Digging the Darling Scarp: Laterite, Clay and Granite in the Perth Hills
The Hills are the opposite of the coastal plain. Gravel that sets like concrete in summer, clay that clogs a bucket in winter, and granite that turns up exactly where the footing was going.
Twenty kilometres east of the city the sand stops and the Scarp begins. From Bedfordale through Kalamunda, Mundaring, Gidgegannup and up to Chittering, the ground is a different country.
The profile
Dig a Hills block and you typically find three layers:
- Laterite gravel on top: the red-brown pea gravel and ironstone rubble of the Darling Range plateau. Hard, free-draining and abrasive.
- Mottled clay beneath, orange and white, formed by weathering of the granite. Plastic when wet, rock-hard when dry.
- Granite at the bottom, sometimes 5 m down, sometimes at the surface as the boulders and outcrops the Hills are famous for.
On valley floors and along the brooks there is alluvium: mixed sand, gravel and clay that behaves unpredictably.
Summer versus winter
The Hills are seasonal in a way the plain is not. In February, laterite and dry clay resist a 1.5 tonne machine; you need a ripper, patience or a bigger excavator. In August the same clay is workable but sticky, balling in the bucket and turning tracks into skis. The best digging is April to June and September to October. See the calendar.
Tools that earn their keep
- Ripper. Fractures laterite and dry clay so the bucket can lift it. The R22 ripper is the first attachment most Hills owners buy.
- Toothed digging bucket. Teeth matter here; a toothless bucket skates on gravel.
- Breaker. For granite in footings and trenches, an R32 breaker or larger. Small breakers polish granite rather than break it.
- Grid bucket. Sorts rock from soil so the fines can be reused. See the R32 grid bucket.
Weight matters more than anywhere
A 1 tonne machine that is perfect on the plain struggles in the Hills. Breakout force comes from machine mass, and laterite does not give way to enthusiasm. The R22 PRO is the practical minimum for regular Hills work; the R32 PRO, with a cab and zero tail swing, is the contractor's choice.
Slopes
Most Hills blocks are on a gradient. Track straight up and down, never across; keep the blade downhill when digging and low when travelling; widen adjustable tracks fully; and treat cut edges and fill batters as unstable until proven otherwise. Rain on a fresh cut can bring the batter down overnight.
Granite surprises
Where the bucket hits rock, stop and probe before you commit. A boulder can be lifted out; an outcrop cannot. Engineers often allow footings to bear directly on sound granite, which can save digging, but the decision needs an inspection, not a guess.
Fire
The Hills are the highest bushfire-risk area in the metropolitan region. Firebreak notices from the City of Kalamunda, Shire of Mundaring, City of Armadale and City of Swan fall due in November, and the same machines that dig your footings cut your breaks. Plan both in the spring window.