Maximum Dry Density (MDD)

Maximum Dry Density (MDD) is the highest dry unit weight a soil attains when compacted at its Optimum Moisture Content under a specified effort, read at the peak of the Proctor compaction curve (IS 2720 Part 7 or 8), expressed in g/cc or kN/m³. It depends on soil type and the compaction effort — well-graded granular soils give high MDD (≈2.0-2.2 g/cc), uniform fine sands and high-plasticity clays much lower.

MDD is the denominator of the field 'degree of compaction' = (field dry density ÷ MDD) × 100%. Earthwork specifications state a minimum percentage of MDD (e.g. embankment ≥95%, subgrade ≥97%, base/sub-base often ≥98% per MORTH/IRC) which the contractor must demonstrate by IS 2720 Part 28 field-density testing.