We’re all familiar with the humble sandbag and it’s part in a flood fight: a natural or synthetic fiber, open-ended bag used to stop bullets, water, or mud. Filled with sand or stone aggregate, they do an adequate job at stopping flood waters if properly filled and stacked. Government agencies provide information to the public on how to use sandbags, such as this Lake County, Illinois video, but did you know that sandbags come with limitations?
Sands through the hourglass
Sand is really just millions of very small rocks. Sandbags let us harness and shape sand into usable shapes and structures. But even confined in bags and interlocked together as best as we can, the sandbag wall, or barrier, is a fragile thing in the face of the power of water. The sandbag’s strength is vertical; it can withstand a great deal of force from directly above. The problem is that flood waters exert sideways, or “shear” force where the sandbag is much weaker.
Also, sandbag walls/barriers typically do not have added reinforcement, such as you would have in a masonry wall. The horizontal and vertical steel reinforcing bars (“rebar”) placed inside masonry or poured concrete walls help absorb forces exerted on the wall from all directions. The necessity of reinforcing masonry is evident when earthquakes occur in other countries, such as Haiti, Pakistan, Iran, Nepal and China. We see from the collapse of multiple buildings what happens when masonry buildings do not have such bracing.
Sandbag structures have none of that reinforcement which makes them vulnerable to flash flooding or currents in water. That being said, they are still the most cost-effective “hasty barriers” when time is of the essence and funding is low.
A New Concept
Disclaimer: The following discussion is meant to provide some more durable alternatives to the basic sandbag wall. Please consult with an architect or engineer before relying on the following procedures to protect life and property.
Don’t you love an article that requires a disclaimer? I included it, though, because I want to make sure you have the right advice for your particular set of circumstances. I’m a practitioner, not a licensed professional, but I have enough knowledge and experience to protect myself, and I’m happy to share that with you.
So let’s talk this through. Let’s assume a huge storm is coming your way, and the National Weather Service predicts flash flooding or a river at flood stage in your area. You and your neighbors have a limited amount of time to prepare and protect your properties.
I’ll tell you what you can expect once everyone becomes aware of the approaching storm. At the grocery store, bottled water and convenience food will fly off the shelves. At your local hardware or big-box home improvement store, sandbags, shovels, bagged sand, and plastic tarps will disappear. However, you’re not concerned. Your home protection plan is a little different than Joe Neighbor’s…you go to the commercial sales desk, because you have a plan. I bet few people are buying concrete as the storm comes in!
Better than Jell-O Instant Pudding
If you own a home, you’ve probably been introduced to “Ready-mix concrete”. It’s a bag containing Portland cement, sand, and gravel aggregate. Need a fence post or mailbox post set? Buy a $3.00 bag of ready-mix and add water. I would argue that bags of ready-mix concrete form the backbone of temporary flood protection that is more durable and reliable than sandbags.
So here’s the concept: Instead of stacking floppy bags of sand, you will lay uniform, rectangular bags of ready-mix concrete end-to-end to form your wall or barrier. The big difference will be the addition of lengths of steel reinforcing bars (rebar) horizontally and vertically as you build your wall, tying the horizontal and vertical bars together with wire just as you would in reinforcing a concrete wall.
As you need to make the wall taller, you add a level of bags that alternate like bricks in a wall. For example, it’s known that a 50-pound bag of Quikrete Fast-setting Mix is 17” long, 10” wide, and 4” thick. If I need a 50’ wall that is 3’ high, I just need to do the math: 50’=600”, divide by 17”=35.5 bags for each course. Three feet=36”, so I need (9) courses of bags to get my desired height: 9×35.5= about 320 bags. Fast-set concrete cures within an hour, probably a good quality in the face of rising flood water.
But you’re not done yet. You’ll need various lengths of rebar and wire to tie it all together. Rebar comes in 10’ and 20’ lengths for horizontal reinforcement, and 1’, 2’, and 4’ cut lengths for vertical bracing. For each level of bags, punch the rebar vertically through the middle of the bag, leaving enough length to make it through the next level of bags. Tie overlapping lengths of rebar together with steel wire to continue the bracing. Investing in a rebar cutter will cut your costs.
On top of each level of bags, lay a long rebar piece lengthwise across multiple bags, tying the horizontal length to the vertical lengths for cross braced strength. If you slit the top of each bag course, the rebar will bond directly to the concrete in the bag below. As water seeps into holes you poke in each bag, the concrete inside will harden. Sounds like a lot, but you’d fill thousands of sandbags for an equivalent wall that wasn’t nearly as strong.
Advantages/Disadvantages
The resulting wall or barrier is not as strong as an engineered concrete wall, nor is it intended to be; its purpose is to provide a stronger temporary barrier. Ordering and handling uniform bags of concrete is much easier than filling and placing sandbags.
Sandbags are the ultimate in flexible temporary walls…when you’re done, load them up and move them elsewhere. My suggested concrete bag structure is less flexible; the hardened concrete will stand strong until you decide to demolish it. That’s cool if you have a long flood season, but for a one-off storm it may be more work than a sandbag wall. The bottom line is to give you some food for thought as you prepare for that next big storm.
Cement starts to set in an hour after it has been poured and is no longer exposed to additional water. That’s why you don’t work with exposed cement in the rain. I’m afraid that under flood conditions, the cement bags with holes in them for water to leak in and mix, would simply remain a soupy mess and not set until after the flood had passed. The packaging might contain the mix, however as cement bags are paper (last I purchased any quick mix) they are more likely to simply fail and again you’d just have a soupy mess on your hands.
But, if you do have adequate warning and the rain has not hit yet (or you simply live in a flood prone region), a cement barrier wall is still a viable flood defense. Just build it before you need it. You can even build two low thin cement walls, one set about a foot behind the other (3 foot tall or however high you wish). Then fill between the two with dirt. This makes for a more cost effective earthen rampart, and you can plant in the center. Both floodwater defense barrier and raised bed for gardening.
It’s good to have different options in mind. Not all solutions are going to work for everybody and the author is right about sandbags. They have their limitations.
Build a monilithic dome 10 x 10 square feet with autoclave aerated concrete AAC the airfoam concrete is cost effective and should be constructed in advance of a pending hurricane, tornado, or earthquake.