Problematic manufacturing slab/mat movements

[RJP78]

To give some background, an assembly table, which is prone to the slightest movement, was placed on an existing 6 inch slab-on-grade within an industrial building. However, the table would become out of level by a few thousandths and fluctuate over time. As a result, the slab was reportedly torn out, some excavation below subgrade occurred, and an 18-inch independent slab (with #5 bar @ 12" OC) was poured. I've been informed that the subgrade consists of relatively fine sand fill, and was compacted prior to slab construction. Nevertheless, the problem persists. As a demonstration, a small forklift was positioned on a corner of the slab and it temporarily settled 0.007± inches.

Some additional information:
- Slab/mat measures 20'x16'x1.5'
- Assembly table weighs approx. 3 tons
- Facility has experienced temporary flooding of the shop floor in recent months.
- Overhead cranes and forklift traffic are present in the area of the table.

I've been asked to evaluate why the slab continues to move even after replacement. Since the slab will remain in place (for now at least), my observations and testing capabilities are limited. To determine the soil and water conditions present beneath the slab, cores alongside the slab are planned, with hand-augered sampling below.

Has anyone experienced similar scenarios? Can this small of movement be stopped, and by what means? Helical piers? Polymer injection?

Thanks for your time. I've been a long time lurker and am thankful for everything I've learned over the years on this site.

 

[hokie66]

The earth is not a static mass. This magnitude of movement is inevitable. If it had been founded on piles, the problem would have been lessened. I doubt there is much that can be done with the existing slab sitting on the ground.

 

[Ron]

This is a prime example of measuring with a micrometer, marking with a crayon, and cutting with an axe.

If you think you can accurately measure 0.007" consistent elastic movement in a fine sand fill...well, you're fooling yourself.

If you have an application that is so critical, then the foundation mass must be increased to several multiples of the activity mass.

You could consider injection of a chemical stabilization grout or an expansive foam injectant. Either will stabilize your condition.

 

[RJP78]

Thank you hokie66 and Ron.
I had my doubts that such small movements could be avoided. Wouldn't the movements have been slightly smaller had the base materials consisted of a well-graded crushed stone, which has a higher coefficient of subgrade modulus? Thanks again.