Your shotcrete Saturday open thread

by liberal japonicus

Perhaps you, like me, weren't expecting discourse about the construction considerations of underground tunnels at Talking Points Memo, but there was this:

Specifically, they are covered in what’s known as “shotcrete”. Shotcrete is nothing more than concrete mixed at a standard 3000 to 4000 psi, but with a very high slump so it can be pushed through a hose and literally “shot” onto a wall or ceiling. (Slump is the measure of a specific concrete design’s plasticity, or workability. Paving concrete for roads, loading docks, aircraft runways and taxiways, and so on, has a very low slump, usually an inch or less, particularly if it’s being slip-formed; the same is true for tunnel “rings” if a tunnel is being cast in place as opposed to having pre-cast concrete panels installed.

By contrast, structural concrete, for walls, culverts, manholes, and the like, has a very high slump, at least three to four inches, since it has to be maneuvered into and appropriately vibrated inside relatively narrow forms.) So a shotcrete mix design is one that includes a fair amount of water, a smaller coarse aggregate (i.e. stone) than other mix designs, and various admixtures and such so that the concrete can be highly and effectively pressurized through a small hose over dozens if not hundreds or feet, and then sprayed onto a wall. So, after enough rock “face” has been exposed (OSHA standard is no more than 4’ if I remember correctly) through digging, cutting, or blasting, the shotcrete crew comes in, drills hundreds of holes where they place steel bolts of varying length and thickness, secure those bolts with grout, hang thousands of square feet of wire mesh on them, and then spray and finish shotcrete. Then the excavation crew comes back in and the whole process begins anew, until you reach the bottom elevation called for in the plans.

In reading about that, I thought of my grandfather. Family history has it that my grandfather went to London to work as a 'navvie' building the London Underground. We have this picture of him, on the right. 

So talk about making tunnels, or infrastructure or family history. Have at it.

Comments

[Brett Bellmore]

Real family history: My maternal grandfather was a plumber in Detroit. On one occasion, he got hired by a mob boss to put a bathtub in a third floor room. (Naturally, this was an "offer you couldn't refuse.) Imagine grandpa going to work, with a hitman conscripted to help carry the cast iron clawfoot tub up the stairs...

Guessing the relationship to the mob wasn't a one time event, as years later, back in the '60s, when his daughter's (My aunt, not mom.) husband ran off on her and the kids, family legend has it that a hitman showed up at her door, and offered to do him for free. Never knew hired killers did pro-bono work.

Shotcrete, as described, is related to "ferrocement", a particular interest of mine. Always regretted I never got to dabble in it, perhaps my wife will let me build an in-ground pool next year.

[Brett Bellmore]

I guess I should add that she turned him down...

[Laura Koerbeer]

My great-granddad was a stone mason and reknown as the strogest man iin Akron, Ohio, He and his brothers immigrated to America during the Franco-Prussian war. They were ethinic Frenchmen in the Alsace-Lorraine, living in an area that was mostly German-speaking. My granddad got conscripted into the Prussian army,which he did not like at all. He got himself locked up in the brig but escape by bashing a guard over the head with a chamber pot. His family somehow raised the money to send him and all of the other boys in the family to America.

That's the story, anyway. Probably ahs as much truth in it as most family legends.

Isn't it funy how often family legends are about the ancestor who did the kinds of things that current family members would be inhibited from doing? Although, had my brother been the right age during the Viet Nam War, he'd be a Canadian now.

[wj]

When we are building infrastructure, we make a couple of implicit assumptions.

First, we assume that the folks doing the construction have not absorbed the "planned obsolescence" mindsent that pervades the rest of American industry. Noticed lately how, if you go to replace something that you bought decades ago, the new one has a projected lifetime of 7-10 years max? Definitely don't want infrastructure construction to give out that fast!

Second, we assume that the design requirements will be successfully communicated (and enforced) through the entire chain of sub-contrators and suppliers of suppliers that are involved in making almost anything these days. If you have been following the fiasco that is the construction of the new eastern span of San Francisco Bay Bridge, you are aware of how badly that can go wrong. (The design specs explicitly call for a certain kind of galvanizing on the critical bolts that hold the whole thing together; a kind which is not going to degrade rapidly under salt water. What was actually delivered were bolts which are already starting to degrade in place, even before the bridge is ready to open. And replacing them in place looks to be on the order of difficulty of building the span in the first place.)

Some of the infrastructure that the Romans built remained in use for a couple thousand years. Would you bet that ours will last even a couple of hundred? I don't think I would consider it.

[Brett Bellmore]

"Second, we assume that the design requirements will be successfully communicated (and enforced) through the entire chain of sub-contrators and suppliers of suppliers that are involved in making almost anything these days."

Indeed, the book, To Engineer is Human; The Role of Failure in Successful Design, relates a similar tale about the Hyatt Regency walkway collapse that killed so many. The design of the walkways was theoretically sound, but it was so difficult to actually build it as designed that the men on the scene made changes to ease their jobs... which dramatically reduced the strength of the structure. The lesson being, it's not enough to design something that will work if designed, it has to be built that way, too, and designers need to take this into account.

The Zilwalkee Bridge collapse was similar: It fell partway through construction, and when the state tried to sue the engineer, it eventually came out that they'd omitted much of the structural reinforcement called for in his design, to economize.

I think you're right about today's infrastructure. To design something to last a few thousand years is not beyond us, but it DOES require 'over-building' for the requirements of the near term to such an extent that you'd never get it past the bean counters.

[dr ngo]

As for family history, one of my grandfathers was a baker (and I still have a medal somewhere to prove it), but otherwise we were never a clan devoted to manual labor. There are ministers and missionaries and teachers and various paper-shufflers, but no one, to my knowledge, capable of doing anything whatsoever (outside of the kitchen) with their own hands. At least not making a living by doing so. Clerks/clerics, is what we are, never wealthy, but never soiling our hands with the work we do . . .
. . . which makes it awkward to be a homeowner, sometimes.

[joel hanes]

Brett, was your ferrocement interest a build-your-own-boat thing?

The boatyards in which I used to spend my summers were rife with ferrorcement projects, only a few of which ever proceeded as far as decking. (Turns out it takes considerable skill to get a fair hull, more than most amateurs could summon. One whole back row was filled with sad, misshappen hulks that their builders were too discouraged to finish, and had too much sunk cost to abandon.) I only saw one actually launch and go sailing.

So maybe you dodged a bullet.

A pool sounds like a much less risky application.

For fifty years, Seacliff beach in Aptos, on Monterey Bay in California has been the resting place of the SS Palo Alto, a 420' ferrocement tanker, one of two that the government built in 1919. The people who bought and beached it intended to use it as an amusement pier, but the hulk decayed much more rapidly than expected, eventually breaking in half, and it has long been fenced for public safety.

[Brett Bellmore]

No, back in college, in the late 70's, I ran across a small book, practically a pamphlet, by some guy who was building the wildest organic shaped buildings using ferrocement. He'd heap up wet sand, cover it with the ferrocement, and then dig it out after the cement set up. It really inspired me, but when the time came to build my own home, I found local building codes made building anything at all unusual practically impossible. A pity, because once you get away from the boring world of stick built homes with drywall and siding, the range of options for cheap quality construction is amazing: Cordwood masonry, ferrocement, straw bale construction, rammed earth. All have their advantages, but building codes keep them from being used in most places.

I've thought of building a boat, but read enough about the realities of doing it ferrocement, that a composite wooden hull seemed more feasible.

[joel hanes]

On Interstate 280 south of San Francisco is an organic-form shotcrete house, locally famous as The Flintstone House for its rounded outlines. My understanding is that maintenance became problematic, and that it's hard to find buyers because the rounded-outline rooms will not actually hold much furniture.

[Brett Bellmore]

That is an issue, no doubt about it. He mostly solved it with built-ins, but there's no question the resulting houses weren't as efficient in their use of square feet. But the square feet were cheaper...

Re maintenance, I've been house hunting the last few months, and have seen enough rotting houses full of black mold, (All because banks flatly refuse to do any maintenance on foreclosed properties, even if a few hundred dollars would preserve tens of thousands of dollars of value. Got me wondering if they're under orders to covertly reduce the housing stock, it's so economically irrational.) that I'm not fantastically impressed with the durability of conventional structures. Walk away from a conventional built house, and it's a ruin in a few years. A properly built concrete structure is inherently more durable.

[Doctor Science]

Open threadily, I am shocked and grieved to learn that John Cole's cat Tunch was killed today. This was just hours after confirming that long-time human commenter "General Stuck" passed away in late June.

The food is lousy, and the portions are too small.

[Turbulence]

The design of the walkways was theoretically sound,

From what I've read, this is not true at all. After the incident, the original design was later shown to be sufficiently weak that it would have collapsed eventually.

but it was so difficult to actually build it as designed that the men on the scene made changes to ease their jobs... which dramatically reduced the strength of the structure.

This is not the whole truth at all. The construction crew asked for engineering changes from the structural engineers, and engineers agreed. They signed off on the changes. As in, the modified drawings literally have the engineers' signatures. The modified design was insufficient to bear the load. But, as I explained above, the original design was also insufficient.

[Brett Bellmore]

Going on what the book had to say, Turb. You may be right, in which case the author was wrong.