Soil stabilization is a process that applies a material such as lime, fly ash, cement or rock to/into the sub-grade when proper compaction cannot be obtained without it. To understand when it is needed, it is first important to understand a few things about the pre and post excavation process associated with building a proper structural and drainage base that goes underneath the synthetic turf field.
Prior to construction, it's recommended that a geotechnical engineer take core samples (typically 6 - 10 15' samples) of the soil where the new turf field will be located. Among other things, these samples are used to identify the amount of top soil that needs to be excavated, as well as the type of material found in the core samples -- such as sand, clay, type of clay, organics, etc.). At the same time, the technician taking the core samples is also determining whether or not the condition of the soil exhibits the proper compaction needed to properly support a layer of 6" - 8" of the base stone that will comprise the above-mentioned structural and drainage stone/layer. While the engineering report will identify this information, it's important to note that the tests are conducted at a very limited number of locations throughout the entire area and thus are only indicative of those specific locations. Indeed, if all of the locations reveal poor soil and compaction issues, it will be "likely" that some type of stabilization will be needed and money should be set aside accordingly. However, the only way to know for sure whether or not stabilization will be needed is after your field has been excavated to sub-grade and after a "proof roll" test is performed.
Specifically, after the field has been excavated, a truck with a load of approximately 20 tons must drive across the field in a grid pattern. As the truck travels, the tester is observing whether or not the tires of the truck are sinking or causing the sub-grade to deflect more than 1/4" over a 10' span throughout the grid. If such sinking or deflection is 1/4" or less, then the sub-grade is determined to be stable (this is known as obtaining at least 95% compaction under the on site "Proctor" compaction test). However, if observation greater than 1/4" is observed, then stabilization will be recommended. If proper compaction is not observed it is advised that a geotechnical engineer be consulted for the appropriate stabilization material/method and quantities and depths for same.
As a final note, if stabilization is required, it will not necessarily be required across the entire sub-grade. Indeed, it is not uncommon that only specific areas will exhibit compaction issues. Again, recommendations from a geotechnical engineer should be relied upon.