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Settlement of Sandy Engineered Fill

Settlement of Sandy Engineered Fill

Settlement of Sandy Engineered Fill

(OP)
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

RE: Settlement of Sandy Engineered Fill

Quote:

There is no compressible material beneath the new fill.

Quote:

My original design report predicted 1/4% post-construction settlement,..

How was the 1/4% calculated, what was the base? What is voids ratio of the fill?

RE: Settlement of Sandy Engineered Fill

Provide typical gradation curves. My bet it is relatively uniform and has a low range between loose and dense densities. Then what you measure makes sense. Sounds like a great place to build on.

RE: Settlement of Sandy Engineered Fill

(OP)
OG - typical grading curve attached - agree the material is good for engineering; I'm just surprised that the monitoring results are that good!!!

Retired13 - the original 1/4% was just an estimate taking into account the materials, engineering and construction duration; it was always qualified with the intent of monitoring. I'm now in a position where I'm inclined to accede that the fill just isn't settling. We've not measured void ratios, just tested the field compactions and monitored the material compliance.

I didn't mention previously, but I also surveyed another high fill embankment in the same industrial development and in that instance, total post-construction settlement of a 100' choked rock-fill embankment was approximately 10mm over a 2 year period. It becomes daunting when the Client asks "so we will be okay using super-flat floors?"...my theory says it's a bad idea, since these platforms straddle cut rock and high engineered fills, but the data suggests no problemo!!!

Best,
Mike

RE: Settlement of Sandy Engineered Fill

Quote (Mad Mike (Geotechnical))


....and the fill beneath the structure is up to 80' thickness

....There is no compressible material beneath the new fill.

2mm has been recorded in the last 3 months- this for a newly constructed earthfill of 50 to 80'



The picture depicts that ,apparently The fill height varies 0 to 80' . That is , the fill area is a valley and the soil bearing pressure under the fill in the range of 0 to 450 kN/ m2. Without knowing the soil properties under the fill, it is hard to estimate the amount of long term settlement what about the GWL? . If the warehouse single storey , flexible structure , there sholud not be any problem.

RE: Settlement of Sandy Engineered Fill

(OP)
This fill is in most cases being constructed on a side-slope of a natural spur. At a single location, it crosses a valley, but there is no perennial water- in all cases the GWL (only present during heavy rains) is perched in the natural soils beneath the fill, and which are themselves sandy and relatively free-draining.

The final platform will be bounded by enormous hard rock cuttings on the upslope side and all final surfaces will be pavement...we will therefore be cutting off any perched groundwater flows.

RE: Settlement of Sandy Engineered Fill

The gradation seems sand heavy. Quite dense by itself, isn't it?

RE: Settlement of Sandy Engineered Fill

My opinion is the spec is a good one and this isn't as uniform a sand as I'd say "No problem". Being well graded, when dense has great shear strength. Thus, likely the high compaction density, results in the low compessibility found.

RE: Settlement of Sandy Engineered Fill

(OP)
Thanks Retired and OG - yes, it is a dense product. I always expected very little settlement out of the new fill; these results however offer me a new perspective on whether a carefully engineered sandy fill will actually settle within itself.

I've reviewed the available studies that detail long-term "hydroconsolidation" with moisture ingress into these sandy fills, but on a well managed cut-fill platform, I just don't see this being a major factor.

I don't suppose many have monitored the settlement of a high sandy fill on non-compressible foundation, to which I could compare my current results, but if any have I would be most interested to hear...

All the best,
Mike

RE: Settlement of Sandy Engineered Fill

I agree. with.
I don't suppose many have monitored the settlement of a high sandy fill on non-compressible foundation, to which I could compare my current results, but if any have I would be most interested to hear...


Guess my experience would say of the hundreds of sites I have used compacted fill for, just the fact it is denser than natural sites of the same material, I've given it no thought. Also, no later problems related to possible settlement. Your jobs will have produced building and other sites better than most natural sites, as my opinion.

RE: Settlement of Sandy Engineered Fill

(OP)
Thanks OG- I'm of the same opinion, though I'm wary too of the ever more stringent tolerances on these industrial slabs and racking systems.

I've always felt that super-flat floors across a cut-fill platform are a bad idea, because settlement would be inevitable.

I'm now very much on the cusp...

Best,
Mike

RE: Settlement of Sandy Engineered Fill

I think your new perspective is quite a good lesson to many practicing engineers. Hope to see more real project report on soil matters.

RE: Settlement of Sandy Engineered Fill

regarding: "hydroconsolidation" with moisture ingress

unless grading of the surface allows the stormwater to percolate instead of run off, I would not see this being a big concern. it would take a prolonged deluge to saturate any more than the outer few feet of the embankment. the moisture content in the center of this fill, 40 feet deep does not feel any effects from the rain

RE: Settlement of Sandy Engineered Fill

Quote (Mad Mike)

I don't suppose many have monitored the settlement of a high sandy fill on non-compressible foundation

I can provide you more references on high rock fill embankment settlements more so than high sandy fill settlements.

RE: Settlement of Sandy Engineered Fill

Sandy soils undergo instantaneous settlement. If placed in layers and properly compacted you will have no issues. There should be some moisture too, hopefully the engineered fill looked at optimum moisture content.

RE: Settlement of Sandy Engineered Fill

(OP)
TheRick109- science says that I agree with you 100%, however the numerous cracked structures I have previously inspected on engineered sandy fills would contradict this. As these fills get thicker/higher, settlement can be a problem even with well-compacted sandy fill on stable ground.

In my case, both moisture and density have been well controlled.

Best,
Mike

RE: Settlement of Sandy Engineered Fill

Mad Mike - am sure you have thought of this - could creep be an issue over the longer term?

RE: Settlement of Sandy Engineered Fill

An observation by OG. When it comes to settlement and cracked structures and compacted fill, my experience with the many structures built on compacted fill where myself or an associate gave the recommendation, never has there been a complaint or notice of settlement. That's roughly forty plus years of experience.

RE: Settlement of Sandy Engineered Fill

(OP)
BigH- thanks, I can't see there being any mechanism for creep in the structural footprint.

OG- I really appreciate that observation of yours. I've always been in agreement, but I'm now dealing with fill embankments higher than any I've dealt with before- they're damn daunting!!!

Best,
Mike

RE: Settlement of Sandy Engineered Fill

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

RE: Settlement of Sandy Engineered Fill

(OP)
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

RE: Settlement of Sandy Engineered Fill

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.

RE: Settlement of Sandy Engineered Fill

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

RE: Settlement of Sandy Engineered Fill

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.

RE: Settlement of Sandy Engineered Fill

(OP)
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

RE: Settlement of Sandy Engineered Fill


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.






RE: Settlement of Sandy Engineered Fill

(OP)
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

RE: Settlement of Sandy Engineered Fill

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.

RE: Settlement of Sandy Engineered Fill

(OP)
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

RE: Settlement of Sandy Engineered Fill

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.

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