Failed Density Test on a Rained-On Building Pad
You are the lead operator on a 3,200 m² building pad that must receive 600 mm of engineered fill under a slab-on-grade. The geotechnical specification calls for 200 mm loose lifts at moisture within 2% of optimum, compacted to 98% of Standard Proctor maximum dry density, with one field density test per 500 m² per lift. Trucks have been arriving faster than the roller can keep up, so the dozer has been spreading 350–400 mm lifts. Two days ago 35 mm of rain fell and the surface is now greasy underfoot. On the inner half of the pad the roller operator made only two passes because the ground was too soft, and the dozer walked that area in instead. The latest test at the north corner returned 96.5% with moisture about 4% above optimum. The rebar crew is scheduled for tomorrow morning.
1. Identify what has gone wrong and explain why each item matters to the density result.
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Four failures stack on top of each other:
- Lift thickness. 350–400 mm instead of 200 mm means the roller's energy is absorbed in the top portion and the lower half of each lift stays loose. Compaction effort falls off quickly with depth - thick lifts hide loose material that later shows up as settlement under the slab.
- Coverage. Two passes on the soft half, with a dozer walking it in, is not compaction. A dozer's tracks knead and remould the surface; they do not deliver the vibratory energy a roller does, and the area has effectively been left uncompacted.
- Moisture. 4% above optimum fills the voids with water, so the soil bulks and 'pumps' under the drum instead of densifying. Greasy surface is the visible symptom.
- Soft subgrade. The roller sank because the layer beneath is yielding. Rolling over a soft spot without proof-rolling and repair just buries the problem.
The 96.5% result is consistent with all of this - it is a true reading of a lift that was never built to spec.
2. Give the corrective sequence you would run before the rebar crew is allowed on the pad.
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The schedule has to bend, not the specification.
- Stop hauling and stop spreading. Adding more fill on top of a failing lift compounds the problem.
- Proof-roll the whole pad with a loaded truck or the roller in static mode and mark every soft area with paint. Probe them to find depth.
- Scarify the failed lift to its full depth - at least the 350–400 mm actually placed - and aerate it. If weather will not allow drying, amend with lime, cement or granular material, or remove and replace with suitable fill.
- Re-spread in 200 mm loose lifts, moisture checked with a gauge or by hand before rolling.
- Compact with the roller at the specified passes and speed, overlapping each pass, and do not roll a lift that is pumping.
- Excavate and replace the soft spots found by the proof-roll with approved material, compacted in thin lifts.
- Retest each lift across the full area, not just at the previous failing location, and get the geotechnical consultant's sign-off before the next lift or the rebar.
Communicate the delay and the reason upward in writing the same day.
3. The general contractor asks you to just re-run the density test at the same spot because the number was close. How do you respond, and what records do you keep?
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A density test is a point sample, not a verdict on the pad. Re-testing the same spot answers the wrong question: the failing lift is 350–400 mm thick, over wet soil, half-rolled. Even if a re-test passed at that one location, the material beneath it and the unrolled half would still be out of spec, and the slab would carry the risk.
What I would say: the failure is procedural and the fix is reworking the lift, not re-testing it. If the consultant wants a confirmation test, it goes in the reworked area after corrections, plus additional tests in the previously unrolled zone.
Records to keep:
- Lift number, station, elevation and area for every test, with the test report.
- Moisture at time of rolling and the weather log (the 35 mm rainfall event).
- Roller type, pass count and speed per lift.
- Proof-roll results and soft-spot repairs with locations.
- Photos before and after rework, and written direction to stop and rework, copied to the consultant and the client's engineer.
That package is what protects the operator and the contractor if the slab moves later.