Why this is a different trade to excavation

Bulk excavation on an open site is a production problem. Tunnelling under a standing building is a risk problem. The house above has to stay level, the neighbour’s wall has to stay uncracked, and the people upstairs usually have to keep living there. That changes the method entirely, and it is why a normal earthworks contractor will either decline the job or learn on your house.

How we break rock without shaking the building

The instinct is a rock hammer. On a sensitive site the hammer is the problem, because it puts energy into the rock mass and the rock mass is connected to the footings. We work down a ladder of methods, using the gentlest one that will still make progress.

• Rock sawing. Cutting the rock into manageable blocks along a defined line. Almost no vibration and a clean, accurate face.
• Hydraulic splitting. Drill a hole, split the rock along its own planes. Silent, and it stops exactly where you want it to.
• Expansive grout. Poured into drilled holes and left to work overnight. No noise, no vibration, no percussion at all. Slow, and sometimes slow is the only option.
• Remote-control breaking. A machine on tracks working in a space no operator should sit in, with the operator standing outside the tunnel. Far more power than a hand tool, and nobody under the unsupported face.
• Hand mining. Still the right answer close to a footing, and still how the last few hundred millimetres get done.

Remote-control machines, and when they earn their keep

A remote-control machine matters on three fronts. It fits where an excavator will not, roughly the width of a doorway. It keeps the operator out of a confined space with poor ventilation, an unsupported face above them and a machine swinging beside them. And it hits harder than anything a person can hold, so a tunnel that would take a crew weeks by hand moves in days. On an occupied house the second point is usually the one that gets the method statement approved.

What a job like this looks like

Before anything moves. The structural engineer sets what the building can tolerate. We write the excavation methodology around that number, sequence the dig so the structure is never unsupported for longer than it can bear, and agree the propping and underpinning with the engineer rather than after them.

Monitoring goes in first. Vibration and level monitoring on the building and on the neighbours, with baseline readings before we start. When someone next door says their wall has moved, and someone always does, we have a reading from the week before.

Then we go slowly and finish early. Sequence, prop, break, muck out, repeat. Spoil comes out by conveyor or barrow because there is nowhere for a truck. It looks slower than a conventional dig and it usually finishes sooner, because nothing stops for a crack, a complaint or an order.

Where we do this

Mostly the Sydney harbourside suburbs, where the houses are valuable, the blocks are steep and the sandstone starts about a metre down. We have tunnelled under homes in Mosman and are working on several more. We also do the same work commercially: lift pits, shafts and excavation beneath live plant and occupied buildings.

Common questions

Can we stay in the house?

Usually yes. That is the normal condition on this work rather than a special request, and it shapes the method from day one.

What about the neighbours?

Baseline dilapidation survey and vibration monitoring before we start, both sides. It is the cheapest insurance on the job.

Do you work with our engineer?

Yes, and the earlier the better. We write demolition and excavation methodologies for other people’s projects as a service, so we are used to sitting on that side of the table.

Is it expensive?

Per cubic metre, considerably more than open excavation. Against the cost of underpinning a cracked neighbour, it is cheap. We will tell you early if we think the job does not stack up.