Sanitary Sewer Settlement

[builditnow]

looking for experience and an opinion on a question from a colleague. A 16 ft deep 8" PVC SS was replaced due to the original installation (over 10 years ago) settling in a 70 ft long area (6" settlement). Ground water is normally 2 ft above the pipe. Pipe was bedded and backfilled full depth with aggregate. The problem: about 1.5 months (recently) after the replacement installation the pipe settled 6" for a second time. The replacement was constructed during dry weather period with no ground water. The pipe is currently uncovered and the groundwater is 2 feet above the pipe. A 3 acre lake is about 500 to 800 feet away form this line. Pipe is about 2 feet below the lake surface water elv. A test pit below pipe subgrade (4ft deep + additional 2 feet of excavator bucket penetration) has revealed no stable soil, the soil is a black shale that is severely weathered. Since the trench is open, in a residential area and roadway we have limited time for hypothesizing. Concrete cradling is possible but we must have a reasonable soil bearing capacity, soil cement - same concern, hanging cradle? This small Town has limited financial capacity and the current alternative suggested by the contractor is concrete piers spaced at small intervals with Ductile Iron Pipe and mechanical joints. Unless founded on stable ground none of these options work. ANy experience out there with similar projects. THANKS

 

[bimr]

How do you know that the pipe settled? Is it possible that it was just constructed poorly?

Do you have record drawings?

 

[rconner]

A suggested design procedure for ductile iron pipe on supports is available from DIPRA at

http://www.dipra.org/pdf/pipeOnSupports.pdf.

I have no reason to suspect that such an installation would not be efficacious in these circumstances.
Contemporary push-on joint ductile iron pipe (that is effective and at least a little more economical in material and assembly) is probably used more often and may be more available than mechanical joint for present-day installations.

 

[builditnow]

Thanks for the replies. The pipe was televised as part of the warranty and payment process, that is how it was discovered. The camera went under water during televising. I beleive there are asbuilts available but I have not seen them nor do I have access to them. I will check the DIPRA site but I still beleive that the support subgrade must be sufficient to be able to carry the load.

 

[rconner]

Perhaps what bimr was getting at was that arguably pipes can be placed at the wrong elevation to begin with, and at least lightweight pipes can also be buoyed (or float upward) in sufficiently fluid and dense (mucky and/or disturbed etc.) ground conditions, according to Archimedes. In any of these situations, if a camera went from a local high point to an adjacent low point, I guess it could go "under water" as described.

 

[rconner]

Oops, I meant to mention also that in extreme circumstances it might even be argued manholes could even move also relative to piping! [Anyway, I guess actual as-built elevations as mentioned might be most helpful to sorting it all out.]

 

[civilperson]

If the pipe is designed as pressure pipe, a force main, then any small settlement is easily neglected. If it is a gravity flow, then it becomes an inverted syphon and the pressure head must form upstream to push the flow through the uphill fraction. This still a workable solution with maintenance to flush solids which are falling out of slow moving sewer. Perhaps a greater slope could be designed for this section of unstable soil to assure no uphill section.

 

[bimr]

Highly recommend that you do a soils investigation, with a boring done every other manhole. The geotechnical expert should be able to provide you with the advice that you need.

Here are some general guidelines for poor soil areas:

The soil classification encountered in the trench would change the trench bedding design. It is necessary to provide a stable formation before pipe laying. Such conditions commonly occur in peat, silty ground soft clays, running sand, or in fill material.

Although sometimes trench formations are filled with concrete, this is unlikely to assure long term stability in all cases, and a form of flexible bedding construction is the preferred method of dealing with the situation.

The trench formation should be over excavated by 24" to 30", depending on the bearing strength of the soil. Gravel reject material is then compacted in layers to form a firm trench bottom. A 2" thickness of lean-mix concrete is then placed as blinding. The pipe is then laid on granular bedding material.

Slurry is usually used as a blinding layer. Clays and silts are highly susceptible to softening when in contract with water. Clay formations are protected with blinding concrete or with foundation concrete as soon as possible after completion of the excavation. If not protected, the clay will swell and then must be removed.