The Weirdest Tool in Underwater Construction

[00:02] In 1989, the Loma Prieta earthquake  shook the central coast of California,   collapsing buildings and damaging infrastructure  across the Bay Area. Bridges, in particular,  

[00:14] suffered extensive damage. In one case, a  major section of the eastern span of the   Bay Bridge's deck collapsed, falling onto the  lower deck like a trapdoor. Sadly, one person  

[00:26] died driving off the upper deck. Crews had the  bridge repaired within a month, but Caltrans   knew that the next earthquake could be worse and  started making plans to replace the structure.

[00:38] Knowing that the replacement project would require  heavy-duty piles, Caltrans developed a testing   program to identify risks and challenges during  design and minimize the chance of unanticipated  

[00:50] problems cropping up during construction. And  they found a pretty big one. In October of 2000,   the barge began the pile driving operation,  dropping a large hammer to drive the 8-foot   (or 2.4 meter) diameter steel pipe deep  into the seafloor. Almost immediately,  

[01:08] fish began dying in the surrounding area.  Biologists involved in the project collected   fish and documented injuries to their organs and  swim bladders. They weren’t being directly hurt by   the hammer itself; it was above the water anyway.  The damage was coming from the intense sound.

[01:26] That massive steel pipe rang like a humongous  bell on every hammer blow, radiating sound   pressure through the San Francisco Bay. It even  had serious impacts on aquatic wildlife up to  

[01:38] a kilometer away, which was a pretty big deal.  Because San Francisco Bay is home to quite a few   threatened or endangered species of fish. The  problem was that the replacement bridge would   need more than 250 of these piles. Caltrans  had to figure out how to install them without  

[01:55] affecting the wildlife in the process, and the way  they did it, I think, is pretty cool. And I even   built a model in the garage to show you it works.  I’m Grady, and this is Practical Engineering.

[02:16] If you want to know the answer right away, it's  bubbles. But I think the most interesting part   is why it works in the first place. And this  matters. Pile driving isn’t the only thing   that creates excessive noise underwater. We  do a lot of construction in waterways, oceans,  

[02:32] rivers, and bays. We also occasionally  have to blow stuff up underwater,   like for demolition of structures or safe  disposal of old munitions and mines. Any   loud work underwater has the potential  to disrupt, injure, or even kill aquatic  

[02:49] wildlife. The phenomenon we know as sound is  just fluctuations in pressure within a medium,   whether it’s air or water (or even concrete). We  sense those fluctuations mainly through our ears,  

[03:01] but pressure fluctuations can do a lot more than  just vibrate the thin membranes, tiny bones, and   hairs. Barotrauma is the term used to describe the  damaging effects of compression and decompression  

[03:13] on wildlife. And it really has only been in  the past few decades that we’ve really started   to apply the science of hydroacoustics to our  own activities and try to mitigate the impacts.

[03:25] You’ve probably heard of sound pressure expressed  in decibels. It’s really just a logarithmic scale   of convenience thing because meaningful pressures  can range across many orders of magnitude. So the  

[03:37] decibel system just makes the numbers easier  to compare. The equation for a decibel is just   20 times the base 10 logarithmic function of the  sound pressure divided by a reference pressure.  

[03:50] Sounds complicated, but it just means  a 1-decibel increase corresponds to an   increase in sound pressure of about 26 percent.  The amount of time over which sound pressure is  

[04:02] measured also matters. Look at a waveform  and you can see there are peaks (both in   compression and rarefaction). But that’s only  for a split second. So a lot of measurements   use a root mean square of the sound pressure  over a given time to provide a better estimate.  

[04:17] We don’t have to go into the math of that,  just think of it as a fancy kind of average. It’s important to point out that, in air, we  use 20 micropascals as the reference pressure,   which is approximately the limit of human  hearing. So that’s 0 decibels. Underwater,  

[04:33] we use a reference pressure of 1 micropascal,  mainly just for standardization purposes,   so just keep in mind that underwater decibels  aren’t really equivalent to sound pressures you   might have as references in your head like the  75-decibel vacuum cleaner or the 140-decibel jet engine.

[04:53] And really, what you think of as sound  has less meaning underwater because our ears and   brains are calibrated for the physics of sound  in air. The underwater version of “loudness”  

[05:05] doesn’t translate well to human perception. But  it matters a lot to fish and marine mammals. Sound behaves a lot differently in water  than air. Of course, water is denser,   and sound moves through it at roughly 4 times  the speed it does in air. Sound also carries  

[05:22] a lot further in water, and importantly, the  acoustic impedance of water is way different   than air. Impedance is basically a  measure of opposition to sound flow,   kind of like resistance in an electrical circuit.  It’s a function of the medium’s density and the  

[05:39] speed of sound through it. And at a boundary  between two media, there are two things that   can happen to sound. It can transmit into the new  medium or it can reflect back, and the difference   in impedance between the two determines how  much of each will occur. If impedances match,  

[05:57] more sound will transmit through the boundary.  If they’re way off, like water and air,   most of the sound is reflected. The practical  effect of that is a transmission loss between   air and water of about 30 decibels. It’s why stuff  happening underwater is quiet above the surface,  

[06:14] and we can take advantage of impedance  mismatch in underwater construction. I built a new acrylic tank for this demo, and  I’ve got a new helper in the shop. This is Brady.  

[06:28] I figured since half the internet calls me that  anyway, we might as well get a Brady in here. He   can wave and nod, and he can probably do a lot of  other stuff too, but that took me several hours,   so he’s just going to bravely hold the hydrophone  for now. And on the other end of the tank,  

[06:43] I have this bluetooth speaker. It claims it’s  underwater rated, so we’ll see if it works out. And here’s the setup; pretty simple. I found a few  recordings of hammering and pile driving sounds to  

[06:55] play on the speaker. And this is how they come  across on the hydrophone, which is connected   to a sound recorder.

[07:11] I also did a frequency sweep  so we can do a little more scientific comparison.  

[07:29] At this point, one of my glue joints on this tank  catastrophically failed and flooded my garage with   water. I didn’t catch it on camera, but Brady took  the brunt of the fall. I got it all fixed up, and now let’s  see if we can soften these construction sounds.

[07:47] I have four air stones made for aquariums  hooked up to an air pump. When I flip these on,   we get a nice curtain of small bubbles between the  speaker and the hydrophone. And I’ll record those   same sounds again.

[08:11] Here’s a look at the waveforms  from the hydrophone with and without the air.   Although it’s not a dramatic difference,  you can definitely see a difference,   especially for the higher pitched hammering  sounds toward the end. And here’s a look at  

[08:23] the waveforms without and with the air for  the frequency sweeps. Even though the sweep   should have had a constant sound pressure  across the full range of frequencies,   the water and demo itself cause pretty serious  distortions. You can see a lot of resonance at  

[08:39] low frequencies, and a lot of attenuation  at high frequencies. That makes it a little   hard to gauge the effectiveness of the bubbles.  It’s similar to the hammering sounds - not much   difference at the lower frequencies, but a pretty  substantial reduction at higher frequencies.

[08:55] This is not an ideal setup for one reason:  even though there’s a big mismatch in acoustic   impedance between air and water, there’s not that  much difference between acrylic and water. So,  

[09:08] it’s pretty easy for pressure waves to propagate  into the acrylic, travel past my bubble curtain,   and back into the water on the other side. So  I’m not getting the kind of sound reduction,   what the pros call attenuation, that  you might expect in the real world,  

[09:24] for example, by surrounding a pile with  a circular ring of air pipes. Thankfully,   the researchers studying solutions like this have  put a lot more resources into figuring out the   right way to do it. The measurements at the Bay  Bridge compared fairly well with mine. Attenuation  

[09:40] was highest as the higher frequencies. But this is  not as simple as just blasting air out of a pipe. These bubble curtain systems require a lot of  logistics. Massive compressors or blowers feed  

[09:53] air sometimes deep below the surface into complex  plumbing assemblies. They usually have filters to   remove oil from the air to make sure the water  isn’t being contaminated. The system has to sit  

[10:05] flush with the bottom to make sure sound can’t  travel underneath the bubble curtain. But also,   there are currents. Any movement of the  water is going to move the bubbles too,   potentially creating gaps in the curtain  or dispersing it altogether. So it’s often  

[10:19] necessary to have multiple levels of plumbing  to keep a continuous screen all the way to the   surface. If that’s not enough, there are ways to  confine the bubbles around a pile or construction   activity using an outer casing or even a flexible  membrane. But how do you know it actually works?

[10:36] Maybe the most comprehensive engineering guidance  on this topic is put out by Caltrans in their   manual on the Hydroacoustic Effects of Pile  Driving on Fish. Appendix 1 in the report is  

[10:48] a nearly 300-page compendium on pile driving  sound data. You might not have known this,   but we’ve been measuring a lot of pile-driving  sounds! If you’re an engineer or environmental  

[11:00] scientist trying to get a permit to build  something underwater and sound is going   to be an issue, this is kind of your bible.  It’s got quite a few ways to minimize impacts,   including timing work when important species  aren’t present, changing designs to reduce  

[11:15] underwater work, using vibratory hammers instead  of conventional equipment, and bubble curtains   that reduce the propagation of underwater  sound pressure. Based on all the testing and   real-world case studies so far, they suggest you  can get about 5 decibels of attenuation this way.

[11:32] Just like my demo, sounds don’t only travel  through the water. They also move through the   sea floor and even through the barge on the  surface, bypassing the bubbles. 5 decibels   doesn’t sound like a big reduction, but you  have to remember that it’s a logarithmic scale.  

[11:47] A 5 decibel reduction means the actual sound  pressure is nearly cut in half. You also have   to remember that what we care about most is area.  For any loud construction or demolition activity,  

[12:00] there’s an invisible ring some distance away that  marks the injury threshold level. Since sound   pressure decreases with distance, eventually  you’re far enough away from the sound that   it doesn’t result in injury. So every foot or  meter that you can pull that ring back toward  

[12:16] the activity through attenuation reduces the  impact area proportional to the distance squared,   dramatically reducing the area in which fish may  sustain injuries. That’s why bubble curtains are  

[12:28] used in so many underwater construction projects  these days, but that’s not all they’re used for. What’s that old saying? If your only tool  is a bubble curtain generation system,   every problem starts to look like a loud  underwater sound. Something like that. It turns  

[12:44] out that bubbles can do a lot more than create an  impedance mismatch for sound pressure propagation.   For one, they aerate water, which can be useful  to prevent algae and other issues with stagnant  

[12:57] pools. For two, they create vertical water  currents. That can help keep things separated,   like trash. You can see it’s a lot harder for  me to move this little boat across the barrier   Of course, a  net or boom or rack can do this too,  

[13:14] but those don’t allow boats to pass through. And  this doesn’t just work for trash. Bubble curtains   have been used to contain oil spills, and they’re  often used in underwater construction not just to   control sound but turbidity. We really don’t want  disturbed sediments clouding up our waterways,  

[13:32] again, primarily for environmental reasons, so  these can be an important tool when booms aren’t   practical. They’ve also been used to control  saltwater and keep it from migrating up rivers   in tidal areas. And they’ve even been employed  to confine herbicides for invasive plants,  

[13:48] allowing for fewer chemicals and less  non-target damage to nearby flora. I’ll definitely be in trouble with the biology  folks if I don’t point out that it’s not just   people who use bubbles as a tool. Humpback  whales cooperate to create bubble curtains  

[14:03] that corral fish to a central point. Then  they lunge into the center to gulp them down,   And we  use bubbles this way on occasion as well, not   for fishing but to keep fish out of certain areas,  usually to prevent the spread of invasive species.

[14:21] By 2005, the pile driving operation on the east  span replacement of the Bay Bridge was complete,   Excellence Award by the Federal Highway  Administration for all the work they did  

[14:35] on minimizing underwater noise impacts  on endangered fish species. And the   lessons from that project have been applied  across the world in the two decades since. You know I love heavy construction. The bigger  and louder the machinery, the better. But I think  

[14:50] that anything we can do to limit the effect we  have on the other things we share this world   with is a win, especially when it’s something  as clever and creative as blowing bubbles.

[15:03] My first exposure to this topic was reading  about how unexploded ordnance is handled in   the Baltic Sea. They use bubble curtains to reduce  the effects on fish, and I’m curious if any of you   know other cool applications in the industry  that I didn’t mention here. I’m so fascinated  

[15:19] to see and learn more about the differences in  techniques and infrastructure across the world.   And there’s probably nowhere you see those  differences better than in transportation. My friend Mike from the DownieLive channel is  on an adventure to travel from the Arctic all  

[15:35] the way to Africa, as much as possible, by  train. The series is called DownieExpress,   and it’s such a cool combination of adventure  and infrastructure. So many cool trains,  

[15:47] from modern high-speed transit to the most basic  coach. I first started watching DownieLive when he   was filming this wild quad tandem bike race,  and this DownieExpress series just   has me hooked. And if you want to watch the  whole thing, it’s only available on Nebula.

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