Road base is a mixture of clay and gravel used in road building throughout the world. As an engineered material designed to withstand extreme loads (trucks and cars), you are assured of good working properties for earthbag building. While site based soils are less expensive and require less transport, the working properties are unknown and so you will want to do a series of tests to determine the best clay to gravel ratio. With road base, no additional tests are necessary and you can begin building right away.
Moisten road base slightly before filling bags.
Road base is typically very inexpensive, largely because the mixture is just clay and gravel. With huge quantities needed for road building, many suppliers are available to meet demand. That often means a truckload can be delivered to your site in short order from a nearby gravel pit.
Road base is a uniform mixture without clumps of clay or large rocks. Using road base eliminates digging for soil on your land, thus preserving the natural beauty of your site and freeing up your time for building. And since it is delivered by truck, you can have it stockpiled around your house just where you need it to minimize labor. Road base excels in compression, and therefore is ideal for dome building. It dries almost rock hard, creating incredibly strong walls. You can use the same material under your house to raise the building site.
Note: road base is a generic term and the mixture may mean different things in different places. Ask for a clay/gravel mixture as used in road building (not just sand/gravel). You need the clay portion, which acts as the binder.
If you live in cold climates, then consider using lightweight fill material such as scoria, perlite or vermiculite for high R-value and ease of use. See Insulated Earthbag Houses for more information.
Earthbag Building Blog 
What about the exact same thing, but with concrete/cement added?
Would the embodied carbon in the cement, if used in a 10-15% ratio, provide enough added longevity/durability to justify itself?
Same question, but in an environment with a LOT of precipitation, tropical zone, etc?
I would use lime instead of cement. And yes, I think it makes sense to use stabilized soil in rainy climates to protect against moisture damage, especially for domes. (And/or consider adding a roof over the dome in high rainfall areas.)
Edited to add: Most houses with wide roof overhangs (= most of my designs) the lime or cement is NOT needed. Just wanted to make that clear.
More advantages of using road base: You can use the same material for floors and earth plaster. I even hear it’s good on pancakes… just kidding.
Tamped earth floors: http://www.grisb.org/publications/pub11.htm (this is my favorite method for making floors)
Earthen plaster: screen out large aggregates or buy it pre-screened to save labor. Need to make some compressed earth blocks (CEBs)? This is the material to use.
I’m guessing that terminology is different from region to region. I called a hauler AND a gravel yard in my area in the Midwest and neither of them was familiar with the term “roadbase”. They seem to refer more to size of aggragate and presence (or absense) of stone dust. Around here you order “53’s” for your driveway, if you want a mixture of gravel as large as 1.5″ down to dust. If you want mostly dust, it’s “quarter-minus”, i.e., gravel no larger than a quarter inch, along with dust.
Most of the stone operations locally are limestone quarries. I’m asuming the “fines” or limestone dust from these can be used the same way? In fact, the code-approved dome house in Berea, Kentucky seems to have used this same material. But I wonder if it would be as applicable in plaster or cob mix as the roadbase you’re referring to. I guess I would have to get some and do the mason-jar test to find out…
No doubt it goes by different names. Ask for the mixture that’s used to make roads. Road departments typically use a clay/gravel mix. The clay holds the gravel together like glue. The gravel is actually a carefully screened mix of a wide range of aggregate sizes for maximum compaction.
What about using acrylic polymers for soil stabilzaton both within the earthen housing structure itself and outside around the yard and around the village to build stabilized earthen waals, embankemnts, drainage, etc.
Also there are now very high quality products to create hydrophobicity within various soil types that eliminate the oils ability to absorb water. These products are a new wave of technology that takes the place of lime, flyash, cement, etc.
I have tons of info on this stuff if it is of interest to others.
I can be contacted at chris@dirtglue.com if desired.
Good luck with he good earth!
This is an area that needs further research as to its use in earthbag building. Feel free to write a report and we’ll consider adding it to our tech articles page at http://www.EarthbagBuilding.com.