relative density to compaction of granular material in embankment 2

[ac4u2nv]

Dear all
Im looking at an embankment which has suffered from susidence.
Possible options are:
1) burrowing by animals has caused voids
2) embankment built in 1940s possibly not compacted properly

CPt results indicate within the fill is silty snds (5/6 robertson material descirptions)
im getting values at low as 25-50% relative density.
From what i know about comapction, OMC and max dry density need to be at 95% with 5% air voids.
Is there anyway to compare relative density to comapction?
Do these realtively low RD values (they go utp 90-100% in surrounding layers) indicate improper compaction?

Please advise

thank you
Ash

 

[fattdad]

In the world of modified proctor, it goes by fives. 100 percent relative compaction is (close) 100 percent relative density. For every 5 points you reduce relative compaction, you reduce relative density by 25 percent. That means, zero percent relative density equates to 80 percent relative compaction. And, that makes sense.

Regarding air voids appropriate for compaction, it has little bearing on stability after compaction. Clearly an embankment dam is constructed with one water content, but after the dam is operational the water content increases. If the density was correct at placement, there'd be no reason that changing water content would affect void ratio or corresponding strength.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

I find it hard to believe that settlement manifest 70 years after construction has anything to do with the embankment material itself- what is underneath this embankment?

How many animals have you got burrowed into this bank that you are considering it to be a factor? Are we talking crabs? Coyotes? Bears?

I'm guessing it's an embankment dam- I would imagine any settlement late into the life-span is related to internal piping or some other erosion-type process- have you got seepage coming through the embankment? Check the seepage for sediment; I would expect settlement from piping to be relatively localized- perhaps focused around an outlet pipe, internal drain or similar ancillary component...

If you have any photographs or sketches that demonstrate the settlement problem it would be most helpful- at least share what the nature of this embankment is (structural platform / earth dam?).

You may want to check the global stability of that embankment too- sometimes big failures are preceded by what can be mistaken for settlement...I've been dealing with some big failures on old earth dams lately- the dynamics change over time so these failures often occur late in the design life...

All the best,
Mike

 

[EireChch]

What is the embankment retaining / supporting, rail or tailings etc? How tall is the embankment, what is its geometry, when has the subsidence occurred? Is this just in the last year or after a big rainfall event?

Provide us with your qc profile through the embankment.

I agree with Mike, any settlement due to poor compaction would have occurred within a few years after it was constructed.

 

[ac4u2nv]

Thanks so much everyone
Embnkment is built either side of a rail underpass supporting a highways bypass above.
it is about 3.3m high and made from 6n fill (although no specs to prove this).
beneath it is weathered Weld Clays.

Burrowing could be rabbits. numerous holes in embankmenet wings. Everything was fairly dry. didnt see any seepage from the embankmenet.
CPTs indicate Rel density to drop to about 30% 2-3m bgl. phi' values also droop from 50 to about 35.

 

[Mad Mike]

I'd imagine that if animal burrows were behind it all you would have intersected voids with your probes.

I'm still guessing you have a long-term settlement beneath the fill foundation, perhaps softening from water ingress. Whatever the case, I would want to cut a trial slot into that embankment to get a better view of the fill materials, core a borehole at the base of the embankment to establish the deep ground conditions beneath it, and then check a theoretical settlement prior to designing any remedial works.

Unfortunately I'm not familiar with your Weld Clays.

Good luck!
Mike

 

[GeoEnvGuy]

Check the state parameter using plewes interpretation and Robertson qtncs. In contractive materials you may find a potential for liquefaction and settlement. As for the settlement of compacted fill you should not have that much long term settlement. Poorly compacted maybe, but likely the settlement would be due to the clay foundation or any softer deposits below the weathered crust of your clays. The other probable explanation maybe erosion of surficial deposits which did not have erosion protection. Also internal erosion or collapsed conduits maybe played a part. Also is there a potential for frost heave of the concrete?

 

[oldestguy]

My guess is that this embankment has settled as a result of the vibration from the railroad. There likely has been a "spreading" of the fill sideways. Don't depend on blow count of sampler in some sands. The range between low and high density in more uniform sands is low and the resistance to sampling is lighter, in spite of the relative density.

 

[aeoliantexan]

In my experience, settlement occurring long after construction usually involves water. I have seen a thick fill that had been supporting buildings for 80 years start settling due to a broken roof drain pipe.

You might want to check for a leaking water main or poor drainage that causes localized infiltration. The "rabbit hole" looks as if water has run out of it at some time.

What are the dimensions of the subsidence? Where is it located?

 

[ac4u2nv]

subsidence is manfiested in cracks on the road surface tarmac, the vrs foudnations becoming exposed and the bridge expansion joint becomining raised slightly affecting ride qualtiy.
on the site walkoker i didnt see any evidence of water seepage. however a drainage survey did indicate one of the drains was deformed

 

[EireChch]

Again its not possible for a 3.3m high embankment to settle 200mm due to poor compaction. It may definitely contribute at max 30mm of self settlement but that would be finished during construction and not long after.

OG raises a good point, what is the distance between the top of tunnel to underside of embankment? Is the settlement localised around the rail underpass and not anywhere else? If so then vibrations are obviously one root cause. If not then rail vibrations are likely not the sole suspect.

What is the tip resistance in your 6N fill and weld clay? Post a pic of your qc profile. Where is groundwater too.

As for animal burrows, I dont think they are the issue but if you need to disprove, you can check these by doing a MASW survey. Would also give you an idea of the various of Vs through the embankment.

 

[ac4u2nv]

i didnt see signs of settlement elsehwere, eg cracks or slips at base of embankment.
no groundwater detected.

 

[Ron]

Agree with OG. Long term vibration

 

[r13]

Just curious, - shouldn't vibration condense the soil, thus less voids, but higher relative density?

 

[Ron]

Not if the vibration leads to raveling or lateral bulging.

 

[r13]

Ron,

Thanks. Would that be like forming of quick sand condition?

 

[Ron]

@retired13....no. It would be much more subtle than a quick condition. Consider that the original slope might be at a 45 degree angle. Instead of maintaining that slope over the years, the vibration can cause raveling of the surface soils away from root mats and can then be susceptible to erosion from runoff or wind. Another effect of the vibration would be to initially densify the soil vertically which can then cause the lateral earth pressure to increase. Since the soil has no lateral restraint other than vegetation ground cover, it can slowly "bulge" outward which will then decrease the relative density and allow vertical settlement.

I would suggest there are multiple actions going on here with regard to load paths, all exacerbated by the vibratory action of the overlying traffic.