If you've ever wondered what goes into getting a site ready for sheet piling, this update walks you through it.
New here? This series follows Project 1, a build with a basement on a sandy site with a high water table. That means the ground around the basement has to be held back with sheet piles (steel sheets driven into the ground) before we dig. It's been a busy couple of weeks: we've moved things around, made some safety upgrades and, best of all, the sheet piling is well under way.
Keeping the site safe and organised
First, the site needed to be safe and clearly marked for everyone, so we've been busy with signage. It might seem like overkill, but you have a duty to run a safe workplace, and you need to be able to show you've met it.
We installed new signs to make delivery points clear, and we keep the main gate locked during the day while workers are inside. That stops visitors or deliveries walking in unannounced while the rig is working.
We've also changed a few other things to make life easier:
- A bigger walkway. We rearranged the layout near the shade area to make more space. It now doubles as the lunch spot.
- A new site entry. We've set up a temporary access point so getting in and out is smoother.
- More safety gear. Cones (I'm a big fan of these for quick barricades), new signs for the fire extinguisher and fire blanket, and a couple of smoke alarms. Bunnings specials, but they do the job.
- Temperature and humidity gauges. These were left over from a previous job, but they're handy to have.
We've also added more fans to keep the electronics cool. One runs all the time, and the other is on a $10 timer. It works a treat.
Sheet piling progress
Now for the big stuff. The sheet piling is progressing, and there's more to it than pushing steel into the ground.
The hardest side of the job is the one with the column capitals (the wider sections at the tops of the columns that stick out). The rig head is about a metre wide, and the crew has to get it as close to the wall as possible to drive the sheets, so working around these takes real precision.
The installation, step by step
Once everything's set up, the process is fairly simple.
- Platform. First we built a working platform and had it certified for 250 kPa bearing, strong enough that the 40-tonne rig can't tip over. That took geotechnical testing and nuclear density testing (a test of how well the fill is compacted).
- Pre-drilling. The rig pre-drills the first section. This brings up a lot of soil and opens a path for the sheets.
- Pushing the sheets. The rig then finds the pre-drilled hole and pushes the sheet down. At this stage we're pushing, not vibrating, and that gets most of the sheet in.
- Vibrating to level. Finally, a smaller vibrating head on an excavator drives each sheet down to the exact level.
In the finished sections, you can see the sheets have been brought down to level and backfilled with the soil from the pre-drilling.
Before installation, we also cut small holes about 800 mm down from the top of each sheet for the rig's safety chains. If a sheet slips out of the rig's jaws, the chains catch it so it can't fall anywhere it shouldn't.
Plant and vibration monitoring
We've got a 40-tonne piling rig on site with roughly 7 tonnes of head force. There's also a 30-tonne excavator, and a cherry picker for maintenance on the rig.
To keep an eye on things, we've set up a couple of vibration monitors. They measure movement across the x, y and z axes, so we can track any effect on surrounding structures. The alerts are set at 3.5 mm/s, well below the 5 to 15 mm/s range that DIN 4150-3 gives for houses (depending on frequency), where the risk of damage starts.
A few other things
The structural engineer came in to inspect the revetment wall, which was a condition of our development approval (DA). It cost $2,750 for them to tell us what we already knew (the wall hadn't moved), but that's how it goes when you need an RPEQ (Registered Professional Engineer of Queensland) sign-off.
We haven't forgotten the tree either. The arborist's report came back this morning, so we're doing the remedial work on the roots it calls for: cutting them back cleanly with sharp shears, then fertilising and protecting them.
What's next
The sheet piling crew has an RDO today, but they'll be back tomorrow to keep going through this section. The one possible hiccup is the palm tree, where the fronds are too low for the rig to get under. We'll probably use the excavator to vibrate those sheets down.
The sewer has been located and camera-checked. We're working within 1.8 metres of it, so the structural engineer has given us a detailed plan to make sure we don't damage the council pipes.
It's been a big two weeks, and it's good to see real progress. More in the next update.
