Total settlement, 90% mdd

[dirtsqueezer]

I had a question. I've seen specs that call out 90% compaction below a certain depth, say 2', for a trench or roadway(95% on top). I am under the impression that this assumes the sphere of influence does not go below this depth, or somesuch. Isn't it the case, wouldn't that 90% compacted material settle over time(especially if your trench is deep), making your 10' wide excavation rip up the pavement someday in the future? What is the purpose for compacting less than 95%? What's the deal?

Thanks.

 

[BigH]

If I remember correctly it has to do with arching and loading on the pipe.

 

[civilperson]

Natural soils are at compaction ranging from high 70's to mid 80's and rarely settle when used as a road subgrade. The percentage used as backfill requirement reflects different government agencies experience as to what results lead to successful long term construction.

 

[dicksewerrat]

The trench will settle. the 90% and 95% will allow the trench to settle. However, I have found that having a testing lab onsite daily to do compaction testing increases the compaction by 5%-15%. My rule of thumb was 90% to 1 foot above pipe and 95%+ to 5 feet below grade, then 100% to surface. Costs a little more at first but the crews will learn quickly. Especialy if you make them scrape off a few feet of trench and redo it. Roadway replacement dropped to less than 10% after 10 years. Ussually you can see the trench reflected in the street surface in 3 years if not very well compacted. After all most trenches are installed for some gov't agency and it is tax money that pays the freight.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[dirtsqueezer]

Wow- 100%. I've tested that before but only for runway strips.

You've jogged my memory on this actually. I tested fill that was placed, and then had a chance to test a year later after seasonal rains. The compaction had actually increased to in some areas over 100%. I understand the difference between laboratory compaction and consolidation of sands, ect. in the field, but that was the first time I'd ever seen it. Makes me wonder about actual settlement in the field, ergo, my question. Anyway, still bewildered. 90%? You're just asking for it, in my opinion.