A quick update on yesterday's progress.
The capping layer is down and compacted
Yesterday we received and placed all the CBR 45 Type 2.3 material. (CBR 45 Type 2.3 is a graded crushed-rock road base: the CBR number is a strength rating, and the type sets the grading.) That brings the capping layer to 300 mm thick, above the geotech's 250 mm minimum. We then rolled it thoroughly with the static roller.
It went faster than expected, with four trucks rotating in from the quarry. We spread and levelled it with the skid steer and a levelling bar.
There was some minor vibration, but only briefly. For the record, the Dynapac (I believe it's a CA4600D) runs at:
- 30 Hz, 0.8 mm/s on the lowest vibration amplitude setting
- 2 mm/s on the highest setting
Our vibration management plan (VMP) follows DIN 4150-3, the German standard most often used for this. For houses it allows 5 to 15 mm/s, depending on the vibration frequency, before there's a risk of damage, so we were well under that. (The mm/s figure is peak particle velocity: how fast the ground or building moves as the vibration passes.)
This morning: the geotech's proof roll
This morning we met the geotechnical engineers and did a proof roll: a heavy roller (at least 12 tonnes) drives back and forth across the platform while they watch how the surface responds.
They're looking for deflection, which means the surface moving or pumping under the roller. If it springs or ruts, the layer underneath isn't tight enough to carry the rig.
The geotech engineer said the roller applies a bearing pressure of about 220 kPa, and they're looking for a platform that can carry around 245 kPa. We went over the whole platform and found one soft spot, marked with pink paint.
Steel plates for the soft spot and the rig
We'll hire two or three 25 mm steel road plates. Digging out and rebuilding the soft spot is an option, but we're using plates instead because:
- We need them to get the piling rig down the driveway and into the machinery area safely.
- We also need them for the sheet piling work at the end of the property.
At the far end, the sheet piling is about 1 metre off the revetment wall, and the piling rig will sit about 1.5 metres back from that. We'll place the steel plates so the rig's outriggers sit on a stable base while it pushes the piles in. A plate spreads the load from the outrigger pad over a much bigger area, so the pressure on the ground underneath drops.
The rig has sensors throughout. If the machine starts sinking or pushing down too much, the safety controls stop it, so a stable base keeps everything moving. The geotech's certificate also says spreader plates are needed wherever the rig puts its outriggers down, and next to the revetment wall.
The nuclear density test
The geotech team is also coming back to do nuclear density testing of the platform. It sounds alarming, but it's a standard compaction test (AS 1289.5.8.1) using a gauge with a small sealed radioactive source.
They drive a rod to the test depth (here, 300 mm), insert the probe and compare the readings with the target values for CBR 45 Type 2.3. That confirms:
- whether we've achieved the right compaction
- whether the moisture content is acceptable
- whether the platform is suitable for the piling rig
Once that's done, I'll sign off and the sheet piling company will have what they need to start. How the platform was built is in Post 22 and Post 23.
Keeping the platform certified
A certificate isn't the end of it. The geotech's letter says the platform must be re-inspected after heavy rain or flooding, kept graded so water can't pond on it, and repaired under their supervision if the rig damages it. You'll see that happen after a storm in Post 26.
Next week
We also had artwork installed on the site hoarding. Hello to everyone involved.
Next week I'm meeting the sheet piling company to walk the platform, talk through their approach and confirm the installation date. Then we'll film the next steps.
