Settlement of Sandy Engineered Fill 1

[Mad Mike]

Hi All,

I'm currently supervising a major earthworks project which involves the construction of high engineered fill embankments, to accommodate commercial warehouse structures. The platform I'm currently dealing with is 10 hectares (useable area) and the fill beneath the structure is up to 80' thickness.

The fill material being used is a granular earth-fill, made up predominantly of SW-SM-SP materials, constructed in layers 500mm thick and compacted to minimum density 95% Mod. AASHTO using heavy pad-foot rollers and a 5-sided impact roller. There is no compressible material beneath the new fill.

We're approaching completion of portions of the fill, and have undertaken precision surveying of a couple of monitoring beacons, to determine their settlement relative to a stable benchmark. The settlement being recorded is very low, for example 2mm has been recorded in the last 3 months- this for a newly constructed earthfill of 50 to 80' thickness across the survey area. We have had some big rains during this period too.

My question is- does anyone have prior experience in monitoring settlement of these sandy engineered fills? Have you recorded similarly very low values of settlement?

My original design report predicted 1/4% post-construction settlement, which would still be approximately 50mm for the fill height in question, and I'm nervous of acceding that the fill is not settling at all, given that it will affect the tolerance of floor slabs in the new commercial warehouse.

This may break down into a debate of whether engineered granular fill actually settles at all, but I'm hoping someone here has actually carried out a similar monitoring programme and obtained similar results.

Best,
Mike

 

[dik]

Looks like you have 'lucked out' on the material and it seems to be great. Gradation must be nearly bang on and material compacts in a dense fashion. With your minimal settlement for such depths of fill, I'd expect no problems with construction on it. My experience with granular fills, and I've not been involved with those sort of fill depths... maybe 1/10 of that... is that most compacted well graded granular stuff works well and I've never had an issue with it. Depending on the lower layer, creep should not be an issue.


Dik

 

[Mad Mike]

Dik- sincere thanks for your input. Perhaps I should just take this blessing as it comes, instead of being purely sceptical about it.

All the best,
Mike

 

[geotechguy1]

I wouldn't expect much post construction settlement within the fill itself for this material. Several of my previous firms have issued standard arse covering statements that post construction settlements of 0.5% of the fill height could be expected for granular materials though. You must have a pretty strong foundation material if the underlying soils aren't settling under eighty feet of fill.

I'd stay on top of the fill material and keep getting more gradation tests though - you could find that this material with about ~10% more clay or silt behaves very differently.

 

[BigH]

What would you say when you will have 1100 ft (335 m) of fill in place?

 

[geotechguy1]

The janky old thing about saying the internal settlement of fill will be 0.5% to 1.0% (i.e not the settlement of the underlying layers) originally came from measurements of settlement histories of large dams so I imagine I would say the same thing but ask for 1000x as much money in order to say it.

 

[Mad Mike]

Thanks guys- I've always hated expressing fill settlement as a percentage of the fill height, and by using the generic 1%, as the development in my area is being driven out onto steeper ground by land availability constraints, we're often getting ridiculous theoretical settlements because of the height of these fills (as BigH says)...

I feel quite disgusted thinking I might tell a Client to expect 200mm post-construction settlement, and he ends up getting less than 10mm...these "judgments" carry serious cost implications...I desperately want to improve my estimation of post-construction settlement, as it occurs within the new fill itself. Estimating settlement beneath the new fill embankment load is easy enough; I don't have a problem with these scientific calculations...it's the arse-covering 1% that I'm starting to bear a grudge against.

To be clear again, my fill is founded on benches cut into weathered bedrock, so there are no compressible soils beneath the fill.

Cheers,
Mike

 

[theCorkster]

Interesting issue that highlights several worthwhile things to ponder.

1. Glad to see you use correctly describe the potential settlement as "estimate."

2. Well-graded silty sand with about 10 percent passing #200 sieve may be the "best" soil you could build with when compacted properly.

3. Why is settlement not meeting estimate? Go back and rerun consolidation tests. How many tests were performed and what was the "precision" of results and range of predicted settlement from the tests? I presume the lab specimens for design were prepared by remolding to some percentage of the maximum laboratory density - how does that density compare to what you're achieving in the field? Likely the earthwork equipment is delivering a denser structure than the laboratory compaction is and hence less settlement under load.

 

[Mad Mike]

Attached is a photo showing the completed fill embankment; the main outer batter in the corner is approximately 130', but within the building footprint the fill thickness is maximum 80' vertically.

Just for interest sake.

Corkster- we didn't run any laboratory consolidation or compression tests- I wasn't interested in racking up large fees on small scale samples, but elected to rather monitor the settlements at the end. One of the trump cards up the Contractor's sleeve is that he's using an impact roller on 500mm soil lifts, in addition to standard vibratory rollers, so I suspect each layer is effectively being compacted twice.

Have a great week!

Mike

 

[Ron]

MM...nice job!

Your material is excellent...your results are great....we've put superflat floors on a lot worse in my area, though typically not on much fill. Commonly done on SP-SM overlying a variety of sands, sandy silts, clayey sands, etc.

My greatest concern for your site would be protection of the slopes and proximity of the buildings to the slope. Even moderate erosion could be a significant issue.

 

[Mad Mike]

Thanks very much Ron- I appreciate the encouragement.

The outer embankment is about to be topsoiled and vegetated, but what you can't see in that plate, and which I've shown in a plate attached to this message, is that the outer embankment shoulders are choked rock-fill. This was my design to make use of the enormous rock volumes on the site...the structures themselves are positioned at least 50' back from the embankment crest, on an engineered earth fill which was the topic of this thread.

So our entire fill embankment has a shoulder of rock-fill (triangular/pyramid profile), and a core of controlled earth fill on which the structures will be supported. We used the rock-fill to construct very steep outer embankments to win platform area for the yard facilities and concrete hard-standing.

I'm off to the pub now- have a great day!
Mike

 

[GeoEnvGuy]

I think that the structure looks well compacted and you can tell it was designed by an engineer because of the efficient pyramid shape at the base. Non of this landscape architect curved nonsense.