• Subsurface drains interfere less with farming operations than surface drains
• Perforated ag-pipe drains are laid at approximately 0.7 m deep, with blue metal covering the pipe
• Intercept drains are installed at the change of slope gradient to intercept the downhill flow /of subsurface water

Subsurface pipe drainage has been referred to as ‘tile drainage’ in the past due to the use of short clay pipes or ‘tiles’. However, clay pipes are expensive to manufacture and difficult to lay and have now been superseded by slotted PVC or flexible corrugated plastic pipes (ag-pipe) of variable diameters. French drains, where the drain is filled with stones rather than a pipe, are still sometimes laid. Subsurface drainage has definite advantages to open trench drains as they interfere less with day-to-day farming operations than surface drains as surface drains require crossings or bridges and/or fences but underground drains still offer added benefits such as better trafficability and improved plant growth.

Ensure that there is suitable surface drainage or a nearby stream to discharge water from underground drains into (Figure 52).

Underground pipe drains can be installed to lower the water table over a wide area or to intercept groundwater flow (see below). Subsurface drainage is best suited in deep permeable soils where their depth allows wider spacing that minimises cost. Subsurface drains can also successfully drain heavy clay, poorly drained soils but in order to avoid having to be install drains so close together that they are often uneconomic, they are often combined with mole drains (Chapter 8). Subsurface pipes can also be used in soils that might have an impeding layer (for example, clay) at some depth and if the clay type and content is suitable, mole drains can be installed above these.

Underground pipe drains can be laid using an excavator digger, continuous trencher or trenchless drain plough. Continuous trenchers and trenchless drain ploughs fitted with laser guidance equipment are operated by contractors in Tasmania (Figures 52 & 53). Chain trenchers cut an open trench whereas trenchless machines use a single tine ripper or “V” style leg to install pipe. The tine is pulled through the soil shattering the profile to provide fissures to enhance water movement. The tine design creates a lifting action in the soil that reduces the draft required. Trenchless machines require more weight and horsepower than trenching machines to maintain the draft required to pull the plough (Gibson 2016). Laser guidance is used on both types of machines to maintain constant depth and fall regardless of ground surface conditions.

Tractor mounted backhoes are of sufficient size to excavate trenches for subsurface drains, but it is difficult to ensure an even grade on the base of the trench with these machines. Also the bucket width on backhoes is often quite a lot wider than the pipe resulting in the use of a much larger volume of permeable gravel that adds considerably to the overall cost. Trenchless drain ploughs and trenching machines are large, tracked vehicles (Figures 53 & 54). The narrow slit created by the passage of the machine makes it the most economic means of installing underground drains, particularly when inserting permeable gravel above the pipe.

When subsurface pipes or moles are installed, a GPS track of the lines should be retained for any future installation to tie into and for any maintenance or upgrades. Outfalls for subsurface drains should be permanently pegged (see front cover) so that they can be checked in the autumn for any blockages due to weed growth or vermin.

Ground disturbance following installation of underground drains can be considerable, particularly under pasture (Figure 55), but most of this will settle over time and if the soil is cultivated, all surface signs of disturbance will disappear.

Drain spacing varies depending on the landscape, gradient and soil types. Local contractors can advise on drain spacing using software packages that they have access to. When drains are placed to intercept seepage or drain smaller areas of wet soils, their location is often derived from experience of the landowner and contractor. Heavy clay soils require closer drain spacing than light clays or clay loams as the draw-down effect in heavy clays extends less distance from the drainpipe than in light clays or clay loams (Figure 56). In heavy clay soils, the theoretical correct drain spacing will almost always be so small as not to be economically viable. Spacing of underground drains in Tasmanian conditions is often at 50 – 60 m but some more intensive jobs have been at 35 m spacing. In soils with stable heavy clays, wide-spaced drains with permeable backfill supplemented with mole drains are the best choice. Pipe drain spacing for a mole drainage system can be as wide as 100 metres, although closer spacing is more effective.

The drainpipe needs to be laid approximately 700 mm deep, with the perforated ag-pipe of 100 mm diameter laid at the base and graded blue metal (20-25 mm diameter) covering the pipe and filling the trench to approximately 300 mm depth, with a final backfill of soil to the surface (Figure 57). If underground pipe installation is aligned directly downhill, water velocity at a change in grade, e.g. where the hills meet the flats, can burst the pipe. When draining steep sections, plan to drain across the slope in order to keep grades constant rather than with abrupt changes.

Permeable backfill refers to the gravel/stone chips applied to the trench above the drainpipe to provide for rapid drainage of water to the drain. Backfill is a major cost of a drainage system but is crucial to the drain’s effectiveness so the quality and price of backfill should not be compromised. The backfill is delivered by trucks or trailers fitted with conveyor belts which feed the backfill into the hopper (Figure 58). The forward speed, hopper channel opening size and material size determine the depth and amount of material laid on top of the pipe. Good quality clean gravel, 20 – 25 mm diameter, is required to prevent clogging up of drains with fines. In very permeable sandy soils, little if any backfill is needed, but in less permeable clay soils, or where moles are to be pulled through above a pipe, the backfill needs to reach the base of the topsoil so that a connection exists between the drain trench and the cultivated layer. As a minimum, the permeable backfill layer should connect with the mole drains or any fissures caused by subsoiling. If mole drains are to be installed over the pipes, the use of permeable backfill is essential to provide a hydraulic connection between the mole channels and the drainpipe.

A drain sleeve, or “sock,” is a geotextile synthetic material placed around a drainpipe to provide a barrier which prevents coarser sand-sized soil particles from entering the drain (Figure 59). Fine-textured soils with a clay content of more than 25 percent are generally considered stable, so they don’t need drain sleeves. A geotextile sock is recommended for sandy textured soils free of silt and clay. These soils are considered unstable even if undisturbed, so particles may wash into pipes.

Intercept drains are installed at the base of slopes at the change of gradient, usually where a steeper slope meets the flats to intercept the downhill flow of subsurface water. Often the soil type on the slope is more permeable than those of the flats and this forces the water to come to the surface, usually at the change of slope (Figure 60). These drains are installed along the contour and their position can be determined when walking slowly up the slope from the base. The soil will be spongey and wet at the base of the slope but will quickly change to be firm and drier above where the seepage is occurring. The intercept drain needs to be placed 1-3 m up-slope of the change and at approximately 700 mm depth (Figure 61).

Intercept drains can also be installed into springs and spring lines to intercept spring water. Grazing animals severely pug the areas around springs and damage is usually more concentrated down slope as the soil is very wet and has little structural strength. This affected area usually spreads unless fenced off. Intercept drains installed as close as possible to the spring and across its downhill flow can be effective in controlling the spread. Drainage reduces stock damage, or pugging, as the soil maintains its strength and structure.