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Soil Compaction

How Do You Evaluate Fill Settlement Beneath Structures? by Focht3
Posted: 16 Mar 04

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:
  1. There are (usually) two kinds of settlement to evaluate: total and differential.
  2. Total settlement doesn't typically pose a problem unless the magnitude exceeds 4 inches (100 mm.)
  3. Differential settlement is usually the concern - this is what causes structural distress (cracks, sticking doors, etc.)
  4. The thicker the fill body, the more important the quality of the fill placement becomes.
  5. Fill bodies with varying thickness almost certainly will experience uneven surface settlement.
  6. Consistent fill density should reduce the differential settlement for fills of uniform thickness.
  7. Increasing the percent fill compaction will reduce - but not eliminate - the magnitude of total fill settlement.
  8. 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.
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:
  • Crushed rock ~ ½%
  • Sands and gravels ~ ½-1%
  • Sandy and silty clays (CL soils) ~ 1-2%
  • Fat clays (CH soils) ~ 2-4%
And, of course, sound engineering judgment is a big part of every fill body evaluation...

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