Our clients, an architect and a structural engineer, are complaining about our recommendation for a 98 percent Modified Proctor dry density in the last foot underneath the floor slab-on-grade for an addition to a high school. It is a 150 psf slab, with no extraordinary criteria. The soils are loose to firm sands to 20 feet. The 98 percent requirement is mostly to get contractors to do their work right. It appears to be fairly easy to get 98 percent if the subgrade is properly compacted. The client's complaint is that 98 percent instead of 95 percent will cost significantly more money. I think this is baloney. In loose to firm sands, 98 percent compaction within a foot of the surface in an area easily accessible to a medium to large compactor, which they'll need anyway, is easily achievable with little to no extra effort. There doesn't seem to be much difference between 95 and 98, but the contractors I have dealt with put a little extra effort into making the slab subgrade evenly and properly compacted if they think they need 98 percent.
Is a 98 percent Modified Proctor dry density specification under slab reasonable, or am I being overconservative?
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