• Mole drains work only on soils with > 35% clay at mole depth
• Mole drains are generally installed at 400 - 600 mm depth
• The ideal time for installing mole drains is in the springtime, as the soil dries

Mole drains are unlined cylindrical channels which function like plastic pipes. They are used when natural drainage needs improving in particularly heavy clay subsoils with low rates of soil-water movement that would require uneconomically closely spaced pipes for effective drainage. Mole drains should fit into a planned overall drainage strategy. Mole drains act similarly to closely spaced pipe drains and conduct water to the permanent pipe drains or direct to open ditches. Mole drains work only on soils with > 35% clay (clay loam, sandy clay loam, sandy clay or clay textures) at drain depth, as the mole needs to be drawn through a ‘plastic’ soil that will leave behind a cylindrical channel with compressed sides. It is critically important that the subsoils are not sodic (low dispersion) as mole drains in sodic soils will collapse very quickly and leave collapsed erosion channels at the soil surface. This happens when fresh water (from winter rainfall or summer irrigation) comes into contact with sodic soil in the mole channel. Mole draining is best suited to paddocks with consistent surface gradients up to 0.2% but can operate on gradients up to 0.6% if correctly installed.

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 (DPI WA 2022).

Mole drains are formed by dragging an implement called a mole plough through the soil which consists of a ‘bullet’ (a round-nosed cylindrical foot shaped like a bullet with slight tapering towards the tail) attached to a narrow leg (Figures 62 and 63). Connected to the back of the foot there is often a slightly larger diameter cylindrical expander. The foot and expander form the drainage channel as the implement is drawn through the soil and the leg leaves a slot and associated fissures (Figure 64).

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. Mole drains are generally installed at depths between 400 and 600 mm below the surface. The ideal configuration for moling at 400–500 mm depth is a mole bullet of 65mm diameter with a 75mm diameter expander. The function of the expander is to smooth and gently compress the mole channel. The expander only needs to be 10 mm larger in diameter than the mole bullet. The function of the leg is to support the bullet without flexing and to create the required network of soil fissures. The fissures extend from the leg and laterally out into the soil (Figure 64) providing the major flow paths for soil-water to move into the mole channels. Any surplus water above moling depth can therefore move rapidly through these fissures into the mole channel. Successful mole drainage depends on the water being able to rapidly enter the mole drain, flow unimpeded down the channel, and exit the system either via an open ditch or into a deeply set pipe system.

Moles less than 400 mm deep are liable to be damaged by tractors and animals causing compaction during or immediately after rain and tend to be short-lived. It is essential however that the minimum depth of the channel is below the ‘critical depth’ of the soil. Below critical depth the mole causes the soil to flow around it in a plastic state (Figure 64). If the mole plough foot is pulled through the soil above the critical depth, soil loosening or brittle failure of the soil down to the depth of the foot will occur and no channel will be formed, and the system will fail immediately. The critical depth is normally at 350 - 400 mm for a narrow tine in clay soils.

Moles should be drawn directly from the outlet, such as an open trench, into which the mole drainer has been lowered behind the tractor and drawn up the land gradient. Moles should be drawn at 2-4 km/hr to ensure good channel formation. If speed is any higher, the expander can tear the walls of the channel resulting in failure within one to two years (Agriculture Victoria 2020). Surface compaction over the mole after installation using the tractor wheels is recommended and this can be done on the trip back to the outlet drain if the paddock is being moled in one direction.

The spacing between the tyres of the tractor determines the minimum spacing, typically 1.5 - 2.5 m. After installation, short lengths of PVC pipe (0.5 m long), the same diameter as the mole channel, need to be inserted in the end of each mole channel outlet. This prevents the ends of the channels from drying out and collapsing and prevents erosion and blockage by debris or rodents. Additionally, both sides of the outlet ditch should be fenced to prevent stock damaging the outlets.

Mole drains can be, and often are, installed over pipe systems with the pipe system installed at least 150 mm deeper than the proposed mole drainage (Figure 65). As the pipes are installed, permeable gravel is laid over them to well above the proposed installation depth of the mole drains (Chapter 7). The moles are then drawn across the pipes, but the moles should be drawn at an acute angle into the gravel rather than at right angles to avoid a pressure dome of water in the soil opposite where the mole drain enters the gravel filled drain. The moles intersect the permeable fill, and this provides a rapid connection for the water to flow from the moles into the pipe system.

When installing mole drains it pays to check the installation by digging a few inspection pits to expose the channel formed and to make sure that it is round and there is good fissuring above it (Figure 66).

Mole drains can be installed at lengths of 20 m - 200 m. The maximum mole drain length should be shorter on low drain grades than for steeper grades. Initially farmers should experiment with varying lengths from 40 metres (0.1% grade) to 100 metres (0.5% grade). These can perform satisfactorily for several years but will survive much longer if the length is reduced (60 to 80 m) because they will empty out more quickly and are not likely to remain saturated. On steeper gradients, moles should cross the direction of the main slope to help intercept subsurface seepage and to avoid the possibility of channel scour and erosion.

The ideal time for installing mole drains is in the springtime, as the soil is drying, however under irrigated pasture, the autumn is also suitable. The soil at moling depth should be plastic or moist enough to be moulded by hand into a ribbon without breaking, and the soil surface needs to be dry enough to ensure good traction and avoid compaction. The drier the soil above moling depth, the greater the fissuring produced and the more efficient the water removal. The mole channel should remain dry for at least a month after installation allowing the soil to ripen before the mole conducts water.

Lifespan of mole drains

The longevity of mole drains depends on several factors, including:

• Soil texture (high clay content is better)
• Climate (wetter conditions will reduce longevity)
• Slope (too shallow or too steep will reduce longevity)
• The moisture conditions in which the moles were formed

Drain life can be reduced due to several reasons (Spoor and Ford 1986) including:

• Patches of sandier soil occur, leading to premature collapse.
• Poor soil management resulting in soil compaction can seal off the routes by which water reaches the mole drains, thus reducing their effectiveness.
• Moles installed when soils too dry or at too shallow depth resulting in roof collapse
• Collapse of the roof and walls due to recurring shrink and swell of the subsoil clays.
• Channel diameter shrinks due to continuous soil swelling.
• Soil falling down soil cracks through the roof of the mole in dry weather and infilling the mole channel.
• Erosion of the mole channel due to too steep a gradient or unstable soil structure.
• Unstable soil structure resulting in moles filling with soil slurry (e.g. sodic soils).

The typical lifespan of mole drains in suitable soils ranges from five to ten years or even longer. Mole channels in very stable, clay soils (clay content > 45 per cent) can last over ten years, but the method can still be effective in soils with at least 30 per cent clay. It is good practice to renew mole drains on a cycle of around once in every five years. With accurate location data, new mole drains can be installed between old mole drains to preserve any of the older mole drains that may still be functioning.