Bay of Biscay Clay: Why Adelaide Footings Are Deep and How to Dig Them
The black, cracking clay under Adelaide's western and southern suburbs moves more than almost any soil in Australia. What it means for footings, underpinning, pools and the timing of every job.
Ask an Adelaide builder about the ground and the answer is two words: Bay of Biscay. The black, self-mulching clay that lies under much of the western, northern and southern suburbs is among the most reactive soils in the country, and it explains the cracked villas, the deep footings and the underpinning industry.
What it does
Bay of Biscay clay takes on water in winter and swells; it dries in summer and shrinks, opening cracks a metre deep and wide enough to lose a phone in. A house that sits across a wet patch and a dry patch is being twisted, slowly, every year. Footings on this clay are engineered deep (often 1.5 to 2 metres) or piered to something more stable, and every job that touches the ground has to allow for movement.
Footings and underpinning
Deep footings and underpinning pits are the classic Adelaide excavator job. The R18 PRO's 2,420 mm dig depth reaches most residential footings; underpinning pits beside an existing wall want the precision of a small machine and the reach to work from a metre away. Dig in short sections, pour promptly and never leave a pit open beside a footing over a weekend.
Pools
A pool in Bay of Biscay clay is dug in the right season and shelled fast. Winter clay is sticky and the walls slump; summer clay is hard and cracked but stable. Autumn and spring are the windows.
Drainage
Clay does not drain. Soakage does not work on the plains; stormwater goes to the street. Keep surface water away from footings with falls and paving, because the wet-dry cycle around the house is what moves it.
Timing
April to June and September to November are the digging months. In July the clay is putty; in January it is concrete and cracked, and a ripper goes on the machine first.
Attachments
Toothed digging bucket for footings, ripper for summer, tilt bucket for retaining walls and drainage falls. A wide mud bucket is for backfilling and grading, not for cutting.
Where the clay is not
The eastern suburbs toward the Hills are red-brown earth, easier and less reactive; the coast from Glenelg to Semaphore is sand. See the Adelaide area guides.
Machine choice
The R15 for landscaping, the R18 PRO for footings and underpinning, the R22 PRO for a block of work. Compare on the comparison page and check your ute on the towing check.