Hitting Rock: Why Digging Stump Holes in Melbourne's West Takes Longer

A city built on old lava

A long way back in geological time, lava flowed across the land west of the Maribyrnong River and cooled into a vast sheet of basalt. That sheet still underlies most of the western suburbs, from Footscray, Yarraville and Williamstown out through Sunshine, St Albans and Deer Park. Over a very long time, the top of the rock weathered into clay. In some streets that clay is deep. In others, you only need to go a short way before the spade rings on stone.

For a reblock, that matters. New stumps sit on concrete footings, and the footings have to reach the depth and bearing the building surveyor or engineer requires. In the west, getting there can mean working through some of the toughest ground in Melbourne.

What basaltic clay is like

The clay formed from basalt is dark grey to black, heavy and very sticky when wet. It is also highly reactive. In winter it soaks up water and swells. In a dry summer it shrinks and splits into cracks wide enough to lose a set of keys in. That movement is what pushes and pulls stumps out of line over the years.

Digging it is hard work. When it is wet it clings to tools and boots. When it is dry it sets like brick. Either way, a footing hole in basalt clay takes more effort than one in sandy or loamy ground.

Floaters and solid rock

Under the clay, the rock is not always one neat flat layer. It is common to strike floaters: loose boulders of basalt sitting within the clay, sometimes small enough to lift out, sometimes too big to move without breaking them up. Below those, there may be weathered rock that can be broken with hand tools, and then solid basalt that needs machinery.

That variation can happen within a single house. One footing hole might go down cleanly through clay, while the one beside it hits a floater halfway. It is one of the reasons reblocking in the west can take a little longer than it does elsewhere.

How footing depth is decided

The required footing depth comes from the building surveyor, and sometimes from an engineer, based on the soil, the structure and the site. In reactive clay, the aim is to get the footing down to where the ground is more stable and less affected by seasonal swelling and shrinking. Where sound rock is reached at a reasonable depth, it can provide an excellent base.

What matters is that each hole meets the specification. The holes are inspected before concrete goes in, so there is no room for cutting corners when the ground gets hard.

How we deal with rock

On a western suburbs reblock, we plan for the possibility of rock from the start. Depending on what we find, that can involve:

• Hand tools for clay and smaller floaters.
• Breaking out larger floaters so the footing can go to the right depth.
• Small powered breakers or drilling equipment where weathered or solid rock is reached.
• Talking with the surveyor or engineer where rock is struck at an unexpected depth, so the design can be adjusted to the ground we have really found.

Because rock can be hard to predict, we look at the site carefully during the inspection and explain how it might affect the job. Neighbouring houses, nearby excavations and the lie of the land can all give clues.

Why the clay still causes movement

You might think that a house sitting close to rock would be stable. Often the trouble is the clay above the rock rather than the rock itself. Stumps sitting in that upper layer of clay ride up and down with it. Where some footings reach rock and others stay in clay, the house can move unevenly. Good footings, placed to the right depth and bearing, are what give a new reblock its staying power.

What this means for you

If you own a timber-floored house in the west, expect your reblock to involve some hard digging. That is normal here. It is also one of the reasons a proper inspection beforehand is so useful: it helps set realistic expectations about time and effort. When you get your figure, ask how ground conditions have been allowed for, and what happens if the holes turn up more rock than expected.