Structural Fill Pad Under Recip Compressor

[sundale]

Situation: Large skid mounted recip compressor founded on a 4'-0" thick moisture compacted ABC fill pad over native sand. Skid rails are buried almost up to strongback beams’ top flange with compacted ABC ramps as well. Skid has developed excess horz and axial movements (and some minor vertical movement) near the compressor end. Mechanics claim (and I believe them) that there are no mechanical issues in the compressor and gas pulsations have been ruled out as well. Skid is seemingly level and plumb per a torpedo level and plumb bob. This skid was earth founded elsewhere and performed OK until it was relocated. Compressor to frame bolts were loose for a few months of runtime, but tightening them halved the horz movements and doubled the axial movements (it helped, but not enough).

I have done >10 of these sorts of foundations under large skid mounted compressors and pumps over the years and never had a problem. Geometrical damping available from dirt foundation is generally an over-damped system and performs well (at least until now for me…). I have always specified a well compacted structural fill pad in lieu of trusting in-situ density of what is there. A 130-ish kip recip compressor skid WILL compact the underlying native soil, if it is not already…

Is it possibly to have a skid “high center” itself over time, if the soil is well compacted like this? I would expect the opposite, but this is a theory offered by others. Another theory from others is that the dirt was compacted “too hard”. I have also heard from others that “in the old days”, one would level the fill pad and then intentionally excavate a shallow depression under the middle of the skid to prevent “high centering”.

Thanks in advance for any diagnostic opinions.

 

[JoshPlumSE]

A Any signs of cracking in the slab? How is the skid mounted to the slab? Any chance the anchor rods are not performing properly? Some anchors loosen up over time when subjected to dynamic load.

 

[cvg]

no slab here, just skid on compacted gravel.
is the ABC confined or unconfined? what are dimensions of the pad and side slopes vs the skid dimension?
was there any compaction testing done? was the ABC per any specs? was the native sand scarifed and compacted first?

not sure how High center would create horizontal movement

 

[sundale]

ABC is unconfined with gentle 5:1-ish slope. Compaction testing was performed (95% Modified) Roadbase spec was NMDOT for 3/4" (-). Native sand was scarified 8" and compacted.