I know it has been really bad – I have been talking to CYP.

Seems that it was not the concrete rings but the ‘blinding layer’. Let me try and explain.

Deep underground they have a dirt surface. They lay a ‘blinding layer’ of concrete. On top of that they lay the steel reo and then the thick layer of concrete that becomes the floor of the concourse.

Once all that thick concrete layer is cured and secured to the walls, they dig underneath it all – that is all the dirt you see coming out. Once they have dug underneath they need to remove that blinding layer, it is just a working layer. It is concrete and they let it fall.

The big thuds are big chunks of concrete falling. I have asked if they can do in smaller chunks and not after 10pm…not sure that they understand the impact.

Several people have asked about building damage, this is the CYP reply (the thud at 930am today was 2.8mm/s)

The limit for the project for structural damage is 5mm/sec with our warning being 3mm/sec. We hit just under 2.5mm/sec, so whilst it was a big thud – it’s very unlikely to cause damage to any buildings.

James has included a bit of commentary of what levels of vibration mean:

~ 5.0 mm/sec Damage to very sensitive buildings may start to occur

~ 4.0 mm/sec and above – scary.  Very concerning.  Uninhabitable if exposed to vibration for long periods

~ 2.0 mm/sec RMS (2.8~3 mm/sec PPV) very annoying, concerned

~ 1.0 mm/sec RMS (1.4 – 2 mm/sec PPV) very clearly perceptible, annoying

~ 0.4 mm/sec RMS (~0.5 mm/sec PPV) clearly perceptible, potentially annoying during the daytime

~ 0.2 mm/sec RMS (0.28~0.3 mm/sec PPV) perceptible, potentially annoying during the night-time in sleeping areas

~ 0.14 mm/sec RMS (~0.2 mm/sec PPV) just perceptible

~ 0.1 mm/sec (0.14~0.2 mm/sec PPV) threshold of perception

< 0.1 mm/sec RMS (0.14~0.2 mm/sec PPV) not perceptible