Sanitary Sewer Settlement
Sanitary Sewer Settlement
(OP)
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





RE: Sanitary Sewer Settlement
Do you have record drawings?
RE: Sanitary Sewer Settlement
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.
RE: Sanitary Sewer Settlement
RE: Sanitary Sewer Settlement
RE: Sanitary Sewer Settlement
RE: Sanitary Sewer Settlement
RE: Sanitary Sewer Settlement
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.