This article is a continuation of “Subsurface Pipe Drainage” please read that section first.
Design considerations:
1. Drain Depth
Uniform permeable soils suit deeper drain placement and wider drain spacing, which reduces cost. Pipe depths are typically a minimum of 700 to 1200 mm for drainage in soils growing pasture. Machines specifically constructed for trenching and pipe laying are used solely by experienced contractors in Australia and can trench to a depth of about 2 meters.
In layered (Duplex) soils, an impeding layer at some depth, such as a clay subsoil or coffee rock layer, causes the water table to build up. This is referred to as a “perched” water table. The impeding layer is situated at 300 to 900 mm depth. In these soil types, drains are usually installed just into the clay layer or some distance below the coffee rock layer and back filled with a permeable material to well above the impeding layer.
The depth of the open main drain or creek (outfall) used to dispose of the water from the pipe system usually determines the depth of the tile drains. In some cases it will be better to use widely spaced deep pipes that drain to a sump. Although water must then be pumped to the outfall, this can be more economic than using closer spaced, shallower pipe drains.
2. Drain Spacing
Drain spacing depends on the hydraulic conductivity of the soil, crop or pasture requirements and the drainage coefficient.
Hydraulic conductivity of the soil
Hydraulic conductivity is the speed at which the water moves through the soil. The more permeable the soil, the more rapid the water moves through that soil, and this allows much wider drain pipe spacing. The minimum hydraulic conductivity suitable for tile drains needs to be about 40 to 60 mm per hour. Conductivity rates lower than this requires pipe spacing to be too close, and is therefore uneconomic.
Crop or pasture requirements
Effect of drain depth on pipe spacing
Orchards for example need to keep the water table lower than do crops or pastures. For pasture the water table should be maintained at least 300 mm below the surface, mid way between drains, to enable grazing and tractor work on dairy farms without causing soil or pasture damage.
Design drainage rate
The design drainage rate is expressed as the amount of water the drain system has to remove over 24 hours to keep the water table below 300 mm (for pasture) midway between drains. Different plants can tolerate water logging to different extents and farmers also require paddocks to be reusable soon after rainfall events. This dictates the removal rate of water from the soil profile after a rain and it’s governed rainfall intensity,(duration and amount), soil type and pipe spacing and depth will influence this figure. Limited trial work in the dairy areas of south west Victoria shows that 7 to 10 mm per day should be used as the drainage coefficient.
3. Pipe diameter
The pipe diameter is dependent on the volume of water to be removed per unit area per day and the gradient. Type of drain such as pipe only, compared to collector pipe system will also influence pipe diameter size. Tables are available from experts to help choose this information.
In pasture situations, most pipe laterals are slotted corrugated polythene pipes of 65 to 80 mm diameter. The main drains to outfalls are usually 150 to 225 mm diameter PVC pipes. However, other pipe sizes and types are available so seek further advice from subsurface drainage contractors. Laterals are the smaller diameter pipes used to collect the water from the area to be drained. These then deliver the drainage water to larger pipes or main drains. It is important to design the drainage system to allow for future extensions when circumstances (e.g. financial, land purchase next door) allow so that the pipe diameter is large enough to cope with the increased flow from new extensions.
4. Pipe length
The maximum length of main pipe lines is determined by the design drainage rate, the area to be drained and its capacity according to the gradient used. The steeper the gradient, the faster the drainage rate. Pipes should not be installed at excessive gradients as fast flowing water may create a suction effect on the surrounding soil and create erosion.
The length of the laterals depends on slope and length of the area to be drained, but a rough rule of thumb is that one length of small diameter pipe drains into the next diameter size up. Realistically, in most dairying areas, the smaller pipe diameter would drain into larger diameter main drains (100 to 160 mm) directly into outfalls
5. Backfill
Permeable backfill should be placed around the pipe. This increases the permeability in the vicinity of the pipe, facilitating much faster removal of the water. It also acts as a filter to prevent fine sand and silt from entering the drains.
Most commonly used backfill are coarse washed sand, graded or pea gravels and blue rock screenings of small diameter. Due to its plentiful supply, scoria (30 to 40 mm diameter) is commonly used in the Western districts of Victoria.
When drainage pipes are installed in the impermeable layer such as clay, the permeable backfill should extend well above the impermeable layer.
Collector pipe drains for mole drains should have screened gravel or scoria (12 to 20 mm diameter) to allow rapid water infiltration into the pipe. Sourcing good quality clean backfill close to the job will keep costs to a minimum.
Filter socks around slotted corrugated pipe are available for some situations to prevent very fine particles from entering the pipes. However, there may be the potential for the fine particles to block the filter itself so consult an expert to determine their suitability.
6. Gradient
Pipe gradient is determined by the layout, outlet depth and drain depth.
Minimum gradients can be as low as 0.1 % but the ground surface needs to be perfect with no irregularities. More often, 0.25 % should be the minimum gradient. Laser graded trenches give more consistent grades and are absolutely necessary for situations with low gradients. Lateral pipes should cut across the slope to maximise water interception.
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