underground pex alternative?

I'll be putting in my under ground pex this week and since the cost of manufactured insulated pex is out of my reach($$) I came up with the following alternative for 1/2 the cost.
2 1-1/4" pex with foam pipe insulation around each individually, pulled through 4" sch 40 pvc pipe. The 4" PVC will be inside 6" sch 40 PVC. In between the 4" and 6" pipe will be great stuff expnded foam. All this will be laid on top of 12" of gravel, pitch to drain water away to daylight(Gravel is left over from another project). All joints will be glues and I'll also apply some silicon caulk to the outside of the seams of the couplers and such. I'll also cover all piping with a thick mill plastic to get any water down to gravel bed before meeting the PVC. It's a straight 75' shot so only one elbow up to boiler shed.
This is costing me half of buying pre-insulated pipe. It also allows me to pull new lines through the 4" pipe should I ever need to. I'll also pull under ground wire through same 4" pipe for electric + a pull string for future needs.
I'll let everyone know how this worked come spring next year but thoughts are appreciated.

6" $20.00 for 10 ft = $2.00 p/foot
4" $11.00 for 10 Ft = $1.00 p/foot throw in the couplings and such and your at about $6.00 p/foot.
I'll post the total when I'm done.
I plan on drilling holes just big enough to insert the straw from cans. After I do the first hole I'll let it expand and then determine the spacing of the spray holes. I plan on doing a 10 ft section at a time.

The price for the spray in foam I got was...you ready for this....$900.00 for 75 feet! Thus the alternative method. I'm not sure how many cans I'll use yet. I have to do the first test spray to see what the coverage is. I'll get back to you on that one. I'll cut a wood disk to support the 4' pipe until the foam stiffens up and holds it . Then move it to the next section.

Tony H you are a wise man. I never gave the electric in the same pipe as the water a second thought! Thank You. At this point I have no intentions of selling the house but who knows............point taken. As far as frost heave goes..I'm burying the lines below the frost footer so frost heave won't be an issue.
I already have a 10' section of 4" pipe and the 2 1-1/4" pipes with foam does fit with a little room to spare. Very little, but room to spare.
The only thing I haven't tested yet is how far a can of spray foam will expand between the 2 pipes. That test will be done within the next 7 days. I'll post back with the results.

Tennman,
I'm not ignoring the alarms. I'm actually trying to sound them. I haven't seen anyone here do a pipe in pipe install. Yes I'd like to save money whenever I can but not at the cost of quality. I'm throwing this out to get people to tell me why it won't work. I know after thinking this through that there is something I could be missing. Woodmaster just gave me the first reason why it's probably not a good idea!

Tennman,
I understand your frustration with the original install and I truly do thank you for the warnings. However, you never mentioned exactly how you installed your original piping? Was it pipe in pipe with foam in between or was it single 4" pvc pipe ? Can you tell me details about your original install?
I want to get my pipe in ditch real soon for obvious climate reasons but I have not bought the PVC pipe yet. I have gone to lowes and made sure a 4" elbow will fit within a 6" elbow. My neighbor will be digging my ditch with his Kabota and he will show up when it's convenient for him, not me as it should be.
One of my personality flaws is I won't take can't for an answer. I need to know why. I hope this doesn't irritate any one on this forum I just can't accept something without a reason. That's why I threw out my ideas prior to installing. A coworker of mine started asking me what I was trying to do with the under ground piping. After describing to him what I was proposing he said that is how they used to do it in the military. The only thing he added was they would spray foam the couplers of the outside pipe and wrap in plastic type material.
From what I can gather from your post is you ran a single PVC pipe and pulled your pex through it. You then drilled holes in that PVC and tried to fill it with foam trying to surround your pex? Yes?
What I'm proposing is running a 4" PVC pipe,sch 40. Within this pipe will be my 2 1-1/4" pex lines. Each with foam pipe insulation wrapped around each(like the piping foam you get from lowes/home depot). Then I will slide 6" PVC sch 40 over the 4" PVC piping. I will then drill holes in the outer most 6" PVC (without compromising the inner 4" PVC pipe) to shoot great stuff expanding foam with all intentions of filling the gap between the 4" and 6" pipe. I plane on siliconing all coupling seams on both pipes and after talking to my co-worker I might as well surround all 6" couplers with foam. This will all be laid on a 6-12" bed of gravel to daylight. It will also be covered in a thick mill plastic so water hits the gravel before it hits the PVC.
I'm confident I can do this for less than I can buy logstor for. The plus to my idea is I can still pull new pex lines if needed. With Logstor you have to dig it back up. I really want to know about your original install that failed so I don't run into the same problems. But please give the details of your original install so we can learn from mistakes.