Vitrified Clay Pipe Overburden Stress...

[LHSVBALL]

We are building a concrete slab for a 29000 pound generator, the only real problem is a 6" vitrified clay pipe 3' below the existing concrete foundation. As the office intern it is my job to find out the overburden stress so we can determine if it will hold the new load. I am having a hard time with the research assignment, and was wondering if anyone on this site might have any experience with a situation like this. Any information or discussion would be great.

Thank you

 

[bimr]

You should remove the clay pipe and replace it.

You want your new concrete slab to be installed on undisturbed material and you have no way to know how the clay pipe trench was backfilled. Remove the pipe and install trench backfill (aggregate) and compact.

Note that the cost of moving the clay pipe will be inconsequential to your project.

The opinion of most people at the present time is that clay pipe is obsolete anyway. It has probably already seen its useful service life.

 

[LHSVBALL]

Thank you for the input, however, the company that own this particular pipe do not want it dug up... We were looking to merely ensure that the pipe could hold the weight. I wouldn't particularly expect this pipe to fail. I don't think the cost to uninstall this pipe, installing a new pipe, and refilling is worth this one project.

 

[cvg]

while I disagree that VCP is obsolete and should be replaced for that reason alone, I do agree that it would be risky to you and to the pipe owner to leave the pipe in place. I would replace with ductile iron pipe or case the pipe to prevent leakage under the slab and allow for maintenance in the future.

Before embarking on a structural evaluation and deciding to leave it in place, I would at least run a camera down the pipe to see
a)is it cracked or broken
b)has it settled at all

The weight of the slab is not excessive, assuming that the slab spreads the load adequately. Analysis of loads on rigid pipe is well documented, do a google search for Marston/Spangler Design Procedure

http://www.ncpi.org/

 

[bimr]

The foundation should weigh at least as much as the generator weight.

http://www.macallisterpowersystems.com/407/generator-set-foundations.htm

The foundation is probably going to cost around $12-15K. Removing the pipe would be maybe 5% of that cost. You surely can't have more than 10 feet of 6-Inch pipe.

What I assumed was that the VCP has probably been in the ground for some time and therefore, the VCP is probably past its service life. Although there are probably a few locales that prefer to use VCP, I don't see anybody installing VCP pipe these days.

It is probably less expensive to replace than to televise.

 

[LHSVBALL]

Sorry for the delay, it's been a crazy few weeks here.

That is very good information, one thing I may not have portrayed correctly is that the concrete pad will be on an existing 6" of concrete. And the pipe is something like 30' long. But sections of the VC pipe have previously been replaced with PVC. It's an... interesting set up.

We will tell the client that we highly advise sending a camera down and viewing the condition of the pipe to better determine its strength.

We are fairly confident that the pipe will be fine as long as we are able to assume that there is no real damage.

 

[rconner]

The new info that some of the pipe has already for some reason been replaced is further worrisome. You don't mention when the pipe was installed, but if it was long ago some VC joints then were not very water proof. Infiltration and/or exfiltration from porous joints has been known to contribute to breakage of at least brittle piping materials like clay at any time now or in the future (due to lack of support), and the image of collapse of a working line apparently also under concrete doesn't sound good.