Fig. 8.4 Average depth, bund internal slope and freeboard.
Cut major factor. Here the aim is usually to withstand 10-year floods (according to local experience), and thus the bunds may be elevated 1 m or more above surrounding levels. Internally, depths may be defined by the relative areas of higher rice-growing areas, which are usually at about the original land level. The excavated areas, typically 0.3 to 1.0 m below this, occupy 15 to 40% of the interior surface.
The slope of bunds is determined according to soil type, relative elevations and practical operational factors. For highly stable soils, the slope of internal embankments is typically at a ratio of 2.5:1 (horizontal:vertical). For unstable soils, the minimum slope should be closer to 4:1. According to Wolf (1994), the best internal slope for prawn pond bunds is 3:1, as this facilitates seine harvesting and avoids erosion (Fig. 8.4). The shallowest regions in the ponds are preferentially occupied by juveniles and recently moulted prawns, while adults occupy deepest areas (Correia & Cavalcanti 1998). Therefore, bunds with a shallower slope at their base may allow better prawn distribution. For exterior bunds the minimum ratio is 1.5:1 for stable soils, though they are usually more gently sloped, typically 2.5 to 3.5:1. Where the external bund of one pond is the internal bund of another neighbouring pond, the slope should be 3:1. In complete contrast to these criteria, small artisanal ponds, such as ghers, are commonly built on floodplain areas whose soils typically have a high content of relatively stiff, cohesive and impermeable clay soils. Simple bunds of up to aboutg 0.5 to 0.8 m maybe built with slopes as steep as 0.1:1, almost vertical, while larger bunds for flood protection are typically set at 0.5 to 1:1. All of these bunds require periodic maintenance and rebuilding, particularly as the clay soils crack widely if dried out, for example during dry seasonal conditions. However, they are generally acceptable for their purpose.
The crest or top width of pond bunds is usually related to their height (Fig. 8.5), and is also defined by the access required. A further factor may be the need to build water control structures such as sluices, where larger and wider bunds may increase construction costs. Crest widths of 3.5 to 4.0 m, suitable to provide a circulation network for vehicles and machinery, are usually required in large farms.
Fig. 8.5 Minimum bund crest. Key: w = width; h = height.
In non-mechanised farms, it is common only for downstream bunds to be wider, thus forming the main access for vehicles. In these cases, lateral bunds are narrower and constitute secondary access, allowing two-wheel vehicles, employees and traction animals to circulate. In very small ponds with clay soils, inner bunds in particular may only be wide enough for walking space. The seepage line of the water from the inner pond level to the outer ground surface is also important; if the overall width of the bund is insufficient, seepage losses may be unacceptably high and, more seriously, the bund may be weakened. For this reason the crest may be widened, or the exterior slope decreased. Alternatively, building a core of less permeable material within the bund will restrict seepage and allow narrower bunds.
Even for small projects, a certain amount of planning and organisation is required before any physical construction steps can proceed. Key points include:
• defining layouts, determining whether construction is to proceed in one or more stages, and defining key target dates for completion, particularly if seasonal constraints exist;
• assessing costs, organising finances for the development of the project, and confirming that the project will be viable, based on the timing and potential level of returns;
• adjusting project plans if necessary to meet timing and financial targets;
• producing the necessary documents and obtaining permissions for site development, for water supplies, power, etc.;
• defining how the project is to be developed, e.g. own build, contracted out or with specific sub-contractors for specialised works, and defining the construction period;
• setting up a project management/supervisory function, and assembling or organising a suitable workforce for own construction work; and
• organising bidding and contract documents, assessing and selecting contractors, and ensuring that finance is available and that the site is ready as required for the contractor to proceed.
Before construction starts, initial access ways need to be made, and the land should be cleared and marked out, with external boundaries clearly defined. Site areas for storage of construction materials and for temporary placement of excavated soils should also be identified. For larger-scale projects, a temporary site office/store and a lockable compound may be required for secure storage of tools, equipment and more valuable materials, and for staff and worker facilities. Shipping containers can be conveniently used for this. Temporary power, freshwater and toilet facilities may also be required. However, if the project is small and simple, and the duration of construction is short, these may not be needed.
A primary site datum point should also be established; this should be a permanent marker, at least during the construction phase, and can either be a prominent and durable natural or man-made feature, or can be fixed once the site is cleared. This provides a reference for distance and elevation for all the subsequent works on the site, and should be positioned so that it is as accessible as possible by line of sight to all parts of the project area. A number of secondary datum points can be referenced to this for triangulation, usually once the site is cleared; where the site has a complex topography, these can also be used to extend the line of sight around major obstacles.
Before clearing the land, the major features ofthe project (roadways, water supply routes, pond areas and building foundations) need to be marked in outline, using simple marker poles or coloured cloth strips tied to trees or shrubs, and decisions should be made about which areas need to be stripped of surface vegetation (trees, shrubs, grasses and other plants) and topsoil. Basic cover or topsoil is typically present as an organic layer of about 10 cm depth, with a mixture of soils, pebbles, decayed vegetable matter and plant or shrub roots. In construction areas this layer should be scraped clear, as it cannot be used for normal earthworks. However, the topsoil can be kept for later use to cover the upper surface ofpondbunds, roadsides andcanalbanks and for general post-construction landscaping. Small areas of land can be cleared by hand using agricultural implements, stump pullers and light tractors. For larger areas, scraper tractors can be employed, with a typical capacity of 1000 to 2000 m2 per 3- to 5-hour work cycle (Wolf 1994).
After scraping, the land should be marked out with wooden stakes to delimit the excavation and construction area. A series of level marks (wooden 'T' or 'H' frames) can also be set out to define key levels and act as local references for excavation, construction or finishing levels. For both horizontal and vertical markers, twine can be stretched between consecutive points to define position and level lines. Forponds, usuallythe main construction feature, Bard etal. (1974) recommended placing stakes within the area marked to build the ponds to form a series of equidistant (usually rectangular) shapes. A line should level the top of the stakes to define the bund crest and depth measurements (Fig. 8.6). Excavation cuts can then be made alongside the position lines. FAO (1992) shows how an outer line frame can be used to define the outside of the bund, then consecutively the outer bund crest, the inner crest and the inner base of the pond itself. Similarly, position and level lines can be set out for canals, roadways and foundations, and trench positions for pipes, cables, etc., can be identified.
As a broad guideline, major earthworks using larger machinery types are carried out first, followed by smaller and more localised works, involving smaller machinery or manual works. The project is built from the lower levels upwards, and so drainage structures, submerged pipes and power lines, and building foundations are usually placed first, pond bunds, roadways, lower-level building works and surface supply lines constructed next, then above-ground building works, and finally various superstructure elements such as fencing, elevated power and lighting systems, pond equipment, etc. However, this order may be changed, depending on whether there are construction risks (e.g. of
heavy machinery damaging newly laid power cables, or according to sub-contractor timetables), or if some parts of the project are to be completed before others.
To reduce earth moving and to ensure that bunds are properly built up, excavation cuts are alternated with earth moving and compaction. According to Wolf (1994), in sloping ground of 2 to 5% gradient, which requires the least amount of earth moving, construction shouldbe made in two stages. The first begins with cuts in the highest area, from which the soil is spread on to the lowest area, forming a flat and well-compacted platform (Fig. 8.7a). In the second step, new cuts are formed, with earth moving to build the bunds (Fig. 8.7b). In slightly sloped land, less than one-third of the total depth of the ponds originates from excavation, generating enough soil to construct the bunds (Fig. 8.8). FAO (1992) provides a more detailed analysis of these relationships, according to land slope, pond size and depth, and bund width and gradient.
The bunds are constructed by building up successive layers of soil, with thicknesses varying from 15 to 20 cm. Each layer is compacted by a tractor driving over it about four to five times (Wheaton 1977) or, preferably, by using a roller. In drier conditions, the soil should also be moistened to improve its compaction. This procedure can also be used to compact the pond bottom in order to decrease seepage. If pond soils are more permeable, it may be necessary to construct a key trench and an impermeable core. In this case, the trench line needs to be defined, and the trench cut by hand or with a narrow bucket excavator. The core material is then packed into place, and the ordinary soil placed and compacted around it.
Fig. 8.9 Nursery ponds being constructed in a Brazilian Macrobrachium farm. (Reproduced with permission of Julio Vicente Lombardi.)
Fig. 8.9 Nursery ponds being constructed in a Brazilian Macrobrachium farm. (Reproduced with permission of Julio Vicente Lombardi.)
Soil removed in excavation undergoes natural increases in volume due to the decompression of soil particles. This phenomenon, which varies with soil type, moisture content and previous compaction history, is known as swell or blistering. A correction factor should therefore be applied in calculating the volumes of earth to be moved. Wolf (1994) suggested a coefficient of 1.43, while Correia & Cavalcanti (1998) considered 1.3 to be appropriate. However, though it is difficult to achieve the original volume after compaction, the final volume may be 80 to 90% of the expanded, loose volume, or around 5 to 10% greater than the original.
As noted, several different types of heavy machine may be used for soil excavation and compaction in larger pond developments. Excavators, scrapers, shovellers and planers are used in excavation, scraping and levelling, as well as earth movement and placing (Fig. 8.9). For larger volumes and longer distances, dump trucks may also be useful. In difficult site conditions (e.g. where excavated zones are seasonally waterlogged) a Caterpillar-tracked bucket excavator maybe useful, in some cases operated from a suitably stable earth or rock platform, with an operating radius defined by the size of the crane arm. Drainage pumps may also be required to de-water construction areas periodically. Standard sheepsfoot or other types of rollers, and Caterpillar-tracked tractors are the most appropriate for compacting bunds and pond bases (Wolf 1994; Correia & Cavalcanti 1998). Tables 8.2 and 8.3 show calculations for earth moving and machine yields under a range of slope conditions and development areas.
For smaller ponds, and particularly in heavier clay soils, manual construction may also be feasible where labour costs are acceptable. Here, excavated soils can be loosened with hoes or cutting tools and loaded into barrows or cribs, and then placed and compacted by hand. Clay soils can be scooped by hand or cut as blocks, and passed hand to hand, to be placed into bund structures. Techniques for building simple earthen ponds are also detailed in FAO (1995).
The features of water supply systems, and options for their development, have been discussed earlier. For gravity supplies, concrete, brick or timber sluice gates can be used to control water intake, flow being regulated by stop-logs placed in grooves inside the sluice gates. Water can be transported to the ponds by means of open channels or pipes (Fig. 8.10). Levels of pipes or channels supplying them need to be carefully checked to ensure that flow conditions are as intended. The position and level of supply weirs and sluice gates need to be co-ordinated with this to ensure that relative water levels are appropriate during all periods of operation. If possible, the construction of water intake structures should be done during low flow periods; in some cases it may be necessary to divert water, or to place temporary protection (e.g. with sandbags or a simple cofferdam) around the construction area to allow the work to be carried out in at least partially dry conditions. Alternatively, it may be possible to build below water and to use pre-assembled structures which can be set into place. It is usually convenient to establish the sluice structures first, and then to link these with the supply channels or pipes.
As noted earlier, channels are usually cheaper than pipes. They may also help to increase water temperatures and DO2 levels (Valenti 1996), but they require more maintenance.
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