Dolerite, Sandstone and Granite: Tasmania's Three Rocks and How to Dig Them
Hobart's dolerite, the Midlands' sandstone and the East Coast's granite each behave differently under a bucket. Recognising them, choosing between ripper and breaker, and the machine weight that makes rock work possible.
Tasmania is a rocky island, and the rock changes with the district. Three types account for most of what a Tasmanian excavator owner hits.
Dolerite
The dark, hard, columnar rock of kunanyi/Mount Wellington, the Hobart hills, the Central Plateau and much of the Midlands. It is among the hardest rock a compact machine will meet. Weathered dolerite breaks into blocky boulders that a toothed bucket can lever out; solid dolerite is breaker work, and slow breaker work at that. Footings on dolerite are shallow because the rock is sound; the difficulty is getting through it for pools, tanks and services.
Approach: probe first. Rip the weathered surface, lift the floaters, and put a breaker sized for the machine on anything solid. The R32 PRO with its breaker is the practical minimum for regular dolerite; the R22 PRO manages weathered rock.
Sandstone
The honey-coloured Triassic sandstone of Richmond, Oatlands, Ross and the older Hobart suburbs. Softer than dolerite, it saws and breaks cleanly and was Tasmania's building stone for a century. A ripper cracks weathered sandstone; a breaker on an R18 PRO or R22 PRO handles the rest. Sandstone spoil is clean and useful for walls.
Granite
The pink and grey granite of the East Coast, Flinders Island and the north-east highlands. Granite weathers into coarse sand and rounded boulders; the boulders lever out, the solid rock is very hard. East Coast pool digs and footings around Bicheno and Coles Bay often meet granite floaters the size of a car.
The soil on top
- Red basalt soil on the North West and around Launceston: deep, rich, easy digging with rock underneath.
- Clay in the valleys and gullies everywhere: wet and heavy from May to October.
- Alluvium on the Derwent, Huon, Tamar and Meander flats: soft and prone to flooding.
Cold and rock
Frozen ground on winter mornings on the plateau and highlands is its own rock. Start later, or rip the frozen crust. Hydraulics are slow until the oil warms; let the machine idle and work gently for the first few minutes.
Machine weight
Rock rewards mass. A 1.5 tonne machine bounces off dolerite; a 2 tonne machine works weathered rock; a 3 tonne machine with a breaker does the job. See the R22 PRO and R32 PRO pages and the breakers that fit each.