Depending on the type of soil, doing much of anything to fill more than 8-12" deep will not get you compaction of the lower layers. If the soil is clay, or clayey gravel, adding water could actually do you more harm than good. Clays are expansive soils, meaning they expand when their moisture content approaches a saturation point. It will then shrink as it dries, losing it's compaction. With expansive soils, generally a technique called "pre-consolidation" is used to drive the water out of the soil and consolidate it with overburden. With sands and sandy gravels, water and vibratory compaction is effective to about 12". The water acts as a lubricator, along with some gravity effect to aid in comapction. You can tell what type of fill you are getting by looking at the particle sizes. Very fine and sticky and soils that stain contain a good deal of clay. Rougher grainy soils have sand and/or gravel, and will compact to a more consitent base and offer better drainage.

For most fill dirt of mixed soil types, I would recommend spreading in layers 6-8" thick, and roller compacting ( driving over )repeatedly before adding the next lift. The soil should be moist, but not saturated. The narrower the tires of the compacting machine are, the better off you are. 3 years will cetailnly help. As rain water or melting snow drains down through the soil, it creates a "gravity effect" to draw down and help compact the soil, but as previously mentioned, erosion can be a big problem over three years.

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