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

 

[r13]

There is no compressible material beneath the new fill.

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?

 

[oldestguy]

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.

 

[Mad Mike]

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

 

[Mad Mike]

Attached is a typical construction photo- the fill material is generally very uniform.

 

[HTURKAK]

....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.

 

[Mad Mike]

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.

 

[r13]

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

 

[oldestguy]

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.

 

[Mad Mike]

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

 

[oldestguy]

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.

 

[Mad Mike]

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

 

[r13]

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

 

[cvg]

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

 

[GeoEnvGuy]

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.

 

[Guest090822]

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.

 

[Mad Mike]

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

 

[BigH]

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

 

[oldestguy]

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.

 

[Mad Mike]

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