Sound generated by pile installation using a down-the-hole (DTH) hammer is not well documented and differs in character from sound generated by conventional impact and vibratory pile driving. This paper describes underwater acoustic characteristics from DTH pile drilling during the installation of 0.84-m shafts within 1.22-m steel piles in Ketchikan, Alaska. The median single-strike sound exposure levels were 138 and 142 dB re 1 μPa2s at 10 m for each of the two piles, with cumulative sound exposure levels of 185 and 193 dB re 1 μPa2s at 10 m, respectively. The sound levels measured at Ketchikan were significantly lower than previous studies, and the sound was determined to be non-impulsive in this study as compared to impulsive in previous studies. These differences likely result from the DTH hammer not making direct contact with the pile, as had been the case in previous studies. Therefore, we suggest using the term DTH pile drilling to distinguish from DTH pile driving when the hammer strikes the pile. Further research is needed to investigate DTH piling techniques and associated sound-generating mechanisms and to differentiate the various types of sound emitted, which has important implications for the underwater sound regulatory community.

Underwater anthropogenic sound and its potential impacts on marine life has become a pivotal issue in marine conservation over approximately the past four decades. Among the various underwater sound sources that have been included in environmental impact assessments, sound from in-water pile driving activities has received particular interest due to its wide use in marine and coastal construction of bridges, ferry terminals, docking structures, and offshore wind turbines (Bailey et al., 2010; Robinson et al., 2012; Dahl et al., 2015b). Sound generated from in-water pile driving has been shown to cause behavioral disturbance (Tougaard et al., 2009; Kendall and Cornick, 2016; Herbert-Read et al., 2017; Branstetter et al., 2018), habitat displacement (Dähne et al., 2013), hearing impairment (Kastelein et al., 2015; Kastelein et al., 2016; Kastelein et al., 2018), and even physical injury and mortality of some marine species (Halvorsen et al., 2012; Casper et al., 2012; Casper et al., 2013).

However, almost all research on sound emitted during pile driving has been limited to two conventional pile driving methods: impact and vibratory pile driving, where the hammer acts directly on the pile head to force the pile into the substrate (Reinhall and Dahl, 2011; Zampolli et al., 2013; Lippert and von Estorff, 2014; Lippert et al., 2016).

Over the past several years, a new method termed down-the-hole (DTH) pile driving has been used to install piles in marine and coastal construction, especially in areas where the underlying substrate consists of hard bedrock. Aspects of DTH pile installation have been adapted from mining, where large diameter holes are drilled through hard rock. DTH drilling utilizes percussive hammering and rotational drilling mechanisms to break the rock into small cuttings and debris, which are pumped out by either air or water (Samuel, 1996; Bruce et al., 2013). When a similar technique is used to directly advance a pile, the DTH hammer bit acts directly on the steel pile shoe at the bottom of the pile and on the rock face. Thus, the pile and the bottom of the hole are advanced together. Alternatively, the DTH hammer bit may act only on the rock face to advance a hole below the bottom of the pile, which typically would then be filled with concrete and reinforcing steel to secure the pile to the rock. In both instances, the DTH hammer uses percussion and rotation while removing the rock cuttings and debris through the exhaust within the pile (e.g., Dazey et al., 2012; Denes et al., 2016; Reyff and Heyvaert, 2019). Based on different applications of pile installation and mechanisms used by different manufacturers, various names have been used for DTH pile installation including DTH drilling (Dazey et al., 2012; Warner and Austin, 2016), DTH hammering (Denes et al., 2019), DTH pile driving (Guan and Miner, 2020), rock socket drilling (Denes et al., 2016; Reyff and Heyvaert, 2019; Reyff, 2020), rock anchor drilling (Reyff and Heyvaert, 2019; Reyff, 2020), and rock anchor installation (Solstice Alaska Consulting, Inc., 2020).

Because of the fundamentally different mechanisms used in DTH pile installation as compared to conventional impact and vibratory pile driving, acoustic characteristics and sound propagation are expected to differ among the three techniques. However, to date, underwater acoustic characteristics from DTH pile installation have not been well described, and their potential impacts on marine species are not well understood (Guan and Miner, 2020). Currently, there is only one peer-reviewed paper (Guan and Miner, 2020) that provides adequate details on the acoustic characteristics of DTH pile installation of 0.46-m diameter steel piles. In that case, the DTH hammer directly struck the steel pile shoe, while also drilling and cutting the rock face to advance the pile. No detailed study has been conducted on DTH pile installation that used a DTH hammer only as a percussive drill to install the pile without direct physical contact with the pile. A schematic diagram of DTH pile driving versus DTH pile drilling is provided in Fig. 1.

FIG. 1.

(Color online) Schematic diagram of DTH pile drilling (A) and DTH pile driving (B), including the (1) drill rod, (2) shaft or pile wall, (3) DTH hammer, (4) pile shoe (used only during DTH pile driving), (5) guide device, and (6) pilot bit that is used for drilling and cutting.

FIG. 1.

(Color online) Schematic diagram of DTH pile drilling (A) and DTH pile driving (B), including the (1) drill rod, (2) shaft or pile wall, (3) DTH hammer, (4) pile shoe (used only during DTH pile driving), (5) guide device, and (6) pilot bit that is used for drilling and cutting.

Close modal

The intent of this paper is to build on the current understanding of sounds emitted during DTH pile installation and to provide new insights regarding the acoustic characteristics of DTH drilling of a 0.84-m shaft to install rock anchors to secure the 1.22-m diameter steel pipe piles without direct contact with the pile. Although industry generally terms any pile driving method using a DTH hammer as “DTH pile driving,” we aim to distinguish between the different uses of a DTH hammer that result in different types of sound emitted, which has profound implications for the community tasked with regulating underwater sound.

The study site was located in Ward Cove on the north side of Tongass Narrows in Southeast Alaska, where sound source measurements were collected by Power System & Supplies of Alaska during construction of a new cruise ship dock (Solstice Alaska Consulting, Inc., 2020). For this project, a conventional vibratory hammer advanced 1.22-m diameter steel pipe piles in 30 m of water through loose marine sediments (approximately 15 m of sand with silt, shells, gravel, and cobbles) to contact hard bedrock. A conventional impact hammer then seated the piles into the bedrock, which was comprised of marine greywacke and consolidated hard mudstone. Following installation of the steel pile, a DTH hammer equipped with a 0.84-m diameter bit advanced a drilled hole below the pile and into the bedrock. After DTH drilling was completed, the DTH hammer was removed, and the shaft and a portion of the pile were filled with a rebar cage and concrete to secure the piles to the drilled hole and the bedrock. During DTH drilling for the rock anchors, the drill bit did not contact the pile, and steel-on-steel hammering did not occur. Drill cuttings were removed by a pump exhaust through the middle of the pipe pile (Solstice Alaska Consulting, Inc., 2020).

Underwater acoustic recordings were collected during DTH pile drilling of two 0.84-m shafts in the 1.22-m piles, denoted as Pile 137E and 131W, on 19 and 20 June 2020, respectively. DTH pile drilling of Pile 137E occurred between approximately 16:50 and 20:30 local time, and DTH pile drilling of Pile 131W occurred between approximately 14:35 and 18:00. Both piles were installed at an oblique angle of 15° from normal. On 19 June 2020, acoustic measurements were conducted simultaneously at horizontal distances of 10, 200, and 1300 m from the pile, along the direction of the oblique angle. On 20 June 2020, measurements were conducted at 10, 90, and 150 m from the pile, along the direction of the oblique angle. Water depth at these locations ranged from approximately 30–60 m, with the hydrophones suspended at approximately 0.5 to 0.6 times the water depth. For all recordings other than the 1300-m location, the hydrophones were suspended from a large floating structure that was in the process of being secured for cruise ship use. The structure was more than 200 m in length, approximately 80 m in width, and very stable. The 1300-m hydrophone was suspended from an assembly of large logs that served as a breakwater. A schematic diagram of Pile 137E is shown in Fig. 2.

FIG. 2.

Schematic diagram of relative positions of the pile, the 10-m hydrophone, and the underlying bathymetry for DTH pile drilling of Pile 137E on 19 June 2020 (distance not to scale).

FIG. 2.

Schematic diagram of relative positions of the pile, the 10-m hydrophone, and the underlying bathymetry for DTH pile drilling of Pile 137E on 19 June 2020 (distance not to scale).

Close modal

The hydrophone used at the 10-m location on both days was a Reson model TC 4040; the one used at 200 m on 19 June and at 90 m on 20 June was a Reson model TC 4033, and the one at 1300 m on June 19 and at 150 m on June 20 was a Reason model TC 4032. The TC 4040 and 4033 hydrophones were connected to a Brüel & Kjær model 2635 charge amplifier, which provided conditioned signals to a Brüel & Kjær model 2270 Class 1 sound level meter (SLM). The TC3032 has an internal pre-amplifier, and it was connected directly to a model 2270 SLM. The receiving sensitivity levels of the TC 4033, 4040, and 4032 hydrophones were −203, –206, and −164 dB re 1 V/μPa, respectively. The sampling frequency was 48 kHz. Each system was calibrated before and after each recording session using two GRAS 42AC high-pressure pistonphones (GRAS Sound & Vibration, Holte, Denmark).

Analyses of acoustic recordings were conducted using custom written matlab (MathWorks Inc., Natick, MA, version R2020a) scripts. For visual inspection of the overall acoustic characteristics of DTH pile drilling, time-varying sound pressure level (SPL) plots based on 1-s averages (Lp,rms,1-s) were computed to produce spectrograms of each pile drilling event over the frequency range from 10 to 22000 Hz. Spectrograms for each DTH pile drilling event were generated using a fast Fourier transform size of 10240 and a window size of 10240 with 50% overlap. For detailed acoustic analyses, root mean square (rms) SPL with 90% energy window (Lp,rms,90%), peak SPL (Lp,pk), and single-strike sound exposure level (LE,ss) were computed for each percussive strike measured at the different distances. The definitions of these acoustic metrics are based on ISO 18405:2017 Underwater Acoustics— Terminology (ISO, 2017). The Lp,rms,90% metric comprises 90% of the acoustic energy in a given strike and was calculated using Eq. (1) based on Madsen (2005),

Lp,rms,90%=10log101TTpt2pref2dt,
(1)

where p(t) is the instantaneous acoustic pressure (Urick, 1983), pref is the referenced acoustic pressure (which equals 1 μPa), and T is the pulse duration that comprises 90% of the acoustic energy. Cumulative sound exposure levels (LE,cum) and averaged Lp,rms,1-s also were calculated for each DTH pile drilling event at various distances. In addition, power spectral densities (PSDs) of a single strike recorded at the various distances were computed to investigate the frequency content of the pulses.

Acoustic recordings that did not contain percussive sounds were manually separated to determine the sound levels associated with other aspects of DTH drilling, such as rotation of the drill bit and airlift of debris and cuttings. These sounds are considered non-impulsive, since there were no detectable pulses. Therefore, they were analyzed using Lp,rms,1-s.

A total of 1280 min of recordings were obtained during both DTH pile drilling events from all locations. The recorded acoustic data covered all DTH pile drilling events except for the 1300-m location on 19 June, for which data after 17:39 were not included in the analysis due to contamination from nearby vessel sound. DTH pile drilling of Pile 137E on 19 June took approximately 220 min, of which 147 min involved active drilling with 51378 percussive hammer strikes. DTH pile drilling of Pile 131W on 20 June took approximately 180 min, of which 156 min involved active drilling with 66702 percussive hammer strikes. Percussive hammering was intermittent over these time periods.

On 19 June 2020, the LE,ss at 10 m from the pile ranged from 126 to 151 dB re 1 μPa2s, with both median (50-percentile of all measured values) and mean values of 138 dB re 1 μPa2s. The 10-m LE,ss measurement on 20 June 2020 ranged from 129 to 157 dB re 1 μPa2s, with median and mean values of 142 and 143 dB re 1 μPa2s, respectively. The LE,cum at 10 m for the entire DTH pile drilling event on 19 June was 185 dB re 1 μPa2s; on 20 June, the LE,cum was 8 dB higher at 193 dB re 1 μPa2s.

The median Lp,rms,90% at 10 m were 150 and 155 dB re 1 μPa for 19 and 20 June, respectively, and the median Lp,pk at 10 m for the two days were 158 and 163 dB re 1 μPa, respectively. The median rms pulse duration, defined as the duration that comprises the 90% energy window of Lp,rms,90% at all measured distances, were between 51 and 59 ms, and a trend of lengthened pulse duration with increasing distance was not observed, as typically occurs for impact pile driving (Amaral et al., 2020) and other impulsive sources (Guan et al., 2015). A summary of the median LE,ss, LE,cum, Lp,rms,90%, Lp,pk, and averaged Lp,rms,1-s at the different distances for both days is provided in Table I.

TABLE I.

Median underwater sound levels from two DTH pile drilling events measured at various distances in Ward Cove, Alaska. LE,cum at 1300 m was calculated using the equation LE,cum = LE,ss + 10log10N, where LE,ss is the median single-strike sound exposure level before 17:39, and N is the number of percussive strikes detected at 10 m for the entire DTH pile drilling event.

Acoustic parametersPile 137E (19 June 2020)Pile 131W (20 June 2020)
10 m200 m1300 m10 m90 m150 m
Median LE,ss (dB re 1 μPa2s) 138 122 109 142 127 124 
LE,cum (dB re 1 μPa2s) 185 170 150 193 180 177 
Median Lp,rms,90% (dB re 1 μPa) 150 134 122 155 140 137 
Median Lp,pk (dB re 1 μPa) 158 143 130 163 149 147 
Median rms pulse duration (ms) 58 59 53 55 51 51 
Median averaged Lp,rms,1-s (dB re 1 μPa) 147 132 119 151 136 133 
Acoustic parametersPile 137E (19 June 2020)Pile 131W (20 June 2020)
10 m200 m1300 m10 m90 m150 m
Median LE,ss (dB re 1 μPa2s) 138 122 109 142 127 124 
LE,cum (dB re 1 μPa2s) 185 170 150 193 180 177 
Median Lp,rms,90% (dB re 1 μPa) 150 134 122 155 140 137 
Median Lp,pk (dB re 1 μPa) 158 143 130 163 149 147 
Median rms pulse duration (ms) 58 59 53 55 51 51 
Median averaged Lp,rms,1-s (dB re 1 μPa) 147 132 119 151 136 133 

The median averaged Lp,rms,1-s were 138 and 140 dB re 1 μPa at 10 m on 19 June and 20 June 2020, respectively, for the periods when percussive hammering was absent. Those sound levels were 7–13 dB less than the median averaged Lp,rms,1-s sound levels when percussive hammering occurred. The median averaged Lp,rms,1-s at 90 m (recorded on June 20) was 123 dB re 1 μPa. At the distances of 150 (20 June) and 200 m (19 June), the median averaged Lp,rms,1-s were 114 and 115 dB re 1 μPa, respectively. Sound levels at these distances were likely just above the local ambient sound levels.

Figure 3 depicts the detailed acoustic characteristics of a 1-s example of DTH pile drilling sound as a time series, SPL plot, and spectrogram. The pulsive structure from the percussive hammer is obvious in the time series (Fig. 3, top plot) but not prominent in the spectrogram (Fig. 3, bottom plot). Throughout the recordings, the percussive hammering occurred at a strike rate of 10–12 strikes/s. It also is evident from the spectrogram that most of the acoustic energy from DTH pile drilling occurs below 1 kHz. In addition, the overall sound levels between percussive strikes are much higher than typical natural ambient sound levels (Fig. 3, middle plot), which generally do not exceed 120 dB re 1 μPa broadband except in areas with large tides and strong currents (e.g., Castellote et al., 2019) or in glacial fjords where ice calving occurs (e.g., Pettit et al., 2015).

FIG. 3.

(Color online) A 1-s example of time series (top), SPL (middle), and spectrogram (bottom) of DTH pile drilling recorded at 10 m from the pile on 19 June 2020.

FIG. 3.

(Color online) A 1-s example of time series (top), SPL (middle), and spectrogram (bottom) of DTH pile drilling recorded at 10 m from the pile on 19 June 2020.

Close modal

Figure 4 shows SPL plots based on 1-s averages, measured LE,cum over each DTH pile drilling event, and spectrograms for the DTH pile drilling events at various distances on both days. As previously stated, recordings at 1300 m from Pile 137E after 17:39 on 19 June were contaminated by vessel sound and thus were not included in the analysis. The high fluctuation in SPLs indicates that the percussive hammer action was intermittent throughout each DTH pile drilling event. The intermittent aspect of hammering also is apparent when reviewing the SPL plot, which clearly show times when the percussive pattern, as shown in Fig. 4, is absent. However, the overall high sound floor (≥130 dB re 1 μPa) at 10 m indicates that other sound-generating activities (i.e., use of air compressors, clanging of air lines and cables, etc.) exist aside from percussive hammer strikes. In most instances, LE,cum levels off after about 1 h into DTH pile drilling. In addition, the spectrograms show that some higher frequency components (>1 kHz) were evident in the first 20 min or so of each DTH pile drilling event (Fig. 4). The median broadband LE,ss was approximately 2 dB higher during this period than during the remaining DTH pile drilling time on 19 June, but more than 12 dB higher on 20 June.

FIG. 4.

(Color online) SPL (line plots, black solid line), LE,cum (line plots, blue dashed line), and spectrograms (color plot) of DTH pile drilling of Pile 137E on 19 June 2020 at 10 m (A), 200 m (B), and 1300 m (C) and Pile 131W on 20 June 2020 at 10 m (D), 90 m (E), and 150 m (F).

FIG. 4.

(Color online) SPL (line plots, black solid line), LE,cum (line plots, blue dashed line), and spectrograms (color plot) of DTH pile drilling of Pile 137E on 19 June 2020 at 10 m (A), 200 m (B), and 1300 m (C) and Pile 131W on 20 June 2020 at 10 m (D), 90 m (E), and 150 m (F).

Close modal

Figure 5 presents the PSDs of median LE,ss at various distances on both days. The solid lines in Fig. 5 denote PSDs for each DTH pile drilling event, while the dashed lines show PSDs for the initial 20 min of drilling, when higher-frequency energies were observed in the spectrograms (Fig. 4). In general, the spectra show that PSD levels follow a general decreasing trend after 100–200 Hz at a rate of approximately 30 dB/decade until 8–12 kHz. An increase in PSD levels with increasing frequency above approximately 15 kHz also is observed. Acoustic measurements collected at 10, 90, and 150 m indicate that PSD levels from approximately 500–7000 Hz were 10 dB higher during the initial 20 min of each DTH pile drilling event (dashed lines in Fig. 5). In addition, occasional tones at approximately 1.2, 2.4, and 3.9 kHz are evident in some of the recordings.

FIG. 5.

(Color online) PSD of median LE,ss from DTH pile drilling of Pile 137E on 19 June (A) and 20 June (B) 2020. The solid lines are the PSDs for the entire DTH drilling event, and the dashed lines are PSDs for the initial 20 min of drilling, when higher-frequency energies were present.

FIG. 5.

(Color online) PSD of median LE,ss from DTH pile drilling of Pile 137E on 19 June (A) and 20 June (B) 2020. The solid lines are the PSDs for the entire DTH drilling event, and the dashed lines are PSDs for the initial 20 min of drilling, when higher-frequency energies were present.

Close modal

The past several years have seen the increased use of DTH pile driving and drilling methods to install and support piles for offshore and coastal construction activities. However, only a few measurements have been reported on the sound levels associated with these activities (e.g., Denes et al., 2016; Denes et al., 2019; Reyff and Heyvaert, 2019; Reyff, 2020), with even fewer detailed studies regarding sound characterization (Guan and Miner, 2020). The lack of detailed characterization of DTH pile driving and drilling sounds has resulted in inconsistencies amongst impact assessments (Guan et al., 2021). As such, this study provides additional data on the sound levels and characterization from DTH pile installation of a different pile size using a different mechanism than previously documented.

Table II provides a comparison of various sound levels measured at 10 m among several DTH pile driving studies. The 10-m sound levels measured during DTH pile drilling of the 0.84-m diameter shafts within two 1.22-m piles at Ward Cove were lower than those reported for smaller piles (Reyff, 2020). Specifically, the 10-m sound levels for 0.2-m rock anchor drilling at White Pass and Yukon Route's railroad dock in Skagway, Alaska (Reyff, 2020), were greater than those reported herein for rock anchor installation at Ward Cove (Table II). The differences are mainly due to the different type of DTH piling mechanism used for the Ward Cove construction project: the hammer was only used to drill shaft holes in the substrate to secure the larger 1.22-m piles that already had been installed to the desired depths with conventional vibratory and impact pile driving methods (Solstice Alaska Consulting, Inc., 2020). A detailed discussion and comparison of all known DTH pile driving studies is summarized by Guan and Miner (2020). Unlike previously reported DTH pile driving activities (e.g., Denes et al., 2016; Denes et al., 2019; Reyff and Heyvaert, 2019; Reyff, 2020; Guan and Miner, 2020), the DTH hammer did not directly contact the pile shoe at Ward Cove. Currently, pile installation using a DTH hammer is generally referred to as “DTH pile driving” by industry and within the regulatory community, regardless of the fundamental mechanism of installing the pile. To differentiate the functions of the two installation mechanisms between this DTH piling activity from the previous ones, we propose to use the term “DTH pile driving” for those activities where the hammer strikes the pile shoe, and “DTH pile drilling” for the current activity, where the hammer does not contact the pile.

TABLE II.

Comparison of sound levels and strike rates at 10 m from this study (Ward Cove) and several other DTH pile driving studies.

Acoustic parametersWard Covea (This study)Skagwaya (Reyff and Heyvaert, 2019; Reyff, 2020)Biorka Islanda (Guan and Miner, 2020)Skagwaya (Reyff and Heyvaert, 2019; Reyff, 2020)Thimble Shoalb (Denes et al., 2019)
Pile or shaft holec diameter (m) 0.84 0.2 0.46 1.07 1.07 
LE,ss (dB re 1 μPa2s) 138–142 144d 145–147 164d 164 
LE,cum (dB re 1 μPa2s) 185–193 193 191–192 207 Not reported 
Lp,rms,90% (dB re 1 μPa) 150–155 Not reported 161–162 178 180 
Strike rate (strikes/s) 10–12 15 13 10 
Acoustic parametersWard Covea (This study)Skagwaya (Reyff and Heyvaert, 2019; Reyff, 2020)Biorka Islanda (Guan and Miner, 2020)Skagwaya (Reyff and Heyvaert, 2019; Reyff, 2020)Thimble Shoalb (Denes et al., 2019)
Pile or shaft holec diameter (m) 0.84 0.2 0.46 1.07 1.07 
LE,ss (dB re 1 μPa2s) 138–142 144d 145–147 164d 164 
LE,cum (dB re 1 μPa2s) 185–193 193 191–192 207 Not reported 
Lp,rms,90% (dB re 1 μPa) 150–155 Not reported 161–162 178 180 
Strike rate (strikes/s) 10–12 15 13 10 
a

Median sound levels were reported.

b

Average sound levels were reported.

c

Ward Cove is the only location where shaft holes were installed rather than piles.

d

The Skagway LE,ss was based on a 1-s average and the strike rate rather than measurements of each single pulse.

In comparison with DTH pile driving at Biorka Island, Alaska, there was a lack of higher acoustic energies above 1000 Hz for Ward Cove (see the spectrograms in Figs. 2 and 3 from Guan and Miner (2020) for Biorka Island vs Fig. 4 in this paper), except for the beginning 20 min. The higher frequency components likely were the result of the DTH hammer striking the pile shoe and the rock surface for Biorka Island and only striking the rock surface for Ward Cove. As DTH pile drilling progressed for Ward Cove, the percussive drilling action moved deeper into the substrate, resulting in attenuation of most of the higher frequency acoustic energies. However, during DTH pile driving at Biorka Island, the acoustic energy generated from the DTH hammer striking the pile shoe was transmitted through the pile wall into the water column with less attenuation. Differences in substrate types and hardness and prevalence of bedrock (i.e., interspersed vs solid rock formations) also could affect the higher-frequency components of the acoustic energies. Such hypotheses need to be validated with additional empirical measurements via well-designed experiments, including site-specific descriptions of substrate characteristics, or numerical modeling of the sound-generating mechanisms.

Sound-generating mechanisms of conventional in-water impact pile driving have been well studied (e.g., Reinhall and Dahl, 2011; Dahl and Reinhall, 2013; Lippert et al., 2016; Deng et al., 2016; Jeong et al., 2019; Tsouvalas, 2020), with relatively fewer studies conducted on vibratory pile driving (e.g., Dahl et al., 2015a). The forcing function for impact pile driving acts directly on the pile head, resulting in an axial stress wave that causes the Poisson effect along the pile wall, forming a series of Mach cone waves that radiate from the pile into the media (Reinhall and Dahl, 2011; Dahl and Reinhall, 2013; Dahl and Dall'Osto, 2017). Based on previous studies, sound source characteristics for impact pile driving of vertical steel piles can be accurately predicted using finite element modeling techniques (Reinhall and Dahl, 2011; Kim et al., 2013; Zampolli et al., 2013). Using a vertical line array, MacGillivray (2018) conducted beamforming measurements of underwater sound during impact pile driving of conductor casings in a deep-water (365 m) environment off California and observed higher-frequency (>1 kHz) secondary waves of higher angles (>45°) that were generated near the surface. Nevertheless, the stronger downward sound from the Mach cone waves dominated the sound field, resulting in a shadow zone near the sea surface (MacGillivray, 2018).

Wilkes and Gavrilov (2017) and Martin and Barclay (2019) conducted studies on raked piles that were installed at an angle away from normal. Their results show that, although the forcing function associated with impact driving of raked piles follows the Mach cone wave model, the directivity of radiated sound depends on the azimuth of the pile. Because the forcing function for DTH pile installation is wholly or partially on the bedrock, it is unlikely similar Mach cone waves like those observed during conventional impact driving would be formed. Instead, the primary sound-generating mechanism would be based on the interaction between the DTH hammer and pile shoe and/or bedrock. Considering this, it also is likely that the source directivity of DTH pile installation would not vary regardless of whether the pile was installed normal or at an angle (raked or battered piles), as in this case. However, since only one hydrophone was used at each distance, the directivity of the sound field cannot be verified for the current study.

Many numerical sound propagation models have been developed over the past several years to predict distances to various impact zones (e.g., Zampolli et al., 2013; Lippert and von Estorff, 2014; Tsouvalas and Metrikine, 2014; Fricke and Rolfes, 2015; Lin et al., 2019). Those models have been described in a comprehensive review by Lippert et al. (2016). Since the forcing function for impact pile driving is applied to the pile head and transmits along the pile wall, sound that radiates from the pile behaves like a line source through the water column. Using a decay rate that is based on the parameters of a Pekeris waveguide with substrate geoacoustic characteristics, Lippert et al. (2018) developed a simple analytical method, the damped cylindrical spreading (DCS) model, to estimate distances to sound thresholds from in-water impact pile driving. Measurements of sound fields during impact pile driving for various wind turbine projects have validated that the DCS model provides more accurate estimates of sound propagation than geometrical spreading models (e.g., Martin and Barclay, 2019; Ainslie et al., 2020; Heaney et al., 2020).

For DTH pile driving and pile drilling, the hammer strikes the pile shoe and/or bedrock, and the axial stress wave propagating up the pile is much smaller that the stress wave associated with impact pile driving. Thus, the source should be treated primarily as a point source instead of a line source. However, given the high variability of the acoustic medium at the source, propagation modeling is complex. Particularly for DTH pile drilling, as percussive drilling progresses deeper into the substrate, a large portion of the acoustic energy likely propagates through the sediment before entering the water column rather than being emitted directly into the water column. This phenomenon may explain the lower sound levels observed at Ward Cove compared to sound levels measured at the same distances during installation of smaller piles using DTH pile driving methods.

Based on sound levels measured at different distances at Ward Cove, sound propagation loss coefficients were computed using the first degree polynomial curve fitting of median values to estimate the sound levels with range (Fig. 6). For example, the decay function for LE,ss on 19 June 2020 was

LE,ss=13.4 log10R+150.6,
(2a)

and on 20 June 2020 was

LE,ss=15.5 log10R+157.6,
(2b)

where R is distance in m from the pile. Propagation loss coefficients for all acoustic metrics were between 13.1 and 15.5 (Fig. 6).

FIG. 6.

(Color online) Median (A) LE,ss, (B) Lp,pk, (C) Lp,rms,90%, and (D) Lp,rms,1-s decay rates with range during DTH pile drilling on 19 June (blue) and 20 June 2020 (red). Circles and triangles indicate median measured values on 19 June and 20 June, respectively. Dashed lines are the least square fits of the medians.

FIG. 6.

(Color online) Median (A) LE,ss, (B) Lp,pk, (C) Lp,rms,90%, and (D) Lp,rms,1-s decay rates with range during DTH pile drilling on 19 June (blue) and 20 June 2020 (red). Circles and triangles indicate median measured values on 19 June and 20 June, respectively. Dashed lines are the least square fits of the medians.

Close modal

The LE,ss propagation loss coefficients of 13.4 and 15.5 are comparable to that from DTH pile driving of 1.07-m steel piles at White Pass and Yukon Route's railroad dock in Skagway, Alaska, which was 16.2 when curve fitted between 12 and 1400 m (Reyff, 2020). However, these values are higher than those reported for DTH pile driving of 1.07-m steel piles at Thimble Shoal for the Chesapeake Bay Bridge Tunnel project in Virginia, in which the propagation loss coefficients ranged between 5.5 and 10.8 for LE,ss (Denes et al., 2019), with the proviso that the curve fitting only analyzed data between approximately 10 and 100 m.

In regard to assessing auditory impacts of underwater sound on marine mammals, sound sources are grouped into two mutually exclusive types, impulsive and non-impulsive, and are evaluated using different thresholds (NMFS, 2018; Guan and Miner, 2020). Although a clear definition that differentiates these two types of sound sources does not currently exist, it is customary to consider broadband, transient sounds with short rise times as impulsive, and the rest as non-impulsive. For example, sounds generated during impact pile driving are considered impulsive, and those from vibratory pile driving are non-impulsive. For DTH pile driving and drilling, the sound-generating mechanism is composed of the percussive hammer striking the pile shoe and/or bedrock, which is similar to impact pile driving sound (i.e., impulsive), and drilling and debris clean out via pumping, which resembles vibratory pile driving sound (i.e., non-impulsive).

For sound sources that cannot be clearly categorized into one of these two sound types, Southall et al. (2007) suggested using a 3-dB difference in measurements between the continuous and impulse settings of an SLM. Specifically, if the readings from the SLM are 3 dB or greater between the impulse setting (a 35-ms window) and the continuous setting (a 1-s window), the sound would be considered as impulsive. A snap-shot analysis of the Ward Cove data showed an average difference of 2.0 dB. Therefore, DTH pile drilling at Ward Cove would be considered non-impulsive. In comparison, the other available DTH pile driving analyses yielded differences of more than 3 dB (Reyff and Heyvaert, 2019; Denes et al., 2019; Reyff, 2020; Guan and Miner, 2020). It is likely that the lack of percussive hammer striking on pile shoe in DTH pile drilling resulted in the overall reduced impulsiveness at Ward Cove.

Although it is appropriate to treat the continuous sounds from drilling and debris clean out as non-impulsive, it is not appropriate to treat the percussive hammering aspects of DTH pile drilling as non-impulsive, as the sound structure clearly contains pulsive features (Fig. 3). Sounds that contain both pulsive structures and steady-state continuous oscillation are referred to as complex in human psychoacoustic research (Ahroon et al., 1993). Studies in human and terrestrial mammal noise-induced threshold shifts (NITSs) have shown that under the same sound exposure level, exposure to complex sounds (i.e., non-Gaussian sounds) are more damaging to hair cells than continuous Gaussian sounds (Lei et al., 1994; Hamernik et al., 2007).

Over the past few decades, a statistical measure of kurtosis (β) has been used to describe the level of “impulsiveness” of complex sounds in human-based impact analyses (e.g., Erdreich, 1986; Zhao et al., 2010; Fuente et al., 2018), and studies have shown that increased kurtosis in the sounds emitted led to more outer hair cell loss under the same SPL (Hamernik et al., 2003; Qiu et al., 2013). Most recently, kurtosis also was proposed for use in determining whether a source is impulsive or non-impulsive for underwater anthropogenic sounds (Martin et al., 2020; Müller et al., 2020).

Given the studies showing the apparent relationship between kurtosis and the degree of NITS, it is imperative that additional research be conducted on the impulsiveness of DTH pile driving and pile drilling using kurtosis measurements. Studies that involve better understanding of the auditory effects from complex underwater sounds, such as those from DTH pile driving and pile drilling, also are needed (Guan and Brookens, 2021). Further, consistent terminology must be used to describe the different DTH piling techniques.

This study describes in detail a novel underwater anthropogenic sound source, the DTH pile drilling technique, that used a 0.84-m diameter drill bit to install a shaft to secure 1.22-m diameter steel pipe piles. The results show that the acoustic characteristics differ from previously described DTH pile driving in which a percussive hammer also strikes the pile shoe to advance the pile. In the current study, all sound levels (LE,ss, Lp,rms, and Lp,pk) were lower than those from DTH pile driving reported by others at comparable distances (see Table II; Denes et al., 2016; Denes et al., 2019; Reyff and Heyvaert, 2019; Reyff, 2020; Guan and Miner, 2020), despite the fact that some piles from the previous studies were smaller. The median LE,ss measured at 10 m from the pile were 138 and 142 dB re 1 μPa2s on 19 and 20 June 2020, respectively. The lower sound levels in this study are likely due to the different hammering mechanism in which the DTH hammer did not strike the pile shoe. Instead, it functioned as percussive drilling, only contacting the underlying substrate (Solstice Alaska Consulting, Inc., 2020). Furthermore, unlike DTH pile driving, where most of the acoustic energies were below 2 kHz (Guan and Miner, 2020), this study showed that the dominant acoustic energy from DTH pile drilling was below 1 kHz (Figs. 3 and 5).

Similar to DTH pile driving, sounds generated from DTH pile drilling include both impulsive and non-impulsive components and are referred to as complex sound (Ahroon et al., 1993). Despite the industry general term of “DTH pile driving,” which includes all pile installation using a DTH hammer, the results from this study show that when the hammer is used only as a percussive drill without directly contacting the pile, the overall sound levels generated are less impulsive. Using the currently accepted method of a 3-dB difference between the impulsive and continuous setting of an SLM, DTH pile drilling sound from the current study would be categorized as non-impulsive under the U.S. regulatory framework for assessing auditory impacts to marine mammals from underwater sound (NMFS, 2018). However, we argue that this categorization may be inappropriate because our analysis showed that DTH pile drilling sound is a complex sound that contains impulsive structure as well as continuous oscillations (Fig. 3), and numerous studies have shown that exposure to complex sound is more damaging than non-impulsive Gaussian sound with the same sound exposure level (Hamernik et al., 2007). Further studies that quantify the impulsiveness of DTH pile driving and pile drilling are needed to provide basic information on auditory impacts to marine mammals exposed to these types of anthropogenic sounds.

The authors thank Turnagain Marine Construction, Inc., and Solstice Alaska Consulting, Inc., for supporting collection of the acoustic recordings. We also thank Paulina Chen, Rodney Cluck, Yoko Furukawa, Stanley Labak, Erica Staaterman, Peter Thomas, and three anonymous reviewers for their constructive comments on the manuscript.

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