You are going to have to support the sandy soils one way or another. The conceptually simplest way to do this is to case the excavation through the sandy layer. If the sandy layer is near the ground surface, then obtaining a 20ft long temporary casing is not a problem. If you can't find one that is only slightly larger than your diameter, then you may want to also use a permanent casing (hung inside the temporary can) to reduce the amount of concrete placed. Again, this is pretty straightforward.
If your sandy layer is at some depth, you may need to telescope two or more temporary casings to reach the required depth. Alternately, you could look at using a slurry to support the sidewalls. See the FHWA Drilled Shaft design guidance for additional details.
