Specifications for any earthwork project in the Fargo metropolitan area must reference AASHTO T-99 or T-180 to establish the compaction benchmark. The lacustrine clays and silts deposited by the ancient Lake Agassiz exhibit high plasticity and a narrow window of workable moisture content, which makes standard Proctor test procedures essential before placing a single lift of fill. We run both standard and modified Proctor tests on material sampled directly from the borrow source or the site excavation, because the difference in compactive effort changes the density target by up to 12 percent in these fine-grained soils. When the project involves structural backfill against foundations or utility trenches, we often pair the Proctor with a sand cone density test to verify that field compaction meets the laboratory-derived curve.
A two-point change in moisture content on a Fargo silty clay can drop relative compaction from 98 percent to below 92 percent, and no proof-roll will catch it until the fill is already in place.
