Piling on berths 201 and 202 has now been running for just over a month and is progressing very well.
By the time we shut down operations for the Christmas and New years holidays on December 20 2012 we will have driven a total of 56 piles, with 30 of those fully driven to depth.
Our site will be closed from the evening of 20 December until we return on 07 January 2013.
Several residents in Marchwood have brought to our attention the noise of a secondary ‘thud’ after the initial hammer strike. After initial investigation, it was decided that we would consult our acoustics specialist Mr Bernard Postlethwaite to determine if there was an explanation for the noise and a possible course of action to prevent the secondary ‘thud’.
After visiting several properties and monitoring the ’thud’, Mr Postlethwaite had the following to say:
“I was able to calculate the time delay between the first and second pulse of each hammer blow, and this enabled me to calculate the speed of sound in the secondary path. This suggested that the secondary sound wave is being transmitted through the ground as what is known as a ‘shear wave’, and possibly as a ‘surface wave’, probably in a sand and gravel strata. As the pile is being driven, acoustic energy radiates both from the toe and the wall of the pile, but the latter increases in intensity as the pile gets deeper into the ground.
Therefore this type of noise may get louder towards the end of driving each pile. The groundborne sound energy propagates away from the pile, reducing in intensity with distance from the pile both through energy spreading and the direct loss of energy in the ground, but is penetrating the foundations of the buildings on the Marchwood side of the river, radiating into the rooms of the buildings as audible sound. There is also some low frequency vibration that can be felt on the floors in some of the properties, but we know from our vibration monitors that the levels of vibration are very much less than would cause any physical damage to a building.
The fact that there is a delay between the two sounds is because the “shear waves” in the ground travel slower than airborne sound waves. The ‘thud’ characteristic is caused by the loss of higher frequency sound that is more readily attenuated in the ground than through the air. As the piling moves away from Marchwood as construction proceeds, this groundborne noise will decrease in intensity for properties in Marchwood, but we don’t yet know whether this type of noise will start to become audible in Southampton, as we get closer to Berth 201.”
We can only apologise for the disruption and inconvenience caused and I know you will be happy to know that the piling programme is still on schedule for completion at the end of February 2013.
We wish you all a Merry Christmas and a Happy new year and thank you for your patience and understanding during the progress of the works.
For and on behalf of VolkerStevin Ltd
Public Liaison Manager
Telephone: 02380 528 068
Mob: 07714 684 744
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