This is a topic that is not well understood by most practicing engineers, but is of critical importance in designing and constructing quality projects!
The following concepts are important to understand when evaluating a compacted fill body:[ol][li]There are (usually) two kinds of settlement to evaluate: total and differential.[/li][li]Total settlement doesn't typically pose a problem unless the magnitude exceeds 4 inches (100 mm.)[/li][li]Differential settlement is usually the concern - this is what causes structural distress (cracks, sticking doors, etc.)[/li][li]The thicker the fill body, the more important the quality of the fill placement becomes.[/li][li]Fill bodies with varying thickness almost certainly will experience uneven surface settlement.[/li][li]Consistent fill density should reduce the differential settlement for fills of uniform thickness.[/li][li]Increasing the percent fill compaction will reduce - but not eliminate - the magnitude of total fill settlement.[/li][li]As the particle size of the soil comprising the fill decreases, the likely post-compaction settlement of the fill body under self weight increases. This settlement usually occurs when the fill body is wetted.[/li][/ol]The post-compaction settlement of the fill body under self weight is frequently termed 'fill shrinkage.' Some of the "rules of thumb" that I use for estimating fill shrinkage of a 'well placed' fill body are as follows:[ul][li]Crushed rock ~ [½]%[/li][li]Sands and gravels ~ [½]-1%[/li][li]Sandy and silty clays (CL soils) ~ 1-2%[/li][li]Fat clays (CH soils) ~ 2-4%[/li][/ul]And, of course, sound engineering judgment is a big part of every fill body evaluation...
