Canberra Clay, Frost and the Digging Calendar
The capital's clay is hard in February and putty in July, and the ground freezes on winter mornings. When to dig, what to dig with and which Rippa machines cope.
Canberra was built on a sheep station because the ground was cheap, and the ground has not changed. Under the suburbs is red, yellow and grey clay over shale, volcanic rock and granite, with a climate that swings from hot, dry summers to frozen winter mornings.
The clay
Moderately reactive, hard when dry and sticky when wet. In February a bucket bounces off it and a ripper goes on first. In July it is heavy, wet and clings to everything. Between those extremes, in autumn and spring, it cuts cleanly and is a pleasure to work.
The rock
Volcanic rock and shale under Gungahlin and the north, granite under Tuggeranong and the south, often within half a metre of the surface on ridges. Granite boulders in a footing are normal in Kambah or Banks. A toothed bucket levers out the floaters; solid rock is breaker work on an R18 PRO or bigger.
Frost
From May to September most mornings are below zero and the top few centimetres of clay freeze. The crust is hard on bucket teeth and slow to dig; rip it, or start at ten when the sun has done the work. The machine needs its full warm-up on those mornings: glow plugs, a few minutes at idle, gentle hydraulics until the oil moves freely. Winter oil grades and a healthy battery matter more in Canberra than anywhere north of it.
The calendar
| Months | Ground | Best for |
|---|---|---|
| March to May | Firm, moist | Everything: footings, pools, retaining walls |
| June to August | Wet, frozen mornings | Drainage, rock, hard-stand work |
| September to November | Firm, drying | Everything again; landscaping before summer |
| December to February | Hard, cracked | Rip first; rock work; avoid lawns in the heat |
Drainage
Clay does not drain, and Canberra's stormwater system is strict about where water goes. Surface falls, ag-drains and a legal point of discharge; soakage pits are rarely approved.
Attachments
Toothed digging bucket, ripper, tilt bucket for the terraces and drainage of Canberra's sloping blocks, and a breaker for the south.
Machines
The R15 handles landscaping on the flat; the R18 PRO is the footing and pool machine; the R22 PRO suits acreage at Hall, Wallaroo and Tharwa. See them on the comparison page and the Canberra area guides.