Hi bimr,
I believe you are correct that pushing or pulling a polyethylene-wrapped ductile iron pipe segment through an expanded path composed of sharp and broken shards of relatively hard pipe material in a pipe-bursting operation would likely result in some damage to polyethylene wrap. I would also agree that it is in the best interests of all when all engineering challenges, including installations in corrosive soil environments, are reasonably addressed. I'm not aware however of ductile iron pipe manufacturers specifically recommending loose polyethylene encasement for pipe-bursting installations.
While I’m not sure what you are inferring or would advocate as preferable with pipe-bursting, I believe it is also true that more than 30 years ago (I think responsive to problems that had occurred prior to then with plastic pipes due to rocks etc. in surrounds) ASTM D2321 e.g. was developed with minimum bedding requirements for thermoplastic pipes. While I believe this standard thus for good reasons requires quite carefully graded surrounds for plastic pipes, I’m not sure however if/how this standard then considered “migration” or movement of rock materials that I have heard may occur in some cases within a soil mass.
In effect quoting a consultant engineer I heard comments from a couple years ago, it is now sort of incredible (to me also) that some plastic pipes are perhaps now most often in some areas in the present day being pulled through and in effect subsequently bedded in shattered, in effect quite un-graded shards of old pipes and sleeves etc., as well as e.g. some encasements and protruding reinforcing steel etc., that are considerably harder than they are! Of course, plastic pipes are also then subsequently much more subject to thermal and Poisson variations/movements than are stronger pipes, due e.g. to lower material moduli, higher Poisson’s ratio and a coefficient of expansion roughly 15-20 times that of steel or ductile iron! In this environment I’m not really sure what good (e.g. to the Owner and tax/rate payers) some specifications I’ve heard, e.g. “allowing” no more than 10% gouging of a plastic pipe wall may be in the long-term (other than perhaps providing some protection/cover and maybe even future replacement business to the pipe manufacturer?) This may be particularly true where the installed, full-length integrity of the pipe wall installed in the ground in th eactual installations is typically not verified after installation.
At least in the case of steel and ductile iron piping, the pipe wall itself is rather impermeable and rugged. Also, commonly in the case of steel and now also for ductile iron pipes, electrical monitoring means and even very corrosive soil protection is available for coated or uncoated pipes via their electrical continuity and/or the application of joint bonding, monitoring, and/or cathodic protection (by means of anodes or rectifiers etc.) Special joint bonding means offering some degree of natural protection from the bell re pulling/dragging is now even also available for pulled ductile iron pipes where corrosive soil conditions are known or suspected.
Interestingly, I have also heard from contractors that small “locating wires” that are required to be pulled most often outside, over or alongside the full extent of plastic piping, where they are directly exposed don’t always survive pulls (the locating wires don’t have near the basic pulling strength etc. of pipe!)
Thus I wouldn’t assume that there is anything necessarily wrong with what the original inquirer wants to do, and even in corrosive soils, but regardless I agree pipe-bursting in general can be a rather rough and risky business compared to other types of pipe-laying (and with a lot to consider from planning to execution).
“Start by doing what is necessary, then what is possible, and suddenly you are doing the impossible”. ...
St. Francis of Assisi
