Stream Crossing Proposal

Irish Bridge at Whitely Woods, Yorkshire.

This is just a proposal, but I’m fairly comfortable with it. There may be an easier way, of course, so, as the Japanese say, if the door won’t open by pushing it, then I may have to try pulling it. Unlike a common Irish Bridge, or low water crossing, which is dry on top most of the year and only under water on certain occasions, I want to do it the other way around. This is simply because I like falling water. I’d like it to look a little bit like the one shown here. The illustrations that follow do not show a stepping stone crossing for people on foot, but it would be easy to incorporate.

This sandbag bridge, by Nobuho Nagasawa, is a work of art.

As people who know me probably suspect, I’d like to incorporate an unusual building technique. The technique is not as unusual as some would suspect, however, as it’s been implemented at an incredible scale during the floods that ravaged Thailand and other parts of Southeast Asia. That’s right, sandbags.  The vision that most people have of sandbags, however, is that of deteriorated polypropylene bags baking in the sun after the flood waters have receded. Making a permanent structure out of them probably seems preposterous to most people. Well, pull up a sandbag and swing the lantern, and prepare to learn something that’s going to be entirely useless to you for the rest of your life.

Polypropylene bags full of gravel being tested. Serious stuff.

Your standard, woven, polypropylene bag for rice, or, whatever, will not rot under water or when buried. I read about a guy who built a dam with them and, disassembling some of it 6 years later, found that the bags were exactly the same as the day he had put them in. They are subject to degradation from extreme temperatures and UV light. Of course, anything can break or burst, but the tensile strength of polypropylene bags is nothing less than amazing. When you give the material a name like “high tensile strength polypropylene geotextile,” rather than “ordinary rice bag,” you’d almost be willing to pay a fortune for them. Under normal temperatures, and when protected from UV light, a one-bag-width wall of these, filled with gravel, has a load strength of over 120kN/m. That means that one linear meter of this stuff can hold up more than 12 tons. That’s about 10 times more than a 2″ x 6″ timber-framed bearing wall. And if this was a wall that could be protected from the weather, you could use moist, clay-rich earth for an even stronger wall without the need for the polypropylene material after it dried.

I've calculated that my ford could easily support this 16-ton APC.

As you will see, if you have the courage and stamina to keep reading, the load bearing  portion of my low water crossing will be (I mean, it’s a proposal, but I’ve pretty much convinced myself that it will work) two walls wide, each. It is also worth noting that I intend to render, or plaster, the outside with wire mesh (that sounds so much better than “chicken wire”) and mortar, as you would a normal house in this part of the world. That not only protects the bags from UV light, but increases strength by leaps and bounds. I’ve calculated that it could easily support the occasional M1126 Stryker ICV, but you’re not likely to see one as the locals can’t quite afford the $3.8 million price tag.

Step 1. Lay down about two courses of gravel-filled bags, add culverts.

It’s the dry season now, so now’s the time to get this done. Also, it will cause a fair amount of disturbance down stream, so I want to get it done before I start producing power. The first step will be to lay down about two courses of bags filled with gravel. Notice the running bond. At this point, the stream water can just run right over it. Then I’ll lay down the culverts. How many? Well, that will depend on how much water. I’d like to use just as many as it takes to channel all the water during the dry season.

Step 2. Build up with more gravel bags, pour the concrete planks, apply plaster skin.

After that bit is down it will just be a matter of building up more courses of gravel bags. Eventually, it will be above the stream’s normal surface level, and, hopefully, all the water will channel through the culverts. Applying the plaster skin will be the tricky part. A plaster skin isn’t necessary on the inside surfaces between the strips, but it looks better in the illustration.

Step 3. Fill the void between the strips with river gravel and local soil.

Next, I think I will be able to get away with simply filling in with river gravel first, and then local soil. And yes, that’s the color of our volcanic soil. It’s fairly important at this point that the stream does not flow over the top of the ford, so I’ve got to be pretty sure about the size and number of culverts. I suppose I could use one more than I think is necessary and plug it up later with a 30cm diameter cork or something.

Step 4. Add river gravel, and voila!

Finally, river gravel fills the 10cm high void between the concrete strips. I know I’m over simplifying the whole thing, but I’d appreciate any questions or comments. I’m particularly eager to get statements of outright praise for the idea, but I won’t get my hopes up.

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5 Responses to Stream Crossing Proposal

  1. Bob in Stow says:

    If you do not want water flowing over the top, your culverts must be able to accept the maximum flow during the wet season.

    • richard says:

      I DO want water flowing over the top most of the time. The one in the first photo from Yorkshire probably doesn’t have culverts, but, anyway, I want it to look sort of like that most of the time. When water is flowing over it, I doubt it would be more than 2 or 3 inches above the concrete planks, easily navigable by motorbike or car. When I know the height at it’s highest time, I can put stepping stones down the middle, as long as a car can get over them.

  2. Mike says:

    You might consider additional small pipes (culverts) through the down-stream side, to drain water trapped between the two strips. Such water might not be a big problem, but the pipes are easy to install during construction, but difficult to retrofit.

  3. Mike Harris says:

    Richard your blogs are good.
    Am going to send this address to occupants in the equally wacky town Hill End in NSW Australia. Some will appreciate your navel/fire gazing philosphy.

    • richard says:

      Hello Mike, thanks. You’ll have to explain to me one day what your relationship with “wacky” occupants of Hill End is. Anyway, I’ve added a visit to my list of things to do and places to go if I ever get to Australia.

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