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.
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.