With conventional pipes, the cost of such systems is usually uneconomic and hence alternative techniques are required.
Subsurface drainage is one possibility:
the other is Mole Drainage.
Mole Drainage systems are unlined circular soil channels which function like pipe drains.
The major advantage of Mole Drains is their low cost, and hence they can be installed economically at very close spacing’s.
The disadvantage of Mole Drainage is their restricted life, but providing benefit-cost ratios are favourable, a short Mole Drainage life can be acceptable.
The success of Mole Drainage system is dependent upon satisfactory water entry into the Mole channel and upon the Mole channel itself remaining stable and open for an acceptable period.
Currently, Mole Drainage systems are most commonly used for surface water control in perched watertable situations.
The use of Mole Drains as a temporary subsurface drainage system for the reclamation of saline and saline sodic soils has been successful in some area’s.
Mole Drain Formation
Mole Drainage lines are formed with a Mole Plough which comprises a cylindrical foot attached to a narrow leg, followed by a slightly larger diameter cylindrical expander.
The foot and expander form the Mole Drain and the leg generates a slot with associated soil fissures which extend from the surface down into the Mole channel. The leg fissures are vertical and are formed at an angle of approximately 45º
to the direction of travel.
The number and size of the leg fissures produced with a given Mole Plough are dependent upon soil conditions.
In SE Australia the right time to install Mole Drains is late spring to early summer.
The success of a Mole Drainage system also depends upon satisfying two requirements:
- Achieving the desired water flow path for the particular drainage situation
- Installing stable Mole channels
The most realistic length of a Mole Drain is approximately 80 – 100 meters.
Longer Mole Drains can be achieved by using Subsurface Drainage systems as interceptors, Were the mole drains to rely for outfall on a system of permanent pipe drains which have permeable backfill ( gravel, sand, crushed stone) over pipes. The Mole Drains are drawn over these pipe drains so that the bullet of the mole plough penetrates the permeable backfill. Water passes down through the moling fissures to the mole drains and then via the permeable backfill into the pipe drains. Having ensured that good mole drains have been formed, the system should provide rapid water movement for a number of years.