MDD is the maximum mass of soil solids per unit volume of soil when compacted at the Optimum Moisture Content (OMC).
It is the highest dry density that a soil can achieve when compacted using a specified amount of compactive effort.
Maximum Dry Density (MDD) is a key parameter in geotechnical engineering that represents the highest dry unit weight (or density) a soil can achieve under a given compactive effort.
How MDD Is Determined
MDD is obtained through laboratory compaction tests.
- Standard Proctor Test (AS 1289.5.1.1)
- Modified Proctor Test (AS 1289.5.2.1)
In these tests, soil samples are compacted in a mould at varying moisture contents using a standard hammer and a specified number of blows. A compaction curve (dry density vs. moisture content) is plotted, and the peak of the curve gives the MDD and corresponding OMC.
MDD is used as a benchmark for field compaction control. On-site, engineers compare the field-measured dry density with the laboratory MDD.
This is often expressed as relative compaction:
\[ \text{Relative Compaction} = \frac{\text{Field Dry Density}}{\text{MDD}} \times 100\% \]For example, if a specification requires 95% MDD, the compacted soil in the field must achieve at least 95% of the laboratory maximum dry density.
If a soil has:
- MDD = \(1.85 \, \text{t/m}^3\)
- Required compaction = 95%
Then the minimum required field dry density is:
\[ 1.85 \times 0.95 = 1.76 \, \text{t/m}^3 \]So the site soil must be compacted to at least:
\[ 1.76 \, \text{t/m}^3 \]Relationship with Optimum Moisture Content (OMC)
- At low moisture contents, soil particles are held apart by capillary forces → low density.
- As water is added, lubrication allows particles to rearrange into a denser configuration → density increases.
- Beyond OMC, excess water fills voids and pushes particles apart → density decreases.
The peak point on this curve is the MDD.
Practical Importance in Geotechnical Engineering
- Used as the reference density for field compaction specifications (e.g., “compact to 95% of MDD”).
- Critical for the design and construction of:
- Embankments and earth dams
- Road subgrades and pavements
- Airport runways
- Landfill liners and structural fills
- Influences strength, compressibility, permeability, and long-term performance of compacted soils.
Typical Values
MDD values vary widely depending on soil type:
- Well-graded gravels/sands: 2.0–2.3 t/m³
- Silty/clayey sands: 1.7–2.1 t/m³
- Clays: 1.4–1.9 t/m³