Non-uniform subgrade slab support

[lowlydirtguy]

Consider industrial slab support via ground improvement, such as soil-cement columns, geopiers, or stone columns. These systems have no rigid connection to a structural slab, but are hard points and a typically very weak soil. Say a geotechnical engineer or design-build contractor proposes a 10'x10' or 12'x12' grid layout of one of these stiff springs. For this project, there's no load transfer platform (such as Geogrid) and only 1' on fill of the ground improvement elements, and the spring stiffness of these elements is, say 100 kips/inch in a "sea" of weak springs (say 15 pci).
1. Is this design exactly like an elevated slab?
2. What is the most appropriate American concrete code to use? Is there a code that combines concrete stress analysis (like ACI 360) with active reinforcing analysis (like ACI 318)?
3. If ACI 318, should the 3" bottom cover requirement still be maintained. It seems like a waste, considering that the slab thickness may only be 7" total.
4. For a gross reinforcing ratio of at least 0.002, is subgrade drag still a concern?

 

[ishvaaag]

Since you can readily model for the weak springs as well the first question has no special implications. Obsiously to have some support at midspans between hasdpoints will influence positively the required reinforcement. Yet at the slendernesses you are naming (7 inches thick for spans 12 ft) the positive effect may turn to be very small, for so is the deflection with null support for loads compatible with good structural behaviour.

Respect the second and third questions I infer from our practice here and your question that being the structural concern the one being represented, a structural code will be applicable and deemed to be used as guide, say ACI 318. This means in all its specifications for consistency. Questions like cover on special situations if not clearly defined in the code could be consulted with the building authority but you always will stay within the general accountability of your design to every other view about. Certainly, say on already poured concrete to be used as form the cover looks hefty, yet even in such cases the general practice in Spain is to provide the demanded structural cover on the main structural body. This has the sense of that protection is counted from its surface, say, crack control, that may become critical in foundations.

Respect the 4th question as in the first any recognizable force on the slab has its effect. However at this thickness and for condominum building sizes in my practice (say till 40 m) when the slab is reinforced in two faces I have not had any problem of this origin.