The same trench, the same day, and a completely different job at 800mm

Most of Sydney sits on sedimentary rock. Hawkesbury sandstone splits along bedding planes, it can be cut, and an experienced operator makes progress in it. It is workable, and every excavation quote in this city is written on that assumption.

Here there is an igneous body in the ground instead. Prospect Hill is a dolerite laccolith, Sydney's largest body of igneous rock, and Prospect Quarry has probably contributed more crushed rock for roads and building than any other comparable quarry in the region. That is not a coincidence. The material gets crushed into road base precisely because it is hard, dense and tough, and that is exactly the property that makes it difficult to dig.

The short answer

An excavation here can change character with depth and without warning. A trench that is straightforward through topsoil and clay at 400mm can become a genuinely different job at 800mm. So an excavation price given on length alone is a guess, and it is the kind of guess that becomes a variation halfway through with the yard already open.

1. Ask what any excavation quote assumes about depth to rock
2. Ask what happens to the price if that assumption is wrong
3. Ask whether the depth has actually been established or estimated
4. Ask whether the job can be done without digging at all
5. Expect the clay above the rock to be its own separate problem

On this page

1. A different rock
2. Changes with depth
3. Quotes on length are guesses
4. The clay above
5. No-dig is worth more
6. What we do not do

1. A different kind of rock entirely

It is worth being precise, because "rock" gets used as though it were one thing and the two behave nothing alike.

Sandstone is sedimentary. It was laid down in layers, it splits along those layers, and it can be worked. That is why colonial Sydney was built out of it, and it is why excavation through it is difficult but predictable.

Dolerite is igneous. It was forced in as a molten intrusion between existing strata and cooled into a dense, tough mass with no bedding planes to exploit. There is a reason the quarry here has supplied more blue metal for Sydney's roads than any comparable operation: the material is hard on purpose, and that is what it is for.

2. Why the trench changes character partway down

This is the specific and expensive problem, and it is different from simply having hard ground.

The top of a hole here is ordinary: topsoil and clay, dug the way any trench is dug. Progress is normal, the machine is the right machine, and the estimate is holding.

Then at some depth the material changes, and everything about the job changes with it. Different equipment, dramatically slower progress, more noise, and a timeframe that no longer resembles the one anybody planned around. Same trench, same day, completely different job.

And the depth at which that happens is not uniform. It is a body in the ground rather than a flat layer, so what is true at one property is not automatically true at the next, which is precisely why estimating it does not work.

3. Why a price on length alone is a guess

An excavation quote is usually built from distance and depth: so many metres, at such a depth, in ground assumed to be diggable.

Take away the certainty about what is at the bottom and that arithmetic stops meaning anything. The same number of metres can be a straightforward morning or a substantially harder exercise, and nobody finds out which until the hole is already open.

That is the worst possible moment to discover it, because at that point the yard is dug, the household is without drainage, and the leverage in any conversation about price has moved entirely.

1. Ask what the quote assumes about depth to rock, specifically.
2. Ask whether that was established or estimated.
3. Ask what the price becomes if the assumption is wrong.
4. Ask what happens to the timeframe as well as the number.
5. Ask whether there is a method that avoids the question entirely.

4. And the clay above it is its own problem

You get two problems in one hole here, and the second one is what causes the damage in the first place.

Dolerite weathers to a heavy clay soil, so the material sitting over the rock is reactive. It shrinks and swells with moisture through the seasons, and buried pipework moves with it. Over enough cycles that works old joints loose, which is a slow, cumulative process rather than a single event.

So the ground here manages to be both things at once: material that moves, sitting over material that will not shift. The first causes the failure and the second makes reaching it expensive, which is an unfortunate combination and worth understanding before anybody starts.

5. Which makes no-dig worth more here than usual

Relining is normally argued for on tidiness: no trench, no mess, no reinstatement, garden stays where it is. Those are real benefits and they are the same everywhere.

Here there is a stronger argument. Repairing a line from the inside removes the uncertainty entirely, because the question of what is at the bottom of a hole never arises. A price for relining is a price, not an estimate contingent on ground nobody has verified.

That is not a claim that relining always suits. A line that has collapsed rather than cracked may not take a liner, and badly displaced sections may need a point repair first. Those are questions for a camera. But on this ground the comparison deserves to be made properly rather than treated as a preference.

6. What we are not going to tell you

We are not going to tell you how deep the rock is under your property. We have not been there, it varies, and a confident number from somebody who has not looked is worth nothing.

We are also not geotechnical engineers. Anything about ground conditions, soil classification or how a structure interacts with reactive clay belongs to somebody qualified in that, and it is worth asking them rather than a plumber.

What we can do is camera a line and tell you what is actually wrong with it, say honestly whether it can be repaired without excavation, and be straight about the uncertainty in any excavation price rather than presenting a guess as a quote.

The short version

• Most of Sydney sits on sedimentary rock. Here there is a large igneous body in the ground.
• Dolerite is crushed for road base because it is hard, dense and tough. That is why it is here.
• An excavation can change character with depth, so a straightforward trench becomes a different job.
• A price given on length alone is a guess that becomes a variation with the yard already open.
• The clay above weathers from the rock, shrinks and swells, and works old pipe joints loose.
• No-dig removes the uncertainty entirely, which is worth more here than the tidiness argument.