Self settlement of engineered fill
Self settlement of engineered fill
(OP)
Hi, I have a site where we have a variable thickness of fill (Class 2C), generally placed to >95% MDD (2.5kg proctor) and generally <5%. Due to some unforeseen circumstances we have had to excavate much deeper in areas to the natural formation (which is a stiff clay). For reference the fill was removed from a backfilled quarry and re-used insitu, compacted in 250mm layers with a vibratory roller (approximately 13T). The fill varies from approximately 4m thick to 16m deep. Whilst most of the self-settlement of the fill would have occurred during compaction and construction (built over approximately 6 months), I need some calculations to back up time to 95% consolidation settlement and calculations for total anticipated self settlement of the different thicknesses of fill (i.e. 4m vs 16m). To put this is context, depending on the final development plan houses may be built over different thickness of fill, so we need to ensure that differential settlement is not going to be an issue (for reinforced strip foundations) and the time it may take, so a construction plan can be developed. Any help/ideas very welcome.





RE: Self settlement of engineered fill
RE: Self settlement of engineered fill
If you want calculations, you can compare the void ratio of the pre-fill material to the post fill material and show how much more compact the material is now as compared to the natural state.
I would anticipate no change from self settlement in controlled, engineered fill....that's the primary purpose of engineered fill.
RE: Self settlement of engineered fill
RE: Self settlement of engineered fill
From my point, I'd have to say the numbers of jobs I've had where building on compacted fill was done, including over very deep marshes (after surcharging), easily runs in the thousands. Never a problem of consequence because of the fill.
RE: Self settlement of engineered fill
RE: Self settlement of engineered fill
Dr. Noorany (retired professor at SDSU) also wrote and ranted a lot about this. Southern California has many case examples of bad outcomes from non-uniform deep files and hydrocompression.
1/2% of 50ft is a big number when it comes to differential settlement.
RE: Self settlement of engineered fill
https://www.engineersireland.ie/EngineersIreland/m...
With it compacted as indicated, this would not appear to be susceptible to further desnsification even if "saturated".
RE: Self settlement of engineered fill
RE: Self settlement of engineered fill
Since moving to Texas, I have seen several cases where the settlement was much larger than that. 9 inches and counting in a 30-foot-deep fat clay backfill. 15 inches in a 35-foot-deep lean clay backfill. Both had reportedly been compacted in lifts and tested. I admit that both the quality of compaction and the testing can be questioned, but unfortunately, they were consistent with the state of the practice.
One other thing learned from those cases is that pushing Shelby tubes into a poorly-compacted fill can make it look dense. We had the 35-foot fill drilled and sampled, and most of the results would plot close to the zero air voids curve. In other words, the sampling compressed the sample to its maximum density at that water content. I do not know the details of the tubes, except that they were 3-inch diameter. A lot of sampling is done with relatively thick-walled tubes, often with blunt ends.
RE: Self settlement of engineered fill
You may not know the modulus value? I'd get that. You may be able to divine some value though? Don't know. Let's say you have a modulus value of 200 tsf within the fill itself. Let's also say the fill is 20 ft thick with a unit density of 125 psf. That'd mean the average stress change in the 20-ft thick layer is 1,250 psf (0.62 tsf). If a 20-ft thick layer with a modulus of 200 tsf receives 0.62 tsf if stress change would return 0.74 inches of settlement (i.e., (20*0.62/200)*12)
That's nothing! Also, as the OP points out, it'd all happen during fill placement.
That's not the end of the story though. . .
Modulus-based settlements are also influenced by time. The parameter, "C-sub-t" is used to anticipate additional, "Modulus-based" settlement that may develop after the immediate settlement is complete. For a forecast of 1 year beyond initial placement the value of Ct is 1.2 (i.e., 20 percent of originally-calculated settlement will occur during the year that follows. For a period of 30 years, that value is 1.5. That means for the example above, you'd forecast 0.15 additional inches of settlement in the year that follows and 0.37 inches of additional settlement in the 30 years that follows.
Class 2C is a cohesive granular fill. It certainly seems reasonable that settlement could, "Lag" from the more typical response of sand. I doubt there'd be a performance issue though.
I'd agree the greater concern being whether the seat of settlement extends into the native soil.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Self settlement of engineered fill
Dik
RE: Self settlement of engineered fill
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