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

 

[oldestguy]

Check out the possibility of water erosion.

Many times the aggregate for cover and bedding is a single sized material with open void spaces

This forms an easy path for water to flow in the trench, after backfilling. It can cause erosion of fines, via that flow system. The place the fines came from settles.

I have seen this mainly recently after construction, since soon the voids get filled and other stable conditions develop. Pumping from the trenches as the job goes along can induce this flow also. In rare cases, artesian flow intercepted by the work can do it also.

For now, I'd examine the pumping practice, since that may be the culprit. It may be necessary to use something other than sump pumping.

In some areas the pumping from open trenches loosens the soil below the trench and so the weight of the backfill compresses that soil and the pipe settles along with it. In these cases the subsoil is usually more granular types than a clay. These situations are difficult to detect, since the excavator takes it down to grade, including "lifted' soil.

If the shale has had the weight of the earth on it before the trench work, there should be no added settlement due to the trench and the backfill, unless some subsoil disturbance is caused by the work.

If the subsoil below the pipe is disturbed and is therefore "compressible", a cradle won't help.

Those members here with experience with expanding shales may have some help.