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Supporting steel structure on compacted crushed rock

Supporting steel structure on compacted crushed rock

(OP)
Hello Civil Engineers. Pardon my ignorance. I am a mechanical engineer designing a steel structure for a construction site.
Basically it is a 30ft x 12ft platform with 8 legs (can be more legs). The fabrication plus the large tooling it carries weighs 30 tons (60,000 lbs).
The thing is intended to sit outside on a prepared compacted area. Here is my question. If for simplicity each leg carries 60000/8 legs= 7500 lbs.
Lets say I put a square steel foot on the bottom of each leg 12" x 12" (144 square inch). 7500 lbs/144 in^2 = 52 psi (or 7500 lbs/square foot) load on the aggregate pad.
Assuming the load is evenly distributed on all 8 legs to simplify. Is there a specification I can use to make sure the compacted pad is up to the job?
I want to be able to say something like " make 20 ft x 40 ft pad 3 feet thick of aggregate x. Compact until it meets Proctor(?) test ?% compacted."
The pad is elevated in an are where water accumulation is not an issue. Again, pardon my ignorance. I would appreciate some help in how to spec this pad
so that it can handle the weight of my structure. Thanks. The contractor uses crushed scoria rock but I can spec something else if need be.

RE: Supporting steel structure on compacted crushed rock

We arn't usually designing things here, but you apparently have. I'd have to assume the site is stable ground where loaded front end loaders can run and not show a rut more than one inch and the main purpose in the crushed rock pad is for leveling. For a construction site I'd also assume the structure can withstand some differential settlement and still do its job. A pressure of 7.5 kip per sq. ft. is pretty light if there is no signigficant vibration from the devices carried.. However, to provide a suitable support,the compaction of the crushed rock could be done to a standard that commonly is used for highway roadway base course. A common specification is 95 percent of Modified proctor density. If it was me I would specify how it is compacted and not run any tests, I'd require a vibrating flat wheel roller weighing at least 12,280 pounds for the drum. Use at least 4 passes in any area on lifts no greater than 10 inches each. If these suggestions can't be met, I'd ask a geotechnical engineer to visit the site and recommend treatment.

RE: Supporting steel structure on compacted crushed rock

(OP)
Thanks for the input. That is a big help. Yes, the pad is something I wanted to add just to make sure we know what's underneath.
It is a surface mine. Construction site was a little misleading.
The open pits definitely employ geotechnical people for a number of reasons.
I'll see what I can find out and post my results.

RE: Supporting steel structure on compacted crushed rock

With "feet" only to support the structure (which I assume is at least semi-permanent), you have no protection from undermining of the feet by erosion. This is one important function of a concrete footing. Your approach will create a high modulus base for the structure and will likely be sufficient from a loading standpoint, but I'd still be concerned about the potential for undermining/erosion beneath the "feet".

RE: Supporting steel structure on compacted crushed rock

assume a safe bearing pressure for the subgrade
assume that compacted aggregate will spread the load at about 45 degree angle.
3 feet thick sounds like overkill, 12 inches would likely be more than adequate, but really depends on your subgrade strength.

wet it and roll it with a vibratory roller until it is well compacted. you really can't use proctor for crushed rock. you might consider using dense graded aggregate base course material.

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