Field Density and Moisture Testing (Compaction Control)
Scope
Field density and moisture testing proves that the compaction achieved in the field matches the laboratory target. The exam tests method selection, sand-cone and gauge procedure, and the arithmetic of relative compaction.
Methods
- Sand cone (AASHTO T 191): the calibrated dry density of the sand and the mass of sand filling the cone must be known; hole volume = (sand used − cone sand) ÷ sand dry density.
- Nuclear gauge (AASHTO T 310 / ASTM D 6938): take a standard count at the start of the day and stay within the manufacturer's tolerance; direct transmission measures density at depth, backscatter reads only the top 50–75 mm.
- Moisture: oven drying (T 265) is the referee method; calcium carbide (T 217) and microwave (D 4643) are rapid alternatives.
Compaction control
Relative compaction = field dry density ÷ maximum dry density × 100. Field dry density = wet density ÷ (1 + w). Moisture is compared with optimum, usually within a specified window such as ±2 percentage points.
Worked example
Sand cone: sand dry density 1.60 g/cm³, sand to fill the hole 1,440 g after cone correction → volume = 900 cm³. Wet mass of soil 1,850 g → wet density = 2.056 g/cm³. Moisture 12.0% → dry density = 2.056 ÷ 1.12 = 1.836 g/cm³. Lab maximum is 1.93 g/cm³ → 1.836 ÷ 1.93 = 95.1% compaction. Against a 95% minimum this passes by a thin margin, so check moisture and test location before accepting the lift.
Traps
Reporting wet density as dry; forgetting the cone sand; comparing a backscatter reading to a lab value from a deeper sample; letting a moisture sample dry before weighing; ignoring oversize particles, which inflate density.