slab-on-grade w embedded rails

huangyhg

slab-on-grade w/ embedded rails
i am a relatively new mechanical engineer doing mostly plant maintenance work.  however i have been assigned the task of designing a slab-on-grade for a pushing a loader along a rock pile.   
the slab should be 12'x12' with 2 rails embedded @ 4' feet and centered along the slab.  
the loader with max capacity filled bucket weight ~100k
my current design based off of the building the loaders are serviced in includes a 12 inch slab with top and bottom #4 bar spaced at 12".  does this seem reasonable?
my other question is how to deal with the embedded rails.  originally they were only going to be 2 inches deep which i figured would be ok.  almost considered a sawcut.
but now they want to use 6" deep 90# rails mainly because we have them on site.  a 6 inch rail embedded in a 12 inch slab seems like a big problem to me.  
i haven't dealt with concrete before and don't know too much about design.  however i feel fairly confident in the slab holding up to the load, not including the rails.
i appreciate any help.  thank you.
i would suggest two concrete pours. the first would be the reinforced base slab - the rails are not embedded in this slab, they sit on top of it and are anchored to it. six thick topping slabs are placed around the rails as needed - these are reinforced also. rebar dowels cast into the base slab could project vertically to tie the base slab to the topping slab. the concrete finish of the base slab is not too important since it will be covered. start wet curing that day. clean the top of the base slab & place the topping slab as soon as possible (probably the day after the base slab is poured) to maximize the bond between the slabs. wet cure everything for at least a week.
depending on your loads, 12 inches thick sounds reasonable for the base slab. put plenty of reinforcing steel in the concrete, maybe #5 or #6 bars instead of #4.
best to be conservative when designing for industrial applications - things like this have a hard life.
get a hold of crsi engineering data report number 33, or pca "concrete floors on ground."  i think you need more reinforcing, like maybe #8@12".
i agree with sliderule.
i would like to say again however,your bottom slab needs to be  12 inches.than cover with 6 more inches.
if you use an over all slab of 12 inches once you put your rail in you will only have 6 inches under the rail.not enough  to last very long.
good luck ed
thanks, for your answers, they have helped a lot.  
one more quick question though.  
what if i used a 2 inch rail instead of the 6.
could i embed the rails in the top layer and bend the rebar around and under them?
or should i just pour an extra 2 inch layer.  the 2 inch layer couldn't really be reinforced.
my guess, is the 6 inch rail would be the way to go so i could still reinforce the top slab.
i would use a single 14 inch pour.  
i have designed for similar situations in two ways.  the first way is somewhat similar to the recommendation by slideruleera and my preferred method.  the main load carrying slab, or bottom slab, is cast first.  the rail is then anchored to this slab.  since this is a loader pushing rock across concrete, the upper slab will essentially be a wearing surface.  although the bucket will be riding on the rails as the loader pushes, the rocks that end up under the bucket will be scraped along the concrete surface causing it to wear.  depending on useage, the slab may not last too many years before it needs replacing.  by having a seperately cast upper slab, it makes it easier to replace when the time comes.  before the upper slab was cast, 4 mil poly was placed on top of the bottom slab for two reasons.  one was to keep the upper slab from bonding to the bottom slab, thus making it easier to remove.  the second reason was for the poly to serve as an indicator to signify that it's time to replace the upper slab.  i've also used some light wire mesh on top the poly to help with cracking and provide an additional method of indicating the need to replace the upper slab.  
the second method i've used is to cast this as one slab.  the rails were welded to steel beams set level on grade.  holes were cut in the web of the beams to allow rebar to pass through.  the beam size was adjusted so that the depth of the beam and the depth of the rail were approximately equal to the thickness of the slab.
as for reinforcing, the least i've used is #5 @ 12" ew and the most i've used is #7 @ 10" ew.  it just depends on the loads, the situation, and engineering judgement.  i've yet to use top and bottom mats.
i agree with ctw and slideruleera.  make two seperate slabs.  for the reinforcing we would typically use #6 @ 12" c-c in a 12" slab.  the way i look at these types of slabs are as a slab with two short uniform loads and supported under the bottom.  slab bottome stresses are usually high.  slab top stresses are usually low due to the narrow axle track of loaders.  be conservative with your design.  a quarry will dish out more abuse than you can ever think of.  a 100,000# loader can dish out some termendous dynamic loads!
do you have good soil conditions? i can't say that i exactly understand "push a loader along a rock pile", but i have had embedded rails in a plant that made masonry block and we did it similar to the two pour method. we had very tight flatness constraints, do you need to worry about levelness and hitting a particular elevation with the top of your rails? if so, think about pouring the slab a bit low (no nore that 1/4") and cound on shimming. (its much easier than grinding)
lastly, depending on your soil conditions, you could consider two strip footings under the rails, then a slab between them.