I look for earth that has a high clay content and ideally some sand in it. It is nice to dig into an embankment, so that Im not having to dig a hole in the ground. There are also many landscape material yards that have big piles of dirt they will sell for about $20 a yard, that often has a good amount of sand and clay in it.
You can do a shake test by filling up a clear jar with half of the Earth and the other half water. You then shake this up vigorously and then let it settle. The sand is heavier than the clay so it will settle to the bottom, and the clay will float on top. This is a way to tell the ratio of sand/clay in your earthen material.
If you are using earth that has hard dirt clods, you need to first break up those clods and soak them in water for a couple hours if possible. You then have the option of either mixing it on a tarp by feet or in a mud box with a hoe. The mixture is 70%sand to 30% clay with straw sprinkled in. On a tarp, its best to have 4 people on each corner, move the tarp back and forth to help mix it, then you stomp on it. Bare feet usually works best.
The 3 rules to ensure the longevity of an earthen structure is that it needs a good pair of boots, a jacket and a hat, namely a sturdy foundation, a good plaster and a roof with an overhang. You have the option of applying an earthen plaster, made of sand, clay, chopped straw, flour paste and manure. this then gets sealed with linseed oil if in direct contact with water. Your other option is to apply a couple coats of lime plaster, ideally hydraulic lime from Trans mineral. I add an iron oxide into the lime plaster to give it some color. You can then seal this with linseed oil for added protection.
It depends on what you are building. If it's an oven or a bench, I usually remove the top 6 inches of topsoil and replace it with baserock and then tamp this down good. I then will stack either broken chunks of concrete ( urbanite) or cinderblocks to build the base of the structure. If it is a cobin, I set the 4 X 4 uprights in a 2 ft deep hole secured with concrete. This is enough to support the weight of the structure even with a living roof.
It is true that there have been numerous adobe structures that have collapsed in an earthquake. However, the way I do my earthen structures is to have wood uprights support the weight of the roof, so this does much better in an earthquake. I have built a mobile pallet/cob "Palletable Cobin" on a trailer that was exposed to much more movement than an earthquake or hurricane and it has faired just fine. I have also built numerous cob ovens on trailers and driven them around bumpy roads without any problems.
So, technically, if a structure is under 120 sq ft and is used as a storage or play structure without electricity or plumbing, it does not require a permit. We have been working with an engineer to have a 160 accessory dwelling unit (ADU) be built with pallets and cob designed to meet code requirements. If the structure is just 120 sq ft, it is usually considered inconsequential and you won't have any problems. .
The cost does depend on several factors, but typically, an oven is about $2500 a bench is about $2000 and a cobin is around $12,000. The cost can come down some if the owners provide some of the labor, or if it is done as a workshop.
They are essentially the same material, except that adobe is made into earth bricks and then sun dried, whereas cob is considered monolithic in the sense that it is all one form with the straw connecting it all together.
If you have a roof built over your earthen structure, you can build during the rainy season. It will need to get some sunlight though, so make sure the structure has some sun exposure.