Subgrade Compaction Fails Density but the Contractor Wants More Roller Passes
A 200 mm thick compacted subgrade layer is being laid over a 1,200 m² area of a colony road. The soil is a borrow material with laboratory MDD 1.85 g/cm³ and OMC 16.0%. The contract requires at least 97% of MDD and field moisture within ±1.0% of OMC at the time of compaction. Ten field density tests using the core-cutter method give an average field dry density of 1.78 g/cm³ and an average moisture content of 13.4%. The contractor has already rolled the layer with eight passes of a vibratory roller and argues that additional passes will fix the density. The junior engineer must decide whether to accept the layer, and if not, specify the correction. The next layer is scheduled for tomorrow morning and the weather forecast is hot and windy.
1. Compute the degree of compaction and the moisture deviation. Should the layer be accepted?
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Degree of compaction = field dry density / MDD × 100 = 1.78/1.85 × 100 = 96.2%. The specification requires at least 97.0%, so the layer is about 0.8 percentage points short. Moisture deviation = 13.4 − 16.0 = −2.6 percentage points, which is outside the permitted ±1.0% band. Both the density and moisture conditions therefore fail. The layer should not be accepted on the present test results. Because the result is an average of ten tests, I would first check the individual test values and the calibration of the core-cutter and moisture determination, but a consistent 2.6-point moisture deficit is a strong physical explanation for the low density.
2. Estimate the water required for the full 1,200 m² layer to bring the moisture to OMC, stating your assumptions.
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Take a unit area of 1 m². The compacted layer volume is 0.20 m³ = 200,000 cm³. Using the target dry density at 97% of MDD: 0.97 × 1.85 = 1.7945 g/cm³. Dry mass per m² ≈ 1.7945 × 200,000 = 358,900 g ≈ 359 kg. The moisture deficit is 16.0 − 13.4 = 2.6% of dry mass, so water needed ≈ 0.026 × 359 = 9.3 kg/m², i.e. about 9.3 litres/m². For 1,200 m², the quantity is about 11,200 litres, or 11.2 m³. This assumes uniform mixing, no loss to the underlying layer, and no evaporation. Because the weather is hot and windy, water should be added in stages, mixed into the soil, and the moisture rechecked immediately before rolling; a small allowance for evaporation and absorption should be made rather than dumping the full theoretical quantity on the surface.
3. The contractor proposes simply rolling the layer more without adding water. Evaluate that proposal and give a sequenced corrective action.
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Additional passes at 13.4% moisture, which is well below the OMC, are unlikely to raise the dry density to 97% MDD. The soil particles are not sufficiently lubricated to rearrange into a denser packing, and extra vibration can cause surface sealing, shearing or over-compaction of the top crust without improving the layer as a whole. The correct sequence is: scarify the layer to the full 200 mm depth; sprinkle and mix water in stages to approach 16% moisture; allow the moisture to equalise; recheck moisture at several points; compact with the approved roller and pass pattern; then retest density and moisture. If the retest still fails, the layer should be reworked or replaced rather than accepted on extra passes alone.