Advantages of mole drainage
Mole drainage is a method of draining clay soils using close-spaced subsurface channels. It is used on soils that experience regular waterlogging from irrigation or high rainfall. The department recommends that mole drainage be considered is part of an integrated water and salinity management program, especially where these conditions are experienced in WA.
Heavy soils with low rates of soil-water movement, such as those found in the South-West Irrigation Area (SWIA), are susceptible to being saturated in the root zone of pasture or crop plants. Surface water drains are often impractical in these situations, whereas properly installed mole drains in heavy clay soil are a cost-effective way of rapidly removing excess water from the root zone.
Notifying the Commissioner is required
For any proposed land drainage, it is a legal requirement to submit a Notice of Intent to Drain or Pump Water (NOID) to the Commissioner of Soil and Land Conservation. This must be lodged at least 90 days before draining surface or subsurface water onto other land, into other water or into a watercourse. It is an offence to proceed without notification, and penalties and prosecutions may be applied. Contact the Commissioner if you are unsure of your obligations.
Notice of intent to drainMole drainage at a glance, Key points on mole-drainage, Key takeaways for mole-draining
- Effective in soils with high clay content of > 30% at 200–700 mm depth.
- Best suited to laser-levelled paddocks with surface gradients < 2%.
- Moling depth should be 400–500 mm.
- Mole spacing should be about 2 m, and not more than 6 m.
- Install collector pipe drainage system across paddock at 50–60 m intervals.
- Make sure adequate discharge point is available.
- Install late autumn or late spring, when soils are moist at moling depth but drier at surface.
- Common permeable backfill in SWIA: Blue metal (crushed granite, gneiss or dolerite).
- Relatively cheap and easy to establish, with life of up to 15 years.
- Best application machines in SWIA: floating beam and scrubbing beam machines.
- Used for 50+ years worldwide; common in SWIA.
- Survey and plan site to fit into an overall drainage strategy.
Stable soils for mole drainage:
Good moling soils that result in mole stability:
- high clay content (30% or more) at 200–700 mm depth
- texture: clays, silty clay loams or clay loams
- most irrigation soils in the SWIA meet this requirement
- yellow, orange and light brown clays and clay loams of SWIA are stable and suitable for mole drainage.
Soil chemical factors affecting mole stability:
- salinity, sodicity and clay type also determine soil stability
- high levels of salinity and sodicity (excess of sodium ions relative to chlorine ions) make soils unstable for mole draining
- salinity and sodicity generally increase with soil depth, so best to keep mole drainage system as shallow as possible on saline soils.
Reliable and practical indications of soil stability:
- observe stability of open drains near area to be moled
- drains maintaining shape and retaining vertical sides over a number of years since construction, likely to indicate stable soil
- drains slumped badly after installation or requiring regular cleaning, likely to indicate soil is unstable and mole drainage unlikely to be practical on that paddock.
Unstable soils and hardpans in SWIA:
- heavy blue, grey and dark brown ‘Bungham’ clays are often unstable and unsuitable for mole
- gravel-filled mole drains are more expensive although their use to date is not well documented
- widespread iron and silica-rich clay hardpans in SWIA soils vary in texture and hardness from rocky to a very hard, dense clay.
- occurring in broad ‘waves’ underground at depths of 400–2000 mm, they can seriously affect moling operations.
- intersecting hardpans (moling too deeply) may result in mole plough riding along top of layers (causing uneven grades and unstable moles) or require a far greater horsepower and traction (for example, a bulldozer) r to pull implement.
- avoid these problems in most areas by aiming to mole at less than 500 mm depth.
System design
For mole drainage to work effectively, your site should undergo a complete and accurate drainage survey, and a whole-farm drainage plan is recommended.
The system design should include outlets to drain completely, and measures to prevent water ‘backing up’ during storm events. Options are presented for mole outlets at the bottom end of the paddock and managing backup up.
Drop the moling tool into the tail drain and mole away from it, effectively using the tail drain as a collector drain. Often the tail drain will need to be deepened to at least 200 mm greater than moling depth. Each mole also needs a length of PVC pipe inserted in the end to prevent erosion and blockage by debris or rodents
A better option is to install a collector pipe 10 m from the end of the bay and mole from soil surface near the tail drain, into the collector and on up the paddock. Moles are likely to be more stable near the end of the bay, and this provides additional deep drainage to the wettest part of the paddock.
Where collector pipes cannot discharge straight into a deep open drain (side of paddock), the options are to dig an open drain, or plumb them into a 150 mm pipe and convey to main outfall point.
Blockage of collector pipes with iron precipitate from drainage water is a potential problem in the SWIA due to high iron concentrations in shallow groundwater. To date, there has been no evidence of blockages (systems of four years old). However, because the possibility exists, it’s wise to incorporate permanent inspection points at ends of lines, and make lines no longer than 300 m. This allows for cleaning of pipe using high pressure air and water scrubbing system (maximum available length 300 m).
Mole plough considerations
Stable mole channels are critical for the success of any system. Channels must have an even, gentle grade, with minimal grade changes that create areas in the channel where water can pond – leading to premature collapse and channel failure.
With a three-point linkage machine, any surface undulations or loss of traction that cause the tractor to pitch, are directly transferred to the mole bullet. This causes sudden grade changes in the channel and variations in vertical soil pressure at the mole point. The result is very unstable mole channels that will collapse soon after installation.
Machines with the best application in the SWIA are floating beam and scrubbing beam machines. With both the mole point is buffered from ground surface imperfections.
The floating beam machine relies on a combination of tow-hitch geometry and depth wheels to control the moling depth. It requires less draught than the scrubbing beam machine, but experience shows it has the major disadvantage of not maintaining depth in hard or variable soil conditions. This is particularly important under SWIA conditions where hardpan clay layers are prevalent.
The scrubbing beam machine uses the full length of the beam, sliding on the ground surface, as both the depth and grade control mechanism. Its major advantage is the geometry prevents any depth variation due to soil conditions and it is usually unaffected by hardpans. It requires more draught than the floating beam and will usually require a coulter mounted at the front of the beam to slice heavy grass thatch (for example, kikuyu). It is generally the best machine for SWIA conditions.
Mole leg, foot and expander design
The configuration of the moling tool is crucial in achieving stable mole channels.
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Size and function |
Configuration and purpose |
Important notes |
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The size of the mole bullet and expander required is governed by the depth of moling.
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A moling depth of 400–500 mm is optimum for most conditions, with a mole bullet of 65mm diameter, and with a 75 mm diameter expander.
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The depth of moling must be at least 6 times the diameter of the mole otherwise poor channel compression will result as evidenced by excessive surface ‘heave’. This is the ‘critical depth’ for moling.
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Function of the expander
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To smooth and gently compress the mole channel. |
The expander only needs to be 10 mm larger in diameter than the mole bullet. It is important to note that a larger diameter, deeper mole does not mean a better drainage effect or a longer lasting mole and can often mean a less stable mole is produced after a more expensive installation.
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Function of the leg
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To support the bullet without flexing and to create the required network of soil fissures. |
The leg should be 200 mm wide and a maximum of 20 mm thick. Any thicker and the leg slot created will be too large, allowing erosion in the leg slot and filling and collapse of the mole channel.
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Leg slot closing wedge where ploughing or reseeding will not be undertaken after moling.
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The ‘wedge’ destroys the vertical continuity of the leg slot and leaves the surface soil in a better condition for compaction. The closing wedge is required to prevent access of surface water directly into the mole channel.
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It is crucial that the wedge is mounted behind the expander at 150–200 mm depth below the surface. Surface compaction over the mole using the tractor wheels is recommended (Note: this can be done on the trip back if the paddock is being moled in one direction).
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Collector drains
- For most laser levelled paddocks in SWIA, a mole drain should be no more than 50–60 m long before it has an outfall.
- Collector drains installed across the paddock at 50–60 m intervals provide an effective network to collect and discharge mole drain water.
- The pipe used in collectors can be 80 mm or 100 mm diameter perforated agricultural pipe. (Pipe size governs trench width.)
- As standard, we recommend the thickness of permeable fill in the trench should be twice the diameter of the pipe. The permeable fill should ideally be crushed rock screenings of around 10 mm size. Blue metal (crushed granite, gneiss or dolerite) is commonly used as the permeable backfill in SWIA.
- The blue metal should be brought to within 300 mm of the surface to ensure the mole drains intersect this layer.
- The top 300 mm should be backfilled with soil and compacted to minimise irrigation water loss into the drains.
- The collector trenches should be dug using laser-controlled equipment for accuracy, as the grades of the collectors (across the paddock) are often very low (less than 0.1%).
- Collector depths will generally only be 900–1000 mm in the SWIA.
FAQs
Mole drainage requires the construction of a series of stable, unlined soil 'pipes’ or moles of even and low grade down the paddock. Creating the mole channel results in the formation of a series of fine fissures or cracks in the soil which provide the major flow paths for soil water to move into the mole channels and then out into the drainage system.
The moling operation itself is relatively inexpensive. Rates of 1 hectare per hour are common using a 120–150 hp tractor with moles installed at 1.5–2 m spacing. The large expense comes with the installation of the collector pipe system by a contractor using specialist equipment. This will cost more than $1,500/ha (50 m spacings between collectors).
Partial-paddock mole drainage systems are a way of reducing this cost and testing the effectiveness of mole drainage. Often the most salt-waterlogging affected part of a paddock is the 50–100 m up from the tail drain. For example, mole drain from a deepened tail drain to 50–100 m up the paddock or install only the bottom one or two collectors of a system initially. Also, using ground geophysics ($10–$20/ha) to find hot spots may reduce the overall project cost.
Deciding which areas should have priority for drainage can be tricky. There are strong arguments for spending money on draining the potentially more productive (lower salinity) areas before the areas that are more saline, waterlogged and which may take longer to become productive. However, many farmers prefer to tackle the most saline areas first. Salinity/productivity mapping services available from private consultants, using the EM38/GPS system is an excellent way of characterising the farm’s natural resource base and assigning priority to future drainage areas.
Mole drains are most effective in these area and instances:
- where water logging is a common occurrence in high rainfall or irrigation areas.
- where surface gradients are less than 2%
- where clays have low sodicity (low dispersion). Dispersive (sodic) soils in Western Australia
- where there are safe discharge points.
Sufficient soil moisture is critical for establishing mole drains. Moling should be done when the soil, at mole depth, is as close to its upper plastic limit as possible. That means when it’s easy to shape or mould by hand. but not so wet that it loses its shape easily. There is often a trade-off between soil surface moisture and traction, and soil moisture at depth. Ideally, the soil needs to be moist at moling depth and quite dry and friable near the surface. This is also important for the development of the soil cracks and fissures above the mole channel.
The best time to mole is in late autumn and early winter, after 25–50 mm of opening rain, but before the full onset of winter. Alternatively, early to mid-spring moling (just after hay season) is suitable. After installation, the moles should be left for as long as possible (1–2 weeks is ideal) to settle and ‘cure’ before they receive large flows of either rainfall or irrigation water. Do not mole when the soil is waterlogged.
Because mole and collector systems are often very efficient, some systems may ‘over drain’ during summer irrigation. If this occurs, installing reducers to the collector outfalls is recommended (for example, 80 mm reduced to 12 mm) during the irrigation season. It is not recommended that the outfalls be blocked off completely because consistent flooding of the mole channels can reduce their stability and longevity. It is also an advantage to remove salt and waterlogging throughout the whole year.
Several factors will influence the success of a mole drainage system and the longevity of the mole channels. When installed correctly and managed well, a mole drain in the SWIA region may last up before remoling is required.
Mole drainage is a completely different operation to deep ripping. Deep ripping is aimed at loosening, fissuring and rearranging compacted soils to allow better infiltration of water and improved root growth. Soil surface ‘heave’ is often a desirable feature of deep ripping (as it indicates good subsurface disruption), but when mole draining it indicates poor technique and results in poor performance of the system.