Mole drainage, on the right soil type and when installed correctly, can help reduce water logging problems substantially. This Ag note aims to help farmers install effective long lasting mole drains in suitable soils.
Introduction
Wet soils in winter are a common problem in the agricultural industry. While surface drainage can remove excess surface water, it is the soil profile itself that needs to be drained, in many situations. This allows the pastures and crops to reach their potential production and stock damage by treading and compaction to be reduced.
Mole drainage is widely used in New Zealand and the United Kingdom in heavy soils to improve productivity of pastures and crops. Mole drainage was popular with dairy farmers in the 1960’s in Australia but these often failed due to reasons now more fully understood. Recent research has resulted in robust guidelines for installing mole drains so that they are more effective for longer, with a greatly reduced failure rate.
What is a mole drain?
Mole drains are unlined channels formed in clay subsoil by pulling a ripper blade (or leg) with a cylindrical foot (or torpedo) attached on the bottom through the subsoil. A plug (or expander) is often used to help compact the channel wall. The foot is usually chisel pointed and the entire point is hard faced by welding. More frequent hard facing of the underside will increase the effective life of the torpedo. The beam is the main rail which carries the leg and torpedo.
Mole drains are used in heavy soils where a clay subsoil near moling depth (400 to 600 cm) prevents downward movement of ground water. Mole drains are a more sophisticated drainage system than open drains. Mole drains do not drain groundwater but removes water as it enters from the ground surface.
Mole drains over a collector pipe system
In soils where mole drains would have a very short life span due to sandy or stoney areas, in heavy clay type soils or lengths greater than about 80 metres to reach an outfall, a ‘mole drain over collector pipe system’ is recommended. This requires the installation of slotted subsurface drainage pipes at approximately 60 to 100 m apart, across which mole drains are pulled. This system is useful where soil may contain stones and/or sandy patches in the profile, at drainage depth, which could collapse when moled. The relatively close spacing of the pipes and shorter mole drain lengths will minimise the area affected by the resultant poor drainage when the mole drain collapses.
Permeable backfill such as washed sand, small screenings or small diameter ‘pea” gravel is placed (backfilled) on top of the slotted pipe in the base of the trench. The collector pipe will have been installed using a laser to ensure a constant fall in the pipe to the outfall. Depending on the clay content and its depth, this backfill must reach at least 150 mm above the moling depth so that the water moves into the backfill via the mole channel.
Mole drains are then installed at an angle (often 70 to 900) to the direction of the pipes. Excess ground water flows into and along the mole drains, then drains into the porous backfill above the pipes, and is then quickly removed to outfalls via the subsurface collector pipes.
Types of mole ploughs
Well designed mole draining ploughs allow the torpedo, when properly set, to maintain itself at the set depth almost irrespective of small ground surface irregularities. The front of the mole plough usually has skids to support the front of the beam, but the body of the beam floats clear of the ground. This allows the blade and torpedo some degree of movement to produce a smooth gradient within the limits of the under-beam clearance.
Trailed mole ploughs
Light Mole Drain Plough
These require higher horsepower tractors than other mole ploughs because this design actually scrubs the ground when being pulled. However they produce more even grades than three point linkage ploughs because of the longer beam (greater than 1.35 m). They are more difficult to manoeuvre compared to the linkage plough but are now being replaced by semi tractor-mounted designs.
Semi tractor-mounted mole ploughs
Although mounted on the tractor three-point linkage, these act as trailing ploughs when working and even out the mole gradient despite ground variation. The longer beam, coupled to a chain with a floating top link, allows a more even mole channel to be formed.
Fully tractor-mounted mole plough
The blade is mounted directly to the three-point linkage, so this type of plough is inexpensive to build and convenient to use. This design of mole plough produces more uneven grades than the trailed ploughs because of the short beam and is one reason for many mole drain failures.
The angle of the torpedo is influenced by the draught control of the tractor so its not free to find its own level. Also being mounted so close to the rear wheels, the torpedo forms a mole channel as dictated by the ground surface undulations.
Ideally the torpedo should be allowed to “float” with no pressure control used to make it go deeper. When traction is lost the linkage arms lift the mole plough to reduce downward pressure and thereby regaining traction.
Automatic draught control is used on most tractors to improve traction. The fully tractor-mounted mole ploughs are better suited for smaller tractors and small areas but are only suitable for even ground surfaces.
Gravel mole ploughs
Gravel mole plough.
Gravel mole ploughs incorporate a hopper to allow finely graded gravel to fall into the mole channel. These ploughs have been used successfully in the UK in heavy soils that cannot hold “normal” mole drains.
Experimental results from north east Victoria and Gippsland show they have promise on unstable clay soils, but are expensive because of the amount of gravel and close spacing needed. Unfortunately very few of these machines exist in Australia.
Tractor power requirements for installing mole drains
Four-wheel drive tractors of approximately 60 to 80 kw minimum power are recommended for installing moles. Crawler tractors of lesser power (above 45 kw) are also suitable. The deeper the mole drain to be installed, and drier the soil, the greater the tractor power required.
Speed of mole ploughing
Recommended speed is 2 to 4 km/hr. If pulled too fast, say 5 to 6 km/hr, the plug can disrupt or tear the walls of the channel resulting in failure within one to two years.
Please continue reading in the article “Subsurface Mole Drainage Sole and Design”
Acknowledgements
The previous version of this Ag note was developed by David Hopkins and published in October 2002
The updated version from the Department of Primary Industries Victoria website was developed by Frank Mickan, Farm Services Victoria/Dairy Ellinbank.
