How To Make Cob & Test Soils | COB HOUSE Part 4

In this tutorial series, we're learning how to build a cob house from the foundation all the way through the reciprocal living roof. And so far in this series, we've gone through a basic overview of the building processes, how to build a rubble trench foundation, and how to pour an earthen floor. In this episode, we'll be learning about how to test your local soils, how to create test bricks, and how to make a cob mix. Before making a cob mix, it's important to be able to source the proper materials necessary for a strong mix. And as we mentioned in previous episodes, cob is a mixture of clay, sand, and straw. For this episode, we'll be briefly stepping away from the Cruising Cob Global Workshop in the North Carolina Mountains and into the woods near my house, where I'll be showing you how to test your own local soils for making a cob mix. The first step of this process is, of course, to source the soil that you have access to by digging it out of the ground. And when you're sourcing your soils, it's important to dig down past the top soil layer to get into the subsoil. Top soils are high in organic matter, which can take away from the structural stability of your mix. Arguably one of the most important materials to have access to when building a cob house is a clay soil. We use clay in our earthen floor mixes, our cob walls, and our earthen plasters as the binder. And in many areas, you'll be able to find a predominantly clay soil on or near your building site. So the first thing we'll be taking a look at is how to identify a clay soil. There are many different ways to test your soil for clay content, and in my opinion, the simplest and most effective way to test for clay is to add water and begin squeezing the soil in your hands. A soil with a high clay content will be very sticky, shapeable, and feel very smooth without much aggregate as you squeeze through it. Another test you can do is called a roll test by rolling the soil into a coil. If the soil holds its shape and doesn't crack very much, this is a good indication of a high clay content soil. The coil should also hold its shape if you pick it up and swing it around. Another test you can do is rubbing the soil on your arm or leg, and a pure clay soil will stain and stick to your skin as it dries. Lastly, you can do a palm test by flattening some soil in the palm of your hand and trying to shake it off. A pure clay soil will be really sticky and hard to shake off. In this portion of the video, we'll be comparing three soils near my house. I'll be showing one more test called a jar test to compare the samples, and since I knew that this soil had a high clay content, I sourced some extra to be able to make test bricks further into the video. We'll refer to this clay soil as soil A. For this next soil test, we'll be visiting a new subdivision under development right near my house, and unfortunately in modern construction, we pretty much bulldoze the entire forest and flatten out each home site with the subsoil left behind. This ends up being the most profitable and efficient way to build, which destroys entire areas causing plant loss and destruction of animal habitat. Here I'm sourcing what we will refer to as soil B for the jar test, and this time I don't have to dig below the topsoil because the topsoil is no longer here. I add some water and begin shaping the soil in my hands, and this soil is very sticky, full of coarse and fine aggregate, and holds its shape very well, which is a great indication of what we refer to as a ready mix. You can actually hear that this mix is sticky and full of coarse aggregate as I squeeze through it. A perfect cob mix will have roughly 50% coarse sand, 25% fine sand, and 25% clay, and we refer to this as a ready mix because you don't have to amend the soil with extra clay or sand. Basically this soil is perfect for building a cob house, and all that would need to be added is water and straw. As you gain experience building with cob, you can actually begin to know what a proper mix feels like purely based on its texture, and still though it's a good idea to make test bricks, so I shape this mix into a mini brick and allow it to dry. For soil C, I stopped at a tobacco field near my house, where again the subsoil was already exposed. I add water to the soil and begin mixing it in my hands, and for this soil, it slightly sticks together and is full of aggregate, so purely based on its texture, I can tell there's a small amount of clay and a lot of sand in the soil. As I begin trying the roll test, you can see that the coil falls apart in my hands, so this would show that there's not enough clay in this soil to keep it held together, so in order to make this into a usable mix, you'd have to amend it by adding more clay soil to bond it together. Lastly, I do a palm test, and you can see that the soil sticks slightly to my hand, but it falls off with very little effort. This again indicates that there's some clay in this soil, but that there's not enough clay in this sample to create a strong cob mix. Here, I gather one more soil sample for our jar test, which we'll use to compare soils A, B, and C. We mentioned that you want to avoid topsoil due to the presence of organic matter, and once you dig into the subsoil, there is one more material that you want to avoid, and this material is called silt. Here's an example of a silty soil, and you can tell based on its texture. It's almost poofy, not very sticky, and not structurally strong. You can see that it does hold its shape, but it doesn't stain my skin nearly as much as soil A when I rub it on my arm. You can also see that the palm test shows that the soil isn't very sticky as well. Clay in our cob mix binds the materials together, and sand provides the structural strength. Silt, on the other hand, adds no benefit and actually weakens the stability of your mix, so it's best to avoid silty soils entirely. So to recap, cob is a mixture of clay, sand, and straw, and so far we've looked at several tests to determine the clay content of your soil. An ideal mix will have roughly 50% coarse sand, 25% fine sand, and 25% clay. We determined that soil sample A was a very high clay content soil that could be amended by adding coarse and fine sand to make a strong mix. We determined that soil B had high clay content, was full of coarse and fine sands, which meant that it was a ready mix, and wouldn't have to be amended with extra clay or sand. We determined that soil C had a low amount of clay content, and was full of coarse and fine sands, so to make this mix usable, you would have to amend it with a high clay content soil. Now that we had gathered our soil samples, we could compare the samples using a test called a jar test. The jar test acts as a visual aid to help identify the proportions of clay, sand, and silt in your samples, and to prepare for this test, you'll want to break apart large trunks in each of your soil samples and fill the mason jars about halfway to the top with each soil sample. I repeat this process for each of the samples and then label them A, B, and C. Now you can fill each jar with water and then shake it vigorously for several minutes to ensure that the soil is well mixed. Unfortunately, when I was shaking soil samples B and C, I made a small mistake, so we no longer have soil sample B. I gave soils A and C another good shake, and then I ran this time lapse for about four hours. For this test, your coarse sand will settle first, then your fine sand, then your silt, and then your clay layer. You can see here that soil C has a much larger variation in its layers of coarse and fine sands, with only a small amount of clay settling at the top, whereas soil A looks almost entirely the same in its settling, other than a few striations of sand pockets throughout. For this build, we had a very rich red clay similar to soil A that had a high clay content and a low amount of aggregate, so to amend this mix, we used a decomposed granite, which is a manufactured sand that we imported from a local landscaping supply. If you choose to import sand for making cob, it's important to find sands that are structurally strong, and ideally you want to have a mixture of coarse and fine particle sizes to make the strongest mix possible. There are many different types of sand that are acceptable, and here's an example that they sell near my house called Chapel Hill Grit, which you can see has a nice variety of coarse and fine sand particle sizes. The sand should also be sharp and angular, which allows the particles to lock into each other almost like jigsaw pieces. The clay surrounds each of the sand particles and bonds the mix together, where the sand actually provides the structural strength. Here's a visualization of why you would want to avoid rounded sand of all the sand particle sizes. So now that we've sourced a high clay content soil and our sand, the last material that we need to source is a good straw. Oat, wheat, rye, and rice straws are all commonly used when making cob, and in our case on the east coast, we normally choose to use a wheat straw. When sourcing straw, you want to make sure that the straw has been stored in a dry area, and the longer and stronger strands of straw in each bale the better, since this is what will give you your tensile strength. This is an example of straw that you wouldn't necessarily want to use in a cob mix, because the strands are all short and broken. You can see when I take a bunch of this and pull it apart that it's not very strong. I go to the back of the truck and I hand select a few of the longer strands, and here you can see that these are much stronger when I pull them apart. Ideally, you'll find bales of straw that are full of long and strong strands, so this way you don't have to go through and pick out what's usable by hand. Now that we've sourced all three of our necessary materials, we can begin making test bricks. Making test batches and bricks is one of the most important processes when practicing natural building, because the world's soils vary so tremendously that there's never an exact recipe when you're creating mixes, so you have to experiment with the materials you have to see what proportions of each material make the strongest mix. For this example, I'm using one part of our Chapel Hill grid, and then one part of our relatively pure clay soil, Soilette. This is almost like making a mini batch of cob, and when you begin, you should add water gradually and continue mixing until the entire batch is homogenous. There's kind of a sweet spot with cob where it's wet enough to be workable and shapeable without having so much water that the mix can no longer support itself. It's best practice to make your cob test bricks without straw at first, because this allows you to dial in the proportions of clay and sand that create the strongest mix. Once you find the strongest proportion of clay to sand, you can then experiment with various amounts of straw and new test bricks. You can see here that this mix has now become homogenous in its consistency and its color, and since it's at the ideal moisture level, I can begin shaping it into a test brick. You would repeat this process with several different ratios of clay to sand, being sure to maintain a consistent shape and size of your test bricks throughout. Lastly, mark each of your test bricks and then give them a couple days to dry all the way through. To test the strength of each brick, you want to drop them each from roughly the same height, and observe how they respond to the impact of the ground. As of 2021, there's now an appendix to the International Residential Code, which is used in 49 out of the 50 states that makes building a cob house to code possible. Part of the requirements for building a cob house to code is creating these four-inch cob cubes that are then put through a compression test, and if you're looking for more scientific analytical data, then this is an option to ensure that you're creating the strongest mix possible. This is especially important when you're building a load-bearing cob house, where the cob walls support the entire weight of the roof. So getting back to the workshop, we made several test bricks and we were ready to perform our drop test to find the strongest mix. The one clay to one sand brick breaks into several large chunks, leaving some pretty fragile pieces behind. The one clay to two and a half sand brick lands on the corner, but it takes the impact very well, and it's close to a full brick after being dropped. The one to three brick breaks into several small chunks, and the one to four brick breaks into even smaller pieces, and some of it even turns to dust. Given our analysis, we found that the one clay to two and a half sand ratio was the strongest, and finally on day seven, we were ready to make our first batch of cob. On the morning of day seven, we had our first demonstration of making a cob mix using the tarp method, and this method was created by a lady named Becky B in the early 90s. It's one of the most accessible ways to It's also possible to make a cob mix using a mortar mixer, using a bobcat, or even an excavator to increase efficiency, but of course there's a trade-off financially as well as environmentally with the use of fossil fuels. Instead of buying new tarps, I'd recommend visiting your local building product supplier where you can source lumber wraps for free and save them from going to the landfill. For the tarp method, you'll want to gather all your materials and lay the tarp in a flat workable area near your building. It's common to use five gallon buckets as our measurement for our materials, and between three to five total buckets is a good amount of mix to be manageable. Here we added our two and a half buckets of sand and our one bucket of clay, and since we're mixing barefoot, we go through and break up some of the large chunks of clay by hand before mixing. The first step of making a cob mix is to dry mix your clay and sand, and luckily Percy gave us a bit of a head start in this process. Rolling the tarp tends to be easier with two people, but it's not too difficult to do it solo as well. With two people, each person grabs two corners of the tarp and runs side to side using their body weight as leverage. You want to do this a few times and then switch corners and mix again until it's well mixed. You can see here that the mix is consistent throughout its color and that the materials are dispersed evenly. Next you want to make a crater in the center of your dry materials and add a small amount of water to the mix. It's much easier to add more water as you're mixing than it is to try to amend your mix with more dry material once it's too wet. You can then give the mix a couple of turns before starting to stomp the mix out with your feet. Here Josh can tell the mix is a little bit dry, so he continues gradually adding more water while he mixes. One of the best parts about building with cob is that each person has their own cob dance and technique. As you continue stomping the mix out, it'll start to flatten out and you'll want to continue flipping and rolling the mix with the tarp to give you more depth to stomp through as well as to help with the mixing process. You can continue gradually adding more water as you mix and repeat the processes of stomping, rolling, and flipping until the mix starts to become homogenous. Starting to take shape. That's the beginning of what you're going for. When you start to see it take shape, start knowing you're getting kind of close to the water amount. Be safe right now. Okay, I'm starting to drift towards the edge there. Bring it back to the center. He also varies which corner he is pulling. So when you are doing this alone, I would prefer you always pull from one corner, only one. Maybe for now this time it wouldn't make such a big difference, but you are gonna see how it makes a difference as we go with cob. So always step and turn. Step and turn because we don't want this to be compacted and look like a pancake. And then it's already time to turn and then step another time and then turn it again. The more wet it gets, the more clumpy it's going to become. The better you can just jump on it and this is when mixing becomes a little bit more fun. Not just like squishing your feet around like that, but more like jumping in and squashing different areas. I think that one thing that it helps if you sometimes we are looking so much like this and then we put our head like heavy here. What I do sometimes is remember to stamp a little bit like this to stretch, but also you need to see because you cannot stamp in the same side always. So I think that people that are not so strong like me, to take the tarp from the edge, you can also... I don't need strength. I can just use my body weight like we did with the tarp. So it's just this that's moving the The cob will get to a point with the proper amount of moisture and mixing that when you roll it, it starts to form a cob burrito like this. After stomping it out one more time and one final roll, we achieve our cob burrito that we're looking for and now we can add our straw. The first few bales of straw that we were able to source for this build weren't the best quality, so we went through and picked out the longer strands by hand as we went. You want to flatten out your burrito slightly and then evenly distribute the straw before stomping it out again. You want to push it in so it gets embedded. A rough guideline that you can use is one compacted bucket of straw for a five bucket mix and the straw should be consistent throughout the mix as you pull it apart to check. Stick your hands in, pull it apart. There should be straw coming out of every little area. So right now, look, I have sections here that don't have straw on them, right? That's a problem. So before you add more straw, maybe this mix needs to be turned more times. So it's more about turning the tarp also than stepping because when we are stepping the straw, we don't want to overstep because we don't want to break it. Well, that's pretty good. So yeah, I would just throw a little bit more on this mix. Not a whole lot. You can get more strategic with it as you go. If you notice, you have chunks of straw. It's still like very concentrated. This wasn't the case. That's why he is adding some more. But if you see, it's very... Now in the beginning, especially, you think you will have already your loaf. You open with your hand in different sections and you check it out. After adding a little bit more straw and stomping it out one more time, we'll give the tarp one last pull and here we have revealed our final cob burrito with the straw well integrated throughout the mix. The final step for making a cob mix is shaping the cob into loaves, which makes building with it and transporting it much easier. To do this, you just grab a chunk of the mix and compress it as you shape it to form your desired loaf size. And this ends up setting you up for another fun part of building with cob, the cob toss. In the next episode, we'll be covering how to build cob walls, which includes shaping, integrating, trimming, framing doors and windows, and we'll be getting all the way through to the top of the wall. Thanks for watching and please subscribe if you're enjoying the series.