Internal Settlement of Rock-Fill Embankments

[Mad Mike]

I'm currently running the geotechnical control on a large-scale industrial development on South Africa's east coast.

The larger development platforms are in the 10 - 20Ha range, developed over a site underlain by hard rock sandstone mantled by sandy soils. Due to the enormous volumes of hard rock, the previous geotechnical consultant incorporated a few rock-fill embankments into the development, which are now being carried over to my involvement.

The rock fill is being constructed by blending sound, blast rock sandstone of acceptable size with the sandy soils and compacting in layers of 750mm using heavy impact plant.

I have drawn the engineer's attention to the fact that rock-fill limits the founding options for the top-structures (which in this case are generally warehouse structures but often with fairly heavy gantries) and he is unfazed, being of the impression that all elements can be founded shallowly on pad footings.

Under normal circumstances I wouldn't be concerned- the fill layers are being engineered to a very high standard. However, the fill thickness in several areas approaches 30m (100ft).

I am now being asked by structural engineers to estimate the settlements beneath shallow foundations, specifically for a platform that has been standing for 1 year, with several good rains having occurred since construction. Note that there are no compressible soils beneath these enormous rock-fill embankments and my gut feel is that internal fill settlement will be negligible.

I would greatly appreciate any advice on fill shrinkage in well engineered rock-fills...I'm aware of all the rules of thumb for settlement estimation but this is not a conventional fill and I'm wary. My recommendation to the engineer / contractor was to monitor the embankment settlement at platform level, which they did, but the readings consistently came back as around 0mm and I have grave doubts as to the accuracy of the survey. I am now in a position where time is of the essence so will have only my own judgement to go by.

Sincerely,
Mike

 

[fattdad]

Virginia Tech, Center for Geotechnical Practice and Research, publication #41, "Settlement of Valley Fills," J.M. Duncan and A. Bursey, 2006. It's all about rock fills, settlements, etc. I can't provide a copy or link. You can purchase the document through the CGPR.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

You should specify the type of compactor, number of passes on each place and the layer thickness. If there is local experience for these items use that which is usually done for compacted fill to support structures. In some cases a sufficient compaction effort is done by routing all (both loaded and empty) equipment over areas not the same as previous travel path and then perhaps more than one of such travel paths on THIN LAYERS (say 1/3 meter thick). This offered only if you have no better local experience. Then the bearing pressure for buildings should be set a that used for LOOSE material.

 

[Mad Mike]

Sincere thanks f-d, I shall look to source that reference.

OG - I am confident that the compaction achieved is of the highest standard. Each lift is being hit with about 12 passes of an impact roller in conjunction with heavy vibratory pad-foot rolling. It won't be feasible to get the layer thickness down below ~500mm due to the size of the rock fragments. My main concerns regarding the fill are;

a) it is a rock-fill, albeit a very good one in my opinion (being comprised of sound rock choked with well-graded sand)

and

b) the exceptional thickness of the fill has me wondering whether there might be some residual settlement occurring even after the highest degree of engineering and a further 1 year free-standing period.

I was hoping someone might have a case history similar to mine. If it turns out there is a real risk of these rock-fill embankments settling significantly (which I don't believe there is), I'd be irresponsible to allow the developer / contractor to construct them any further. As I alluded to, rock is in excess on this site and by contrast, conventional earth-fill is limited.

Mike

 

[fattdad]

Oh, I have a case history! I have several. Power retail in West Virginia with controlled lifts of rock fill placed and compacted with a field density program. It all seemed so good on paper. Earthwork in February. Clasts too large for the prescribed lift thickness. Frozen ground and rubber tire compaction using calibrated eye.

Development complete and the loading docks began to settle, slabs cracked and the lawyers circled.

We are also in the midst of building the Coalfield expressway in Western Virginia (at the Kentucky border). 300 to 400 ft of end-dumped rock fill placed in uncontrolled 50-ft lifts. Two years of settlement, monitoring and then completion.

The CGPR paper cites some other case histories and their findings.

It helps to be practical in these regards. Citing Proctor density, lift thicknesses and other such, "Normal" stuff reads well, but it's not going down that way!

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

From Fattdad's post
Frozen ground and rubber tire compaction using calibrated eye.

That is sufficient info to explain a lot. Trouble is the below freezing temp is difficult to detect, since the water easily sits as a solid on the surfaces of particles. It may look bone dry, but it "ain't". I've seen many a failed fill done when the guys think the stuff is not frozen because it is not stuck together. One pretty sure rule is if the temp is below 20F you won't get your compaction. Even between 20 and 32 and any wind, again it won't work.

As to building on your fill, considering the care being taken, I'd expect no problems. Using average footing pressures of say 4,000 psf and re-bars in the floor slabs any settlement won't be noticed.

 

[Mad Mike]

You guys are great, thank you!

There is no such thing as freezing where I come from. I will go with my gut feel on this one and monitor the progress of the development...to cover my bases I will let the structural guys know that some slight differential settlement can still be expected across the finished platforms, make sure they articulate and reinforce to accommodate it.

Will let you know if anything goes horribly wrong!

Sincerely,
Mike

 

[BigHarvey]

Think about RIC (rapid impact compaction), it should be more efficient for thick layers. You can even think of increasing the thickness of the layers to be compacted.

 

[BigHarvey]

For monitoring I suggest magnetic settlement gages :

https://www.sisgeo.com/fr/produits/extensometres/item/tassometre-magnetique-brs.html

This will allow you to identify in which layer settlements are occurring.

 

[Okiryu]

I think that settlement may occur due to lack of interlock of rock pieces, but you are using a SW for chocking it. Based on these conditions and since your project is for a large area (no lateral confinement issues), I would not be worried about settlements of the rock-fill embankment. I am just curious what type of test are you doing for checking compaction?

 

[fattdad]

Oops! I meant to ask, "What sort of rock is it?"

In our spec for the Coalfield Expressway, we had some requirement on soundness - e.g., a minimum value as a limiting criterion.

So, what's your rock? 'Cause if it's shale you have a problem!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

f-d, totally agree!

 

[Mad Mike]

F-d / Okiryu,

Thanks guys- I don't play around with shale...the rock in question is all relatively sound, mostly quartzitic sandstone recovered from blasting. So I would expect the rock to meet any general soundness criteria.

No testing is being carried out for compaction, which is being done based on a method statement...each 750mm lift is being compacted by 15 passes of a heavy 3-sided impact roller prior to being sealed by a few passes with a 20ton pad-foot roller.

Following my previous posts, I advised the structural design team to work on a residual fill settlement of 0,25% the fill thickness...the differentials using this value translate to about 1:2000 across the filled platform, which shouldn't be a problem to design for. Despite my gut feeling I am very cautious of stating "no further settlement is expected in the fill".

I think it takes some balls to apply that statement to significant fill embankments, regardless of the materials and compaction!

Cheers,
Mike

 

[Okiryu]

Hi Mike, you are constructing warehouses (which I expect are flexible structures) so, I think that the amount of settlement (differential) that you are indicating above can be taken okay by the structures. But, who is inspecting/approving your proof rolling tests ??

 

[Mad Mike]

Okiryu,

We have an experienced engineer running the operations- the method adopted for compacting the rock-fill is modified from one of our national standards- modified in a good way to ensure a high compaction. I provide the geotechnical input and visually approve the rock-fill material. The geological conditions are such that only sand (SW/SP/SM) and rock are readily available. Natural geology was a 2 - 3m (6 - 10ft) sand mantle overlying viciously hard sandstone bedrock that couldn't be ripped with a CAT D11- topography was moderate.

The fines used to choke the rock-fill are being used across the site in conventional earth fill and lower road pavement layers- it has been no problem achieving a 95 to 98% compaction with the fines by heavy impact rolling...it is my belief that a similar density of the fines would be achieved in the rock-fill. My only hesitation from the beginning is that I would personally never recommend rock-fill beneath structures- I relayed to the engineer that settlements "could" sometimes be anomalously high, but he wasn't worried at all given the very heavy compaction plant being used.

The politics surrounding this project are unusual- the developer owns both the construction fleet and the contracting firm- so the developer IS effectively the contractor. Thus, there is a clause of liability which the developer has to manage within his own firm...the contractor is therefore exercising a great deal more care on this job than he usually would on a fly-by-night job.

I've attached a photo of the rock-fill being processed...the contractor is initially placing and spreading the sound rock, following which the fines are being added and compacted until the rock is completely choked. On the left hand side of the photo you can see the finished surface of a choked rock layer.

You can also appreciate from the photo the scale of these fill embankments.

Mike

 

[Okiryu]

Looking at your photo, appears that there is an existing warehouse. Does it have the same subgrade preparation? is it performing okay ?

 

[cvg]

No testing is being carried out for compaction, which is being done based on a method statement...each 750mm lift is being compacted by 15 passes of a heavy 3-sided impact roller prior to being sealed by a few passes with a 20ton pad-foot roller.

the method adopted for compacting the rock-fill is modified from one of our national standards- modified in a good way to ensure a high compaction

so if I understand correctly, you are using a generic method which is somehow "beefed up" for this project with absolutely no testing being done and you want to get a warm and fuzzy that it is "good enough"? Personally, I would not accept a method spec as being adequate if it was not validated on a test fill.

 

[Okiryu]

Mike, also how about groundwater? Look at the choke layer of sand and potential soil grains migration. Perhaps you can also consider smaller size rocks as the choke layer.

 

[Mad Mike]

Thanks Okiryu,

The existing warehouse is on conventional earth fill- it was constructed prior to my involvement in the job and is in my mind precariously close to the edge of the embankment (about 12ft) - it has performed well but has only been in place for about 6 months.

Regarding groundwater- since these are filled embankments, and the platform surface is almost completely sealed by hardstand, I don't believe there will be significant infiltration into the fill material. All due subsoil drainage / surface drainage measures are in place.

Regarding the rock size- what you see are the natural joint blocks...achieving a smaller size would require processing in a crusher. For me I don't believe this is necessary- although there is no scale in the photo, average rock size is around 500mm...sometimes bigger sometimes smaller. Anything larger than 750mm is supposed to be bladed off the side of the layer. Our local standard specifies 750mm as the maximum particle size in rock-fill, so we are within reasonable limits.

CVG - a settlement monitoring programme was set up on the embankment in the photo prior to my involvement. The settlement monitoring showed almost zero settlement which I refuse to believe.

Sincerely,
Mike

 

[Okiryu]

Mike, is the settlement monitoring part of your tasks? Or it is done by a third party company? Just curious to know why you are hesitant to believe the (good) results of this monitoring. Are your field observations or existing foundations performance showing the opposite?