We recently tested a commercial pad near the Oakway Center where the contractor had completed compaction on a silty clay subgrade typical of the Willamette Valley. The project spec called for 95 percent of modified Proctor density under the slab-on-grade, and the nuclear gauge results submitted by the earthwork sub were inconsistent along the east lot line. Rather than accept questionable numbers, the structural engineer asked us to run parallel ASTM D1556 sand cone tests at six grid points to verify field density. Eugene sits at about 430 feet elevation on Quaternary alluvium, and the local clay can retain moisture late into August, which affects compaction readings if not accounted for. We combined the sand cone verification with a grain-size analysis to confirm fines content, because high plasticity silts in this part of Lane County often require moisture conditioning beyond standard rolling patterns.
ASTM D1556 sand cone testing gives a direct, physics-based density measurement — no radiation source, no correlation curve, just calibrated sand and a field scale.
