pipe encasemen

huangyhg

pipe encasement
we have an 18" diameter hdpe drainage pipe passing about 4 feet under a thickened slab supporting a train track (cooper e80 live load).  the allowable soil bearing pressure is good for 2500psf.  what's the best way to protect the pipe from crushing?  could i design the thickened slab to span the 18" over the pipe?  could i encase the pipe in reinforced concrete?  how do i design the encasement?  do i assume the 2500psf bearing pressure from the thickened slab needs to be supported by the encasement and design the rebar in the encasement for an 18" span?  does anybody know where i can find a good encasement detail?  i asked if the 18" hdpe pipe could be encased in a steel pipe, but it's already in place and the contractor wants to use concrete encasement.  thanks.  
a steel pipe sleeve is the best solution. however, since you can't do that, a properly designed reinforced slab over the pipe is probably next best. encasing an hdpe pipe in a block of concrete can create problems if the block settles under load - the differential settlement can shear the pipe where it enters/exits the encasement.
suggest that you download the "performance pile engineering manual" at this link
we usually require a sleeve around the pipe (1" clear of the pipe all around) and then pour a concrete encasement. a split sleeve can be used around an existing pipe as long as it is bolted or welded back together before pouring the encasement.
thanks.  what do you use to maintain the 1" clear all around?  do they make 22" diameter steel pipes?  do you typically reinforce the concrete encasement?  is the concrete encasement really needed if the pipe is steel encased?