Sawing a Dam in Half (on Purpose)

0:01 Concrete is the second-most-consumed substance on our planet.

0:04 Only water beats it,

0:06 and actually water is a major ingredient of concrete anyway.

0:11 Every year, humanity mines, mixes,

0:13 and places roughly three metric tons for every person on Earth.

0:18 It’s ubiquitous.

0:19 Most of us hardly even think about all the concrete around us.

0:23 We’ve all seen the grey lumpy mixture flowing

0:26 down chutes into formwork to become a road,

0:29 sidewalk, footing, pile, patio, or foundation.

0:32 It’s easy to think of concrete as a single,

0:35 uniform substance used around the world.

0:37 But it’s not.

0:38 The only reason we are able to use

0:40 so much concrete in construction is that it’s cheap.

0:43 Of the four main ingredients- sand, gravel, cement,

0:46 and water- two of them come directly

0:48 from the ground with little need for processing or refinement.

0:53 One is water.

0:54 Cement is the only ingredient that requires a significant manufacturing process,

0:59 but the raw materials for it are fairly widespread across the globe.

1:04 Many building materials are constrained by geography.

1:07 They only grow, occur mineralologically,

1:09 or are manufactured in specific locations.

1:12 Then they have to be transported, often at great cost, to where they’re needed.

1:18 It’s not true for concrete.

1:19 No matter where you are on earth,

1:22 there’s a pretty decent chance that somewhere nearby exists a ready source

1:25 for at least most of the raw ingredients you need to make it.

1:29 That simple fact has significantly contributed to its widespread use,

1:33 but it’s done something else too.

1:36 Take a look at any geologic map.

1:38 If you’re like me, you do this in your spare time anyway.

1:41 You realize pretty quickly that there is tremendous variability in the different

1:46 kinds of materials that make up the surface of Earth’s crust.

1:49 And the practical result of that, at least for the purposes of this discussion,

1:54 is that every batch of concrete is just

1:56 a little bit different depending on where you go.

1:59 In a way, that’s kind of special, right?

2:01 In most cases, the concrete you see

2:04 around you represents a particular place on Earth.

2:07 Its strength, durability, appearance,

2:09 and essence are highly local characteristics.

2:12 It’s literally made from materials that were sourced not too far away.

2:16 But, in some cases, we’ve learned too late that local

2:20 materials had some hidden problems when used in concrete,

2:23 and the ways we’ve worked to fix those problems

2:26 have created some of the most interesting stories.

2:29 I’m Grady, and this is Practical Engineering.

2:40 This is Fontana Dam on the Little Tennessee River in North Carolina.

2:45 At 150 meters (or nearly 500 feet) in height,

2:48 it’s the tallest dam east of the Mississippi River.

2:51 The north shore of Fontana Reservoir forms

2:54 the border of the Great Smoky Mountains National Park.

2:57 And if you’re through-hiking the Appalachian Trail,

3:00 the famed 2200-mile path through the wildest parts of the eastern United States,

3:05 you have to walk right over the top of it.

3:08 Built by the Tennessee Valley Authority (or TVA), Fontana was completed in 1944,

3:14 just in time to provide hydropower to the Alcoa

3:17 aluminum smelting plant at the end of World War II.

3:21 It’s a concrete gravity dam, meaning that it derives its stability to hold

3:26 back Fontana Reservoir entirely from its own weight.

3:28 And boy does it have a lot of weight.

3:31 More than 2.1 million cubic meters of concrete

3:34 went into the structure before it was finished.

3:37 That’s well over half the volume of Hoover Dam,

3:40 and if you watch the same kinds of videos I do,

3:43 you know that putting all that concrete in Hoover Dam was a major challenge.

3:49 Concrete heats up as it hardens, which can negatively affect the curing process,

3:53 but more importantly, it causes the concrete to expand.

3:57 For a structure like a dam sandwiched between two rocky abutments,

4:01 that expansion can lead compressive stress to build up in the concrete.

4:06 Then, after curing, when the concrete starts to cool back down, it shrinks.

4:11 That shrinking can lead to cracks,

4:13 especially in mass concrete structures that heat up and cool down unevenly.

4:17 And cracks are not ideal for dams.

4:20 To mitigate this issue, pipes were installed within the concrete at Hoover Dam,

4:26 and chilled water was continuously circulated during

4:29 construction to pull heat out of the concrete.

4:31 The same thing was done when they were building Fontana Dam.

4:35 In fact, in addition to the cooling lines,

4:37 the dam was built with deliberate expansion joints that would

4:41 allow each separate concrete block to cool off and shrink.

4:44 Once the concrete cured,

4:46 those joints were grouted to add strength and make the dam watertight.

4:49 It was a pretty robust and thoughtful plan to avoid

4:53 the buildup of stress in the structure, or so they thought.

4:56 In 1972, engineers inspecting the drainage gallery,

4:59 a tunnel through the concrete dam used to collect and redirect drainage,

5:05 noticed unexpected cracks right where the dam curves.

5:08 Later investigation revealed that the cracks extended

5:11 through a large part of the structure.

5:13 At this point, the dam was still less than 30 years old.

5:17 It shouldn’t be deteriorating this quickly.

5:19 But the cracks were serious enough that something needed to be done.

5:23 Engineers initially blamed the Tennessee sun.

5:26 Fontana Dam runs almost perfectly east to west,

5:29 with its broad downstream facing directly south.

5:32 That means a huge area of concrete is exposed to sunlight for most of the day.

5:38 The sun heats the concrete, causing it to expand,

5:41 and over thousands of cycles, cracks are inevitable.

5:44 The curved section of the dam was most vulnerable.

5:47 Reaction forces from the abutments align with the axes of the dam.

5:51 Instead of pure compressive stress,

5:53 the expansion of the concrete created bending stress

5:56 (a combination of expansion and contraction) at the corner.

6:00 In addition to the cracks,

6:02 the movement was also causing the spillway gates to bind up.

6:05 After instruments were installed on the dam,

6:08 the scope of the problem became clear.

6:10 Thermal movement is cyclical with the seasons.

6:12 Concrete may expand in the summer,

6:14 but it returns to its original size in the winter as temperatures cool.

6:19 Fontana had some of that, but underneath

6:22 the cyclical changes was a continuous one.

6:25 The concrete was permanently growing.

6:28 TVA took some cores of the concrete to start planning a repair,

6:31 and sent them out for testing.

6:33 When the results came back, the reason for the unexpected growth was discovered.

6:37 The laboratory that examined the concrete under the microscope noticed

6:41 that some of the aggregates inside had dark rims around them.

6:45 That is a classic sign of alkali-silica reaction,

6:49 or ASR, sometimes known as concrete cancer.

6:53 The fundamental components of concrete are aggregates, large and small,

6:56 bound together by a paste of cement and water.

6:59 As the cement paste hydrates,

7:01 potassium and sodium hydroxides dissolve into the water

7:04 within the tiny pore spaces of the concrete, creating an alkaline solution.

7:09 In some cases, this is a good thing.

7:11 The alkaline environment is great for steel reinforcement,

7:14 helping to prevent rust.

7:16 But for some types of aggregates, it causes a serious problem.

7:21 Specifically, if reactive forms of silica are present,

7:25 they can more readily dissolve in the high-pH water,

7:28 combining with the alkalis to form a kind of gel.

7:31 As that gel absorbs moisture,

7:34 it swells and expands, causing internal stress and cracking.

7:38 This is an extremely widespread problem that has caused structural damage

7:42 in every state in the US and many countries around the world.

7:46 You usually don’t have to search far for an example of a cracked up bridge,

7:51 broken sidewalk, or ruined building foundation that resulted

7:54 from an alkali-silica reaction in the concrete.

7:57 Fortunately, the reaction requires three conditions,

7:59 so there are quite a few ways to deal with it.

8:03 For one, an alkali-silica reaction requires

8:06 the aggregates to actually contain silica, also known as silicon dioxide.

8:10 Well, 90 percent of the Earth’s crust is made up of silicate minerals,

8:15 so this might not seem possible to avoid.

8:18 Luckily, only certain forms of silica are significantly reactive in concrete.

8:22 We have tests we can perform ahead of time

8:25 to identify quarries or sources of rock that react with cement,

8:29 allowing us to just avoid the issue altogether.

8:32 But like I mentioned before,

8:34 the cost of concrete is really sensitive to transportation costs.

8:37 The farther you have to go to get suitable aggregates,

8:40 the higher the project’s costs rise,

8:43 so avoiding local materials is not always ideal.

8:46 The second condition required for an alkali-silica

8:49 reaction is highly alkaline cement.

8:52 So, we have ways to control for that too.

8:55 Cement can be manufactured to have lower alkali content, and we can use what are

9:00 called “Supplementary Cementitious Materials,” like fly ash,

9:03 to replace some of the cement in concrete.

9:06 Those solutions only work if the concrete isn’t already in place, though.

9:10 The third factor of an alkali-silica reaction is excess moisture.

9:14 You can just keep the concrete dry with waterproof coatings or membranes.

9:18 Without moisture, the gel can’t expand, so the problem is solved.

9:22 But there are some structures where waterproofing is a pretty big challenge.

9:28 So TVA was in a bind, literally.

9:30 They were facing the possibility of just having to perpetually

9:34 repair cracks and equipment as Fontana continued to expand.

9:38 Then they decided to get creative.

9:40 Kristen Smith is the Senior Program Manager for Dam Safety at TVA,

9:45 and she explained the thought process: Kristen: You know,

9:48 the impacts on the spillway and powerhouse equipment.

9:50 That led to major maintenance and repairs[...] Need to move from the reactive

9:57 approach- that's not a long-term solution- to a more proactive approach.

10:04 The proactive approach they landed on was a fourth option for dealing with ASR:

10:10 Rather than trying to stop the reaction,

10:12 TVA decided to just give the concrete more room to grow.

10:16 The solid rock abutments at each end of the dam had no room to give,

10:20 so that space would have to be found in the dam itself.

10:24 In 1976, they embarked on a fairly novel

10:27 operation to cut a relief slot all the way

10:31 through Fontana Dam and do it without draining

10:34 the reservoir or causing any disruptions to the hydropower plant.

10:38 The idea was pretty simple:

10:40 instead of building up axial stress as the concrete expands,

10:44 the dam can expand into the newly cut slot.

10:47 Simple in theory; pretty challenging in practice.

10:50 How do you saw a dam in half?

10:52 Luckily, TVA has done this at two of its other dams in addition to Fontana,

10:57 and shared some footage of that so you could see it happen.

11:01 These are big dams, so this isn’t

11:04 sawing with blades you find at ahardware store.

11:06 The tool used for cutting through the concrete

11:08 looks more like a rope than a saw blade.

11:11 Kristen: It is diamond wire, and it's really neat.

11:16 It's, if you touch it, you know, it's 15 millimeters,

11:21 which is a little over half an inch.

11:25 It's abrasive.

11:26 I mean, you know, it would rub your skin if you drug it across your skin,

11:31 but you can touch it.

11:32 You can run your hand along it and it's not going to cut you.

11:35 It can cut through concrete.

11:36 It can cut through steel.

11:39 It looks like a big necklace.

11:42 That big diamond necklace runs along pulleys strategically

11:45 installed on the dam to advance the slot downward.

11:48 The saw pulls the wire in a loop,

11:51 managing the slack and keeping constant tension against the bottom of the slot.

11:55 There are a lot of advantages

11:57 to this, in addition to the practically unlimited depth.

12:00 It causes very little vibration or dust,

12:03 and provides a clean cut without breaking the edges.

12:06 But, there’s a pretty obvious challenge of cutting a slot in a dam:

12:10 how do you deal with the water?

12:12 Turns out, it depends on the dam.

12:15 At Fontana, crews installed a cofferdam on the upstream face

12:18 of the dam to hold back the reservoir during the operations.

12:21 It’s basically half of a steel pipe that seals

12:24 against the concrete face on the sides and bottom,

12:27 just big enough for access to adjust the pulleys.

12:30 At Chickamauga Dam, the geometry made a cofferdam less feasible.

12:33 So instead, they broke the process up into three

12:37 sections separated by boreholes drilled downward into the structure.

12:41 One section could be cut by the diamond

12:43 wire while the other borehole was sealed,

12:45 preventing water from moving through the slot.

12:47 That’s easier said than done, but you can look to your feet for inspiration.

12:51 The seals installed in the boreholes are long rubber tubes called sock seals.

12:57 Kristen: Well, it's like a sock you put on your foot,

12:59 but a half-inch thick rubber and a hundred feet long.” [Grady laughing] Kristen:

13:04 And I've heard it described as kind of like an inside-out fire hose.

13:11 Very strong and waterproof, but to some degree flexible.

13:19 The mess is another problem.

13:21 The dust from the fresh cut concrete mixes with lubricating

13:25 water to form a slurry that runs out of the slot.

13:28 Concrete slurry isn’t good for the environment.

13:31 It mucks up the water and changes the chemistry.

13:34 So the slurry generated by the cutting process

13:36 has to be captured and pumped to holding tanks.

13:39 After the concrete particles have settled out,

13:42 the water can be recirculated to control

13:44 the dust and lubricate the wire as it cuts.

13:47 And this whole process happens essentially non-stop.

13:50 Time is of the essence so that the internal stress

13:54 doesn’t close the slot while the wire is still inside it.

13:57 Slot cutting is relatively low impact on the dam operations,

14:00 but parts of the dam have to be shut down to avoid an accident

14:05 like a broken wire being pulled into a hydro unit or spillway gate.

14:09 One of the reasons this is possible at all is

14:12 that TVA’s concrete dams experiencing ASR are all gravity dams.

14:16 In essence, that means that any vertical slice of the dam

14:20 is theoretically stable on its own without lateral support.

14:23 Cutting a slot in an arch dam wouldn’t work,

14:26 because they depend on axial thrust forces for stability.

14:30 Before, during, and after the slot cutting operation,

14:33 there’s an intensive monitoring program to keep an eye

14:36 on how the dam is behaving and methodically

14:39 measure the movement and strains to make sure

14:41 the dam responds in the way the engineers predict.

14:44 Kristen: We have hundreds and hundreds of instruments

14:48 on the concrete portion of the dam.

14:52 We measure the slot that we've cut.

14:55 Is it closing?

14:57 Is it opening?

14:59 At what rate is it closing or opening?

15:02 We measure our spillway piers.

15:04 Are they moving?

15:06 We measure expansion joints.

15:09 Everything in every direction we measure.

15:13 And those measurements are important because

15:15 the slot cutting isn’t a one-time permanent solution.

15:19 This doesn’t slow down the alkali-silica reaction in the concrete at all.

15:23 It just mitigates the stress building up

15:25 in the structure as the concrete expands, which is basically a non-stop process.

15:30 Over time, the slots close.

15:33 That means that TVA has to go through the operation regularly.

15:37 Kristen: Every approximately five years, we update,

15:41 we use finite element analysis models on our concrete growth projects.

15:47 So they take all of those years

15:51 of new information data from the instruments and they

15:55 recalibrate and they rerun these models and they

15:59 can tell us how effective the slot cut is.

16:02 They can tell us when we need to do it again.

16:05 Whatever we need to do to ensure that we are maintaining

16:09 the integrity of our dams and the adjacent equipment, that's what we do.

16:15 I was curious why they don’t just cut a big slot

16:18 to get a longer period of relief before having to do it again.

16:22 In hindsight, it was kind of a dumb question: Kristen:

16:25 The simple answer is so we don’t leave a big hole in the dam.

16:28 The slot cut at Chickamauga is approximately a half and inch.

16:33 It's a lot easier to stop water from flowing through a half an inch

16:37 slot in a dam than it would be maybe a six inch wide slot.

16:43 In addition, slot cutting is expensive.

16:46 In other words, TVA wants to disturb their structures as little as possible,

16:50 while still mitigating the problems AAR causes.

16:54 It’s a back-and-forth thing.

16:55 You cut, observe, wait, and only cut again when it’s necessary.

17:00 It’s good stewardship of the resources available

17:03 to take care of the structures we’ve already built.

17:06 Alkali-silica reaction in concrete is a huge problem.

17:09 It’s something engineers have to consider

17:12 when designing basically any concrete structure,

17:14 which means it’s something that quarries, batch plants,

17:17 testing labs, and contractors have to think about as well.

17:21 Since the 1970s, we’ve gotten pretty good at avoiding it in our structures.

17:25 But since it’s often a slow-growing issue,

17:27 we’re still figuring out how to deal with the problems it’s causing

17:31 on the stuff we built before we really had a handle on it.

17:36 On mass concrete structures, like TVA’s dams, it could have been a death blow,

17:41 significantly shortening the lifespans of these massive projects.

17:44 But they figured out a creative solution to live with it.

17:48 Kristen: “I mean, it's cool.

17:50 And when you think about a dam, it's a water barrier.

17:54 It is designed to hold back water.

17:56 So the last thing you expect to do is to cut a piece out of it.

18:00 But we do.

18:02 We do.

18:04 Reactive aggregates are a hyper-local phenomenon.

18:07 Go a few miles in any direction,

18:10 and the composition of rocks can completely change.

18:12 That’s true for a lot of parts of life,

18:15 but one thing I never considered was how specific

18:18 a sports stadium is to the city it's based in.

18:21 There are huge differences in how they’re built,

18:23 where they’re located within a city, and how it feels to watch a game.

18:28 One of my favorite channels, Maapify, produced a 3-part video series called

18:32 Beyond the Bleachers that explores the people,

18:34 policies, and priorities that shape the differences

18:37 in stadiums between the US and Europe.

18:40 And if you want to check it out, it’s only available on Nebula.

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