Jay Nelson's soil report was generous in that it provided four alternate solutions to the foundation upgrade.  In order of Jay's preference:

1. Deep-drilled piers. Drilled piers are the Cadillac. These particular drilled piers needed to be a minimum of 18 inches in diameter and a minimum of 20 feet into the hill or extend at least eight feet into bedrock. These are expensive to create, and a lot of dirt would need to be removed from the hill, and a lot of concrete pumped up the hill.

2. Helical screw anchor piers. These are long metal rods with screw blades on the end. The specs for these were blades that were 10" minimum diameter and driven into the ground to a resistance of at least 3,000 foot-pounds. The assumption was that this resistance would be achieved at a depth of 20-30 feet.

3. Hand excavated bearing piers. These were described as being a minimum of 7 feet in depth and a minimum of 3 feet by 3 feet in size, and they would be connected by grade beams. This solution would require the removal of a lot of dirt. 

4. Stiffened continuous grid footings. This is a series of grade beams, minimum of 3 feet in depth and a minimum of 2 feet in width, located around the perimeter and in a grid pattern within the perimeter. Lots of dirt removal, and not really impressive resistance against sliding...

The engineer, Henry Larsen, suggested that if I was not going to spend the money for the drilled piers, that the helicals were a good solution. A main driver in his mind was the cost of removing dirt off the hill. 

Helical screws it would be.