Basalt and Bluestone: Digging on Melbourne's Western Plains
Werribee, Tarneit, Melton, Craigieburn and Sunbury sit on one of the largest lava fields on earth. Thin reactive clay over hard rock, and what that means for every footing, pool and drain.
The bluestone in Melbourne's laneways and church walls came from under the western suburbs, and it is still there. From Werribee and Point Cook through Tarneit, Truganina, Melton, Caroline Springs, Sunbury and up to Craigieburn, the ground is a thin skin of black clay over basalt, the remains of lava flows that ran across the plains a few million years ago.
What you dig into
The clay. Black and grey, heavy, cracking in summer and swelling in winter. Reactive enough that most houses on the plains are on stiffened rafts or piers. It is fine to dig in autumn and spring, slow and sticky in winter and concrete in February.
The rock. Basalt appears anywhere from 300 mm to 2 m down, as floaters (loose boulders in the clay), as a weathered crust or as solid flow. A toothed bucket lifts floaters; a ripper cracks the weathered crust; solid basalt needs a hydraulic breaker and time.
Footings and piers
Engineers on the plains specify to the rock or to a depth in the clay, and either way the excavation is deeper than an eastern-suburbs block. The R22 PRO's 2,365 mm dig depth reaches most residential footings; piers into the rock are a bored-pier rig. Dig and pour in the same week.
Drainage that will not soak
Basalt clay does not drain and the rock beneath it drains not at all. Soakage pits are useless here. Every block needs surface falls, ag-drains and a legal point of discharge to the street stormwater, which the council will specify. A tilt bucket shapes the falls; a narrow bucket trenches for the drains.
Wind and weather
The plains are exposed. Winter mud, summer dust and a wind that dries an open trench in hours. Keep spoil covered, and keep the machine's radiator clear of the grass seed that blows across the estates in November.
Attachments
Toothed digging bucket, ripper, tilt bucket and a breaker sized for the machine. A grid bucket sorts basalt floaters from the clay so the rock can be reused as fill or sold.
Machine choice
The R18 PRO is the minimum for footings in the clay; the R22 PRO is the plains machine; the R32 PRO with a cab suits contractors working through a Melbourne winter. Compare them on the comparison page and see the Western Suburbs area guide.