Trench Settlement
Trench Settlement
(OP)
One year ago, a new sewer line was installed down the middle of an existing street. The total length of the trench is about 2 miles. The width of the trench was about 10 feet. The backfill material consisted of gravelly soils and compacted to about 90 percent according to the inspector. However, there was no full-time inspection during compaction. The top of the pipe is about 25 feet below street grade. Now the trench has settled in some areas as much as 12 inches. Has anyone come across this problem before, if you have, what were your recommendations for the repair of the trench settlement.





RE: Trench Settlement
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Trench Settlement
RE: Trench Settlement
Your situation sounds very much like a few problems I have investigated. The typical scene was a moderate to deep (10' to 20') backfill was placed, compacted and tested. Ground water was very close to the bottom of the trenches and surface waters were usually not well controlled. Within a few months, Settlement was about 5% to 8% the total fill depth.
After the initial finger pointing and gnashing of teeth,there was very little doubt that the fill was placed to the original project requirements, 90% of Proctor. The contractor complained the soils were wet and slightly pumping. The majority of the test results were slightly dry of 'Optimum Moisture and Density was usually less than 94%. Due to observations of others (not the contractor or the testers), it is not believed the contractor was severely 'cutting corners' or the testing was compromised.
However the job compaction requirements were were not well considered. The 90% referred to the Std Proctor, ASTM D-698. The soils were 'Gravelly' but had clayey and silty fines. The fines are often sandy CL-ML (My practice has been blessed with an exceptional amount of CL-ML in the area). When the Modified Proctor Test ASTM D-1557 was performed, the Optimum Density increased 7 to 11 lbs/ft3 and the Optimum Moisture dropped 4% to 6%.
When the repairs were made, the contractor favored placing the fill slightly wet of the 'Optimum Moisture' and the compaction test results were mostly in the 94% to 96% range, without a lot of effort. The soils did not have a tendency to 'slightly pump' during compaction. The Dry Unit Weight of the fills increased about 6% to 12%. Fills are good after 2 to 4 years.
I know that at least one of my cases is a rather exceptional soil for my area, a gain of 11 pcf and drop of 6% moisture when going from a D-698 to a D-1557.
Just a suggestion.
RE: Trench Settlement
RE: Trench Settlement
RE: Trench Settlement
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The repair alternative that we are incorporating starting in 2oo7 is to reclaim the asphalt pavement section to a depth of approximately 12-inches and stabilizing the resulting mixture with approximately 6-percent (by weight) Portland Cement. After shaping and compacting the mixture, we will allow the material to cure (i.e. - hydrate) for 1- to 3-days, depending on the ambient and roadway temperatures, and then overlay with 2- to 3-inches of hotmix asphalt. Hope this helps.
RE: Trench Settlement
There's another problem with the practicality of soils testing in a trench. OSHA safety standards specify that none may enter a trench greater than 4'(I believe that is the figure). Therefore, trenches over 4' are not tested, at least by me. That leaves your 25' trench 84% untested.
I have seen this recently on a project with the same order of depth. The contractor pretty much took advantage of this, filling the trench in one big lift about halfway, then to 4', then 2' then subgrade. For some reason, with the proctor we were using, this seemed to work- this is a new company for me, and, well, it wasn't my proctor. The godsend was it was just temporary fill; it was to be reexcavated to install a local sewer line about 5' above the installed pipe. Didn't make me feel that much better about it. -not a very salient entry; sufficed to say contractors can take advantage if you aren't able to test at depth.
RE: Trench Settlement
Who paid for the inspector? What was the inspector's instruction or requirement for being on site? Can the inspector be held responsible for letting the contractor backfill improperly?
You can dig a test hole or do a test boring to check the backfill density. Or, you can dig up the trench and backfill it again with proper compaction (not likely to happen). Or, you can compaction grout the backfill material above the pipe. Or, you can keep repairing the pavement for several years. The question is, Who pays for the repair or maintenance?
RE: Trench Settlement