For others I will add that the thumbnail standards are related to compaction test measurements. Compaction as Standard Proctor (the test method) is without soil being brought to optimum moisture content for the specific material used and compaction at Modified Proctor requires you measure how you approach the optimum moisture content for the tested soil type and then you measure compaction as a percentage of ideal compaction at that moisture content. You end up with something like 98% Modified Proctor under a strict standard.
In certain conditions, you want to wet or dry out the soil during compaction procedures to reach that optimum moisture as lab tested for that soil being used.
As an “armchair engineer” I will make some comments that may simplify this into what I call “the classic comic” version in honor of the old comics that simplified complex stories in the 50s. I will add to the above that placing the materials in proper lift thickness and measuring and repeating proper compaction yields an “engineered fill” rather than a bank fill or a non-compacted fill.
An engineered fill will perform to a certain standard and characteristics like “differential settlement” (the variation in settlement amounts over a certain time and load in different parts of a fill or earth mass) in a predictable, reliable and allowable manner.
When ER talks about differential settlement he is talking about settlement that is one amount in one area of the dam fill and a different amount in another area and the difference produces a shear or twist between them or other areas not settling. This produces a weakened path for water or stress to work its way through the material.