/engineer will need to supply the responsible authorities with a brief report outlining the design of the storage to obtain authorisation to commence construction of a dam. The report must contain the details shown below.
• Location of the proposed dam using the following parameters – Australian Map Grid co-ordinates, Australian Height Datum, Land Title description and distance from major landmarks or roads. It is desirable that other improvement sites are located within the proper section and subdivision as accurately as possible from existing map or field data.
• A brief description of the catchment area, its soil type, vegetation and topography. This may also contain legal land descriptions and the dam shown on the catchment or location map.
• Description of foundations and foundation material of the dam site, including the downstream area as far as the point of return of the spillway channel to the waterway.
• The nature and properties of the proposed embankment material (laboratory test- ing of material to be carried out by a registered company experienced in this field). • An indication of the embankment design methods and cases, and the factors of
safety adopted (including earthquake prone areas).
• Calculations of the design storm inflow (for at least a 1 per cent chance in 100 years of a recurrence interval storm) or such longer period as may have been specified. • Calculation of the earth or culvert spillway size to pass the design outflow, includ-
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spillways are usually 1 m deep with a freeboard allowance of 500 mm under design flood conditions) and an assessment of possible erosion effects under design conditions.
• Plans of the proposed embankment showing full details and dimensions, including spillway, capacity of storage, compensation pipe and trickle pipe, berms, beaching, crossing culverts.
• A brief outline of housing, utilities or other development downstream of the storage.
• Details of the proposed method of construction and equipment to be used, the degree of supervision to be provided, and the names of the supervising engineers. • When the dam has been constructed, the engineer will be required to certify that such works have been completed to design and to provide ‘as constructed’ details. • On request the engineer may provide a maintenance and safety surveillance
program for ongoing use.
5.3 HOW TO BUILD A DAM
Most of the element of risk can also be eliminated from the building of a dam by careful attention to several basic requirements. The builder should ensure that:
• the site selected is the most economical, that is, a natural depression is usually best because it considerably reduces the amount of excavation;
• the storage has low seepage losses and the soils present are capable of holding water – clay is usually the best (sometimes a sealer can be used with sandy clays, but this increases costs significantly);
• data relating to dam capacity, that is, the limitations of the drainage area are available;
• the foundations are watertight and capable of supporting the dam; • there is sufficient dam-building material; and
• 75 per cent of the drainage area is permanent pasture or woodland to prevent excessive silting.
The cost and effort expended on making sure that these conditions are met will, in the long run, save the builder both time and money.
5.4 STEPS IN CONSTRUCTING A DAM
Building a good dam embankment is rather like making a layer cake; it is a methodical process requiring exactly the right ingredients and careful attention to building each layer. However, unlike a cake, the aim of building a dam wall is to force air out from between soil pores, making the soil denser and less permeable. The following steps should be considered before constructing an embankment, as listed in Appendix 2.
5.4.1 Setting out
Setting out or pegging out transmits the information on the small dam plans to the ground. This information provide lines, grades, and elevations for construction of the
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works in accordance with the plans. Consideration should be given to the contractor’s requests to ensure optimum efficiency of the set out. The quality and appearance of the completed work will reflect the care and thoroughness exercised in the set out procedure (SCA, 1983).
The areas to be cleared usually will consist of the embankment site, the spillway site, the borrow pit, and the area over which water is to be stored. Each of these areas should be clearly marked with pegs. In the case of the storage area, the proposed waterline should be located accurately with a level. Clearing pegs should be at least 4 metres outside this waterline to give an indication of the area to be cleared around the edge of the storage.
The embankment is located by setting pegs along its centre-line at intervals of 10 metres or less. Usually this will have been done during the course of the initial planning survey. Fill and slope pegs are then set both upstream and downstream from the centre- line pegs marking the points of intersection of the side slopes with the ground surface. The earth spillway is located by pegging the centre-line and then setting cut and slope pegs along the lines of intersection of the spillway side slopes with the natural ground surface. The procedure for setting these pegs is the same as for pegging the embankment, except that they are cut pegs rather than fill pegs. They should be offset so they will remain in place for referral during construction.
Where suitable fill material must be obtained from a borrow pit, it is essential that this area be clearly demarcated. Cut pegs should be set to control excavation within the limits of suitable material and to drain the borrow pit.
A spillway or trickle pipe should be located by pegs offset from the centre-line of the pipe and placed at intervals not exceeding 10 metres. The spillway should be located where it will rest on a firm foundation. Cuts from the tops of the stakes to the grade elevation of the tube should be plainly marked on the pegs. The locations of the low level pipe and gate valve, cut-off collars, outlet structure, and other appurtenances should be identified by clearly marked, additional pegs.
Setting out of a dam site can vary from the pegging of the Full Supply Level (stor- age area) at each end of the embankment to placing pegs all along the toes of the embankment and spillway and the placing of offsets. For small dams offset centre-line pegs (steel posts), full supply pegs (coloured) and one upstream toe peg are normally sufficient.
Centre-line pegs should be placed well away from the construction area. Two pegs placed 6 metres apart at one end of the bank enable the centre-line to be easily established during construction. Batter peg distances can then be calculated simply from a longitudinal centre-line survey. It is important that all pegs and the bench mark (see Section 4.2.2) are shown on the dam to minimise confusion.
The distance of toe pegs from the dam centre-line is calculated from a longitudinal centre-line survey as follows:
Distance of toe peg= (Height at centre-line × batter slope) + (half of the crest width).
= (6 × 3) +3 2 = 19.5 m
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Figure 5.1 Location of toe pegs on slopes (Source: SCA, 1983).
This formula is only applicable to non-sloping sites. Adjustments must be made to correct for the slope, when measuring a site for a high dam or where steep slopes occur (for an example with 5 per cent slope [1:20, vertical : horizontal], see Figure 5.1).
In this example the correction is distance= 19.5
20 × 3 = 3 m (approx)