Trailed mole plough.
Soil suitability for mole drainage
Soils should have a clay content in the range of 30 to 35 % to ensure long lasting mole channels. A soil with a clay content above this range is likely to collapse quickly due to swelling or shrinking as they “wet up” and “dry down”. Clay gives the soil the ability to hold together and reduces the chances of it collapsing after the mole is pulled. Sand content should be less than 30 %. Ideally, the soil should be free of stones at the mole drain depth. If not, ensure the soil at moling depth is fairly wet so that stones are more easily pushed aside by the plough torpedo/plug.
Testing of suitability for mole draining
Although not always foolproof, two simple tests can indicate a soil’s suitability for mole drainage:
1. Test the soil at mole draining depth by rolling out a pencil thick rod and try to form a 40 to 50 mm diameter circle. If this can be done without crumbling or cracking then it may be suitable for mole draining. If the soil sample has dried out before conducting the test, mix in some water until the soil is of plasticine or soft putty consistency.
2. Another test is to find out if the soil at mole drain depth will slake or disperse. Small golf ball size balls of the soil are placed in distilled or rain water and observed over a day or two. If the water becomes cloudy and the ball soften or slumps, then this indicates a dispersive soil. These soils are prone to tunnel erosion and should not be mole drained. If these ball falls apart quickly it is has a tendency to slake.
Soils which tend to slake may be successfully gravel mole drained (actually a gravel slot), albeit expensively. Gypsum may be useful in dispersive soils to suppress clay dispersal, but is difficult to get the gypsum into the subsoil.
Cross section of mole plough effect in soil.
It is wise to tap into the experience of a subsurface drainage expert and, failing that, only mole drain a small area initially to gauge the likelihood of success in the long term.
When to mole drain
To achieve satisfactory results, the soil in the vicinity of the mole channel needs to be moist enough to form a channel. The soil must not be dry enough to crack and break up, and not soft enough to slough off and form a slurry. These conditions usually occur during the drying cycle in late spring to early summer.
The action of the mole plough is to form a channel in the area of the profile with a specific clay content, yet produce upward cracking of the soil profile immediately above the mole channel. These cracks allow water flow to the mole channel.
The upper section of the soil profile needs to be dry enough to form cracks at the time of mole draining and the ground surface dry enough to allow traction. If too moist then the cracks can “heal” over and reduce water intake. It is extremely preferable to have a warm drying period with no rain to allow the cracks to dry and the mole channel itself to harden.
Usually when the clay at mole draining depth has a moisture content of 20 to 25 %, conditions are satisfactory. Test the soil by kneading between the fingers. If a ribbon can be rolled out without it sticking to the fingers the moisture content is right.
Mole draining in autumn is not recommended, as the topsoil is wet and subsoil is too dry. The subsoil is difficult to mole and to dry out and it’s difficult to achieve the desirable depth. Mole channels will tend to slough off and fail.
Length and gradient
The length of mole drains varies with the slope (gradient) of the ground, surface evenness, soil textural changes in the profile and soil suitability. If mole channels are too long, the channels could remain wet at the lower end for too long, gradually softening over time and collapsing. If outfalls dictate long mole drains, then a collector pipe system is highly recommended.
Many mole drains have been installed at a length of 200 m and some up to 400 m have been pulled at Cooriemungle in south west Victoria. These have performed satisfactorily for a number of years, but would survive much longer if the length was reduced (80 to 100 m) because they would empty out quicker and are not likely to remain saturated.
Steeper gradients
Steeper gradients of greater than 3 % should enable relatively trouble free moles because minor surface undulations won’t cause blockages with negative gradients, and the risk from erosion is reduced. On steeper gradients moles should cross the direction of the main slope to help intercept surface runoff and to avoid the possibility of channel scour and erosion.
Lower gradients
However, in most situations surface slopes are usually below 3 % and drains should run parallel to the slope fall. The flatter the gradient, the more even the soil surface has to be and the use of collector pipe drains may be needed to achieve good results.
Depth
Optimum mole drain depth depends on soil type, and the conditions when moles are installed. Generally moles are pulled at 400 to 600 mm deep. For most pasture situations a torpedo diameter of 65 to 75 mm is recommended, although the channel diameter will be slightly less immediately after moling. A plug or expander attached to the rear of the torpedo will tend to maintain the channel size and smear the channel itself increasing its longevity.
Moles less than 400 mm deep are liable to be damaged by tractors and animals during or immediately after rain. However, if mole drain depth is slightly less than 400 mm, use a smaller diameter torpedo (35 to 45 mm) to reduce ground heave during mole draining. Torpedoes of 100 mm have been used but require substantially higher powered tractors and have not been proven to be an advantage over the smaller sized torpedoes.
A rule of thumb is that the expander to mole draining depth ratio is 1:7 i.e. a 70 mm diameter expander should have mole depth of approximately 490 mm.
Very heavy soils and those with high contents of suitable clay down to moling depth may benefit from its first mole draining at a shallower depth due to tractor limitations. Also as the soil structure improves over time subsequent moles can often be pulled at deeper depth.
Spacing
Spacing is designed to promote an interlacing effect of the fracture pattern from the moling and so encourage maximum water flow opportunity. Spacing between moles is usually about 2 m in dairy pastures. In less intensely grazed areas spacing may be up to 5 m apart, but performance falls off markedly with wider spacing.
Outfall
The drain outfall or outlet is the most important part of the system. If this fails the whole system fails.
Mole drains can discharge to open drains, into interceptor drains filled with gravel, or preferably a collector pipe system. The latter two systems are more expensive, but protect the mole outlets and the only maintenance required is at the tile outlet.
Open drain outlets should be fenced off from stock and kept clean so the outfall is above the drain water level. This prevents water backing up into the mole outlets, softening the clay channel and eventually causing them to collapse. Short lengths of plastic pipe inserted in the ends can protect them.
Another advantage of gravel filled interceptor drains is that moles can be pulled both ways, instead of the one way trip from open drains, speeding up the job.
Effective life
The effective life of a mole drain can be from hours (if done in the wrong conditions to 10 to 15 years (if done in the correct conditions). Moles in some soils have lasted longer. Even some soils that in theory shouldn’t support moles are still operating after 5 years.
Other factors.
Improved drainage not only takes away surplus water but takes away some nutrients as well. Fertiliser should not be applied immediately before rain events when run off is likely. Good management practices, such as not applying fertiliser within four days of predicted rain should reduce nutrient run-off to a minimum. Consider changing paddock layout to make mole draining easier. Water pipes often have to be re-laid. Drainage trenches can outfall to dams, so that more water can be harvested.
