Why Trenches Collapse in Perth Sand, and How to Dig Them Anyway
Bassendean and Spearwood sands have almost no cohesion. Here is what that means for trench walls, footings and pool digs, and the techniques that keep a hole open long enough to use it.
Anyone who has dug a soakwell hole in Wanneroo or Cockburn knows the feeling. The bucket comes up clean, the hole looks perfect, and thirty seconds later the walls slide in and you are digging the same sand twice.
What Perth is built on
Most of the metropolitan area sits on two sand units. Bassendean sand is the pale grey, leached sand of the inner and eastern suburbs. Spearwood sand is the yellow-brown sand over limestone closer to the coast. Both are almost pure quartz, well sorted and rounded. There is next to no clay to bind the grains, so dry sand has an angle of repose of around 30 to 35 degrees and nothing more.
A vertical face in dry Bassendean sand is not a wall. It is a pile waiting to happen.
What actually holds a sand face up
Moisture. Damp sand has apparent cohesion from surface tension between the grains, which is why you can dig a neat vertical trench after rain and watch it fail as the face dries in the sun. Once the surface dries, grains start rolling and the collapse works its way back.
That gives you three levers: keep the sand damp, keep the face from drying, and don't rely on it at all.
Techniques
Batter or bench
Slope the walls back to the angle of repose, or step them. For a soakwell hole this means a wider hole and more spoil, but the hole stays open. A tilt bucket shapes a batter without repositioning the machine.
Dig and use immediately
For footings and pipe trenches, dig only what you can pour, lay or backfill within the hour. Have the pipe cut and the fittings ready before the bucket goes in.
Wet it
A hose on the face keeps apparent cohesion in the sand while you work. It is the oldest trick in Perth landscaping and it still works.
Shore it
Anything deeper than 1.5 m that a person will enter must be shored, battered or benched under WA work health and safety rules, no matter how good the sand looks. Trench boxes and hydraulic shores are hireable; on a pool dig the shell goes in before anyone works at depth.
Choose the bucket
A wide, toothless mud bucket moves sand fastest and leaves a flat base. Teeth do nothing useful in sand and scratch pipes.
The winter version
From June to September the water table on the coastal plain rises to within a metre of the surface in low-lying suburbs. Trenches then fill with water and the walls turn to slurry. Dig drainage and deep footings in autumn or spring where you can, and if you must dig in winter, plan on dewatering.
Machine choice
A 1.5 to 2 tonne machine like the R15 or R18 PRO has enough reach to sit back from the edge and enough dump height to stockpile spoil clear of the hole. Keep the tracks at least the depth of the hole away from the edge; the ground under them is the same sand.
See the WA earthmoving calendar for the best months to dig on the coastal plain.