Supporting steel structure on compacted crushed rock

Question

Supporting steel structure on compacted crushed rock

engineer1968 (Mining)

(OP)

12 Nov 14 12:26

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.

oldestguy (Geotechnical) · Answer 1 · 2014-11-12 19:54:46

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.

engineer1968 (Mining) · Answer 2 · 2014-11-12 20:36:24

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.

Ron (Structural) · Answer 3 · 2014-11-15 21:56:52

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".

cvg (Civil/Environmental) · Answer 4 · 2014-11-16 17:23:15

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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Supporting steel structure on compacted crushed rock