ZAV; field compaction precautions

Zero air void

The zero air void (ZAV) line is the plot of dry unit weight against water content for a fully saturated soil ($S=100\%$, no air):

$$\gamma_{d} = \frac{G\gamma_w}{1 + wG}$$

It is the upper limit of the compaction curve.

Precautions during field compaction

General: use the correct roller, lift thickness (150 - 300 mm) and number of passes; keep the water content near OMC; check the density with field tests (sand replacement, core cutter); remove stones and organic matter; mix the soil to get uniform moisture; avoid over-compaction.

Homogeneous earth dam:

• Compact on the wet side of OMC (about OMC + 1 to 2%) to get a flexible, low-permeability fill that can adjust to settlement without cracking.
• Use sheep-foot or pneumatic rollers; bond each layer to the next by scarifying the surface; keep a uniform water content; compact in thin layers; provide a filter drain to control seepage.
• Avoid over-wetting, which would cause pore pressure build-up and slope instability during construction.

Highway subgrade:

• Compact on the dry side or at OMC to get high strength and stiffness (CBR); the top 300 mm should be compacted to 95 - 100% of the standard/modified Proctor MDD.
• Protect the fill from rain and keep the drainage working, because later wetting can cause swelling and loss of strength; adjust the moisture to OMC by sprinkling or aeration.
• Use vibratory rollers for granular soils.

Dry side or wet side?

For earth dams and canal linings (where impermeability and flexibility are needed), prefer the wet side of OMC. For highway subgrades and wherever strength is important, prefer the dry side up to the OMC, as the soil has higher strength, but there is a risk of swelling and water absorption if the soil gets wet later.