Settlement of the Embankment (not underlieing it)

[garrettk]

I recall seeing in a publication (maybe NAVFAC) that provided methods of estimating settlement of embankment soils - but alas I can't find the reference now that I need it.

We will be placing approximately 12 feet of engineered fill on top of 50+ blow material. I'm not concerned about settlement of the foundation soils, but would like to estimate any settlement of the engineered fill. It will be monitored during placement, with 95% compaction (standard) or greater.

Is there an easy estimating method, or should I complete a more rigorous analysis?

 

[BigH]

Several points come to mind, if I may.
Firstly, what is the purpose of the fill? To support a roadway? To support a structure?
Secondly, what material will you use to construct the fill? A compacted clayey material? A well graded sand and gravel? Crushed stone aggregate?
Thirdly, how will you place and compact the fill? In thin loose lifts compacted with a heavy (say 10 to 15 ton vibratory roller)? Or, will you just end dump the material?
Fourthly, is this related to "payment" to a contractor? - i.e., he would have to "make up" the settlement with fill as he progresses in construction.
The first three points will determine how much settlement you get of the fill. If you use an engineered fill and compact it well and in thin lifts, your settlement should be small (say using well graded sand compacted to 95%MDD D1557). Crushed stone would be much less.
For instance, we compacted a flyash (85%) and 15% fine sand to 97% ASTM D1557 MDD and had N values of 40 of the placed fill. Would you worry much then about settlement - even if for a moderately loaded foundation? Doubtful. Highway abutments in Ontario and other places are many times of the floating variety placed as footings high in the fill but of a crushed stone base course gradation well compacted - in such cases allowable bearing pressures of 300 to 350 kPa or more can be used. Also, don't forget, with granular fills, settlements of the lower layers will be ongoing due to the loading imposed by upper layers.
If, on the other hand, you are compacting a wet clayey fill, you can get fairly large settlements. If I remember right, there is something in a study on the compaction characteristics of Bearpaw Shale and it is something like 2%*H for embankment settlement. (It was a Bureau of Reclamation report from the late 1970s.)
Normally, embankment fills of only 3 m height are not judged critical with respect to settlements if properly constructed. And, if you are using granular fills with proper compaction, you are approaching 40 blow material itself.
The fourth point is covered by paying according to the final in-place volume. For a hard base as you have, this would be nearly insiginficant. If you had a soft compressible base, i.e., the foundation was settling, you could get several inches of settlement within a year or so of the base and this might be significant - especially if the contractor (or client) has to make up the difference by an asphalt leveling course.

 

[jdonville]

All,

I am going to resurrect this thread as I have a current project where the current design is for a variable height MSE wall founded on a variable thickness of new embankment fill up to 40 ft high. The MSE wall height will be the difference of the fill height below the wall and the approximately 43 ft of the overall embankment height.

The settlement of the large amount of fill due to self-weight and the settlement of the new fill under the load of a relatively high MSE wall both have me concerned.

The fill is anticipated to be on average A-6 (silty clay) soil. (Unfortunately, there is no obvious local borrow source, so nobody knows for sure what the embankment fill will be.) Based on conservative estimates, we have modelled the soil strength for other fill areas on this project as c=1000 psf and phi'= 28 degrees for undrained and drain analyses respectively. Atterberg limits for near-surface soils in the area are typically LL 25 to 50%, PL 15 to 20%, so it is a good bet that optimum moisture contents will be in the mid to high teens near the plastic limit.

Any references to papers, texts, etc. discussing the settlement behavior of new cohesive embankment fill would be much appreciated.

Thanks in advance for your responses.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.