Laterite and Black Soil: Digging Around Darwin and the Rural Area
The orange gravel of the Darwin plateau and the black soil of the flats behave in opposite ways. Footings, drainage, bores and the attachments that work in each.
Darwin's ground comes in two colours. Orange laterite on the higher ground of the city, Palmerston and most of the rural blocks; black soil on the flats, floodplains and low ground between. They want different machines, different attachments and different months.
Laterite
An iron-rich gravel and clay that formed under millions of wet seasons. Dry, from June to October, it is hard: a 1.5 tonne machine skates on it and a ripper is the first attachment on. Wet, from December to March, the surface turns to a greasy orange slick that rubber tracks slide on and that clings to every bucket.
Footings and pools are dug in the dry, in laterite that holds a vertical face well. Drainage through laterite is easy to cut and stays open. Bores and water lines are trenched to 450 to 600 mm before the wet so the pipe is safe from traffic and heat.
Attachments: ripper, toothed digging bucket, tilt bucket for driveway crossfalls and drains. Laterite spoil makes excellent driveway gravel; keep it.
Black soil
Cracking clay on the flats at Humpty Doo, along the Adelaide River and on every low block in the rural area. Dry, it opens into deep cracks and is workable but heavy; wet, it is bottomless. It floods in the wet, every wet, and anything built on it needs to be above the water or designed for it.
Drainage is the first job on a black-soil block and the one that makes the rest possible. Pads for sheds and tanks are built up, not dug down. Footings are engineered for the movement.
Attachments: wide mud bucket for volume, tilt bucket for drains and pads, and a skid steer to move fill onto the block.
Rock
Sandstone and quartzite outcrop toward Litchfield and on the escarpments. A breaker on a 2 tonne or bigger machine handles what a ripper cannot.
Termites and buried surprises
Termite mounds in the rural area are concrete-hard and full of surprises for a bucket. Old wet-season debris, buried tree trunks and abandoned fencing wire are common on rural blocks; a rake and a grab clear them.
Machines
The R22 PRO with a ripper and toothed bucket for laterite, a wide mud and tilt bucket for black soil, and an RS10 skid steer for moving fill and gravel. The R32 PRO with a cab suits contractors working through the build-up. See the Darwin rural area guide and the attachment finder.