Long sandbags are filled on-site and arranged in layers or as compressed coils. Stabilizers such as cement, lime, or sodium carbonate may be added to an ideal mix of 70% sand, 30% clay. Straw may also be added. The earthbags are then plastered over with adobe. Arquitectura en Equilibrio (Architecture in Balance).
Plastic bags recycled into plastic bags — if plastic does not break down for a thousand years, this building is sure to last several lifetimes. Of course covered with adobe or plaster, so that the plastic does not offgas or degrade.
Foundations differ as per site. Here, in a rainy locale, rocks were placed under the earthbags for drainage. Note the barbed wire which keeps the bags from slipping and creates an earthquake resistent structure.
The time consuming part, filling the bags. The bags are filled in place on the wall. The CalEarth site says that three reasonably-fit persons can lay 100 linear ft of bag per day.