Compaction — OMC & Maximum Dry Density (Proctor) — Parts 7 & 8
Parts 7 and 8 establish the compaction characteristics — optimum moisture content (OMC) and maximum dry density (MDD) — by light (standard) and heavy (modified) Proctor respectively. OMC/MDD are the laboratory targets against which field compaction is controlled (degree of compaction = field dry density / MDD), governing the strength and settlement of every fill, embankment and subgrade.
Key Requirements
•Light/standard Proctor (Part 7) or heavy/modified Proctor (Part 8) per the specified compactive effort
•Determine OMC (moisture at peak dry density) and MDD from the compaction curve
•Field control: degree of compaction = (field dry density / MDD) × 100 ≥ the specified percentage
•Field moisture controlled near OMC (a band) — dry of OMC under-compacts, wet of OMC traps pore pressure
•Specify which Proctor (standard vs modified) — they give different MDD/OMC
Formulas
Degree of compaction (%) = (field dry density / MDD) × 100
Field-compaction acceptance against the laboratory maximum dry density
field dry density = in-situ dry density (e.g. core-cutter / sand-replacement)MDD = maximum dry density from Proctor (Part 7/8)
Practical Notes
✓Always state standard vs modified Proctor — modified gives a higher MDD/lower OMC; field acceptance against the wrong reference is meaningless.
✓Compaction at the wrong moisture is the No. 1 earthwork failure: dry of OMC won't densify, wet of OMC builds pore pressure and won't achieve density.
Common Mistakes
⚠Field compaction judged against the wrong Proctor reference (standard vs modified).
⚠Compacting outside the OMC band (too dry or too wet).
⚠Using one MDD for a variable borrow source (re-test when the soil changes).