Without recapping all the data in the previous post, we would like to update further work on the Weymouth Harbour Wall repairs at the midway stage (last week of pile-driving?) During 19th November, the contractors swiftly moved the piling jig another 10 metres or so along the Harbour Wall repairs site to the final position (Western end, 74m).
Very quickly, the piling rig was then fitted with piling guides and another 6 piles were lowered into position. Ancillary work onvolved spot-welding and cutting where the “ears” of the “staples” were deformed and manually “knocking” the piles into alignment, against the wind and crane movements till they could slide smoothly into place. Below is a “sequence” (actually compiled from 2 adjacent pile emplacements).
You can easily see from the above why the Council and the Contractors advised us to close for the duration of the works! (See here for a contemporary update from Weymouth Harbour website). We are also being kept informed regularly by Dorset Council, in turn advised by Knights Brown Construction, the contractors. Also, note the dust and grime adhered to the outside of the windows. Together with extra internal dust generated by pile driving vibrations, this is another reason for closing our business during the works. Latterly, we have discovered TV interference effects.
Note the occasional transient excursion in vibration records below. In this example, the peak-to-peak transient was approximately + 50 to – 80 mGal, or about MMI 4 (possible damage). Peak-to-peak the gap was about 130 mGal – or MMI 5 (persons may lose balance, significant building damage) – BUT – it was only a transient, not continuous, not repeated many times and not resonant – so no observable damage likely. Also note, our measurements are NOT accurate, NOT ground displacement figures and NOT directly relatable to MMI values. MMI itself is a scale more usually associated with earthquake events (running from 1 – no effects, to 12 – total destruction). We only use these results as rough indicators.
Pile driving vibrations and resulting damage to nearby structures is a long investigated subject and surprisingly complex (Some accessible texts are – http://onlinepubs.trb.org/Onlinepubs/hrr/1967/155/155-002.pdf https://www.diva-portal.org/smash/get/diva2:610771/FULLTEXT02.pdf https://en.wikipedia.org/wiki/Pile_driver )
Update: 22nd November
We shall not bore you with another series of vibration graphs – just some typical data as the crew near completion of the piling stage (about 12 metres distant).
As with the vibration graphs, the sound level graphs are NOT accurate and depend significantly on factors such as microphone quality, sampling rate and software rendering. As a very rough guideline, the sound and vibrations are disturbingly loud indoors and sufficient to dislodge some items from shelves, rattle toilet seats, windows, doors, floorboards and plumbing against fixings. However, we see no significant damage yet.