Under contract to the Washington State Department of Transportation (WSDOT), Battelle, Pacific Northwest Division, conducted a re-analysis of dynamic pile driving and impulsive underwater sound data acquired at WSDOT construction projects (Hood Canal Bridge and Friday Harbor Ferry Terminal) to better understand the mechanisms of impulsive sound generation by pile driving in support of efforts to determine the effects of impulsive sound on fish health and behavior. Analysis focused on derivation of statistics from impulsive sound and dynamic pile driving data sets that permitted evaluation of the amount of variability in impulsive sound metrics that might be driven by variability in pile driving mechanics metrics.
The energy required to drive a pile at various depths and substrates and an index of the sound energy produced during the pile drives were also compared. These comparisons yielded the conclusion that most of the variability in impulsive sound during driving of a pile can be accounted for by changes the impact hammer operator makes to overcome resistance to increases in pile depth. Thus, it is the operation of an impact hammer in response to changes in substrate, not the substrate itself, that is responsible for changes in impulsive energy metrics during driving of a pile. A recommendation of the study is that any future data acquisition and analysis efforts to improve understanding of linkages between pile driving mechanics and impulsive sound or underwater sound monitoring activities in support of construction activities include hammer stroke data as a basic element of underwater sound data sets.
As an element of comparison of data sets to assess the relationship in variability between impulsive sound and pile driving mechanics, the importance of wetted pile length was evaluated. It appears, based on the data sets analyzed for this study, that the wetted length of the pile is not related to impulsive sound metrics such as peak pressure. The lack of relationship between impulsive sound metrics and wetted pile length probably results from the way sound is produced by the pile when it is deformed by a hammer impact. As a consequence, when evaluating the potential for sound generation during project planning it should be assumed that a pile with minimum wetting length may produce impulsive sound levels of the same magnitude as piles with significantly greater wetted length. Environmental factors not evaluated in this study will determine how the generated impulsive sounds propagate.
Analysis of the cumulative energy required to drive a pile and an index of the cumulative sound energy produced during driving of a pile revealed a relationship between the diameter of a steel shell pile and the amount of energy transferred to the pile at impact to obtain an incremental increase in pile depth and the amount of sound energy produced per incremental increase in pile depth. It appears, logically so, that the energy required to drive a pile an increment in depth and the sound produced during that process are directly related to pile diameter. This being the case, we recommend that sound mitigation measure development, such as bubble curtains, focus on piles 30 inches or larger in diameter. It is unlikely that sound mitigation measures that would result in reduction of energy transfer to a pile, which will be necessary to reduce sound production, will be acceptable economically for larger piles because of the rapid increase in energy per foot of drive with pile diameter.