Backfill compaction

[karma134]

Dear fellow Engineers

Our designer has proposed a backfill on top on concrete and confined by concrete on 3 sides by concrete. The designer has proposed to use a Cohesion less/ free draining material for this purpose and relative density of 80%. The backfill will be carrying the weight of the whole RCC intake structure along with intake gates and trash rack.
Now my query lies :
1. in the settlement of the backfill and how can I determine its expected settlement before the job.Or more specifically should I be worried about the settlement in this case.
2. Also since the backfill is exposed on the abutment on one side(as said earlier the backfill is confined on 3 sides by concrete)should I be worried about the ingress of water into the backfill and fear the possibility of bulking of the cohesion less backfill.However there is drainage pipe provided at the bottom.
3. Also am I wrong to assume that the RD of 80 % which is minimum which could be achieved without compaction and by the virtue of its own weight. I am asking this because the designers has asked to compact in layers of not more 300 mm. Since a CL material needs a vibration for compaction and I am reluctant to use a roller in a concrete structure. Will the manual hand compactors work in this case?

PS: I have uploaded a a rough drawing of the structure for your reference.

KARMA

 

[karma134]

this is the link to the drawing:

http://files.engineering.com/getfil...-409e-8872-683042af42e4&file=ROUGH_SKETCH.png

KARMA

 

[Ron]

Settlement of the fill will be related to two things....first will be additional compaction above the 80% relative density from the initial compaction. 80% RD is not that great, so additional compaction will likely occur from the weight of the structure above, vibration, change in moisture content, etc. The second will be elastic settlement from the load on the granular material.

 

[cvg]

sounds like a poor idea, use concrete

 

[oldestguy]

What if there is a break in the piping or a joint leak? Will there be any possibility of water flowing in the fill? Erosion of the backfill may be serious then.

Rather than worrying about percent compaction of a "free draining" fill, go for a pipe bedding material that you probably already have specs for and one that won't erode with some water flow (probably some form of open graded coarse material). Single sized coarse material has very little potential for settlement unless disturbed and then not much. This then can be worked around under the pipe with little risk of subsequent movement.

 

[karma134]

1. I agree with @cvg but my hands are tied.I am just guessing that it was just to save some money.
2. @oldest thank you for your view. We are using geotextile drainage pipes for this purpose.Since the construction has already started.
3. @Ron I am also concerned about the consolidation by the weight of the structure.I also believe that for CL material the water content have minor implications to its dry density upto certain moisture content maybe upto 6%.

4. The only thing which I can do right now at the site is checking the degree of compaction of each layer.And I have proposed for a Sand Replacement Method rather than core method since extracting core from a CL fill would be impractical.

5. I would be pleased to listen to other precautions to take for this small project.


THANK YOU ALL

KARMA

 

[Ron]

Why not use a material that is better than CL? If you can't get better material, then mix cement or fly ash with the CL material to better stabilized it.

 

[BigH]

Why does a CL material - a low plasticity clay above the A-line need vibration for compaction? That is what CL stands for - not CohesionLess.

Your sketch doesn't include any dimensions and you have not indicated if the abutment is rock (which I might presume); nor do you indicate what this "fill" box is to be used for - if anything. As such, it is difficult to give specific recommendations, in my view, although you have hinted that there are some pipes involved.

However, in viewing the sketch, I would be more concerned about the hard point between the Weir RCC and the "fill" as you will be placing rigid materials above. This is not typically an acceptable design siutation. You have a non-yielding RCC and then the "yielding" fill of the box on which the rigid RCC is to be placed. I am assuming that the box is being filled with cohesionless material as a cost saving measure . . . but if this is the case it is a matter of "shillling wise/pound foolish." Since you have so much RCC, why not just fill the box with it? or use, as Ron suggested, a stabilized material such as cement sand mix with 4 to 5% cement. You pipes could still be placed inside and not be worried about erosion or materials washing into the pipes.

Since it appears that the material is already being placed, the above suggestions by others and myself are sort of moot. I would consider, though, giving thought to stabilizing the fill's mass using a grout that will make the fill more rigid before you start putting on the RCC above. The hard point, in my view, make be asking for cracking and this isn't a good idea with a dam - again, in my view.

 

[JedClampett]

If they're bound and determined to fill the cavity, maybe you can talk them into supporting the building to the lower level using columns, and spanning the building to these columns. Or possibly you can span across that backfill zone, although it's hard to know what the dimesnions are. As a matter of fact, why backfill at all? It's just adding weight to be supported.
If you can't do that and you can't fill with concrete or slurry, you better start getting your resume in order. This has forensic investigation written all over it.

 

[fattdad]

80 percent relative density is akin to 95 percent relative compaction, so I'd think that's reasonable for compaction. Problem is nobody uses relative density for lean clay soil. Relative density is used for open-graded materials - soils/aggregates that don't produce moisture-density relations.

Whether it's 300 mm lifts or 50 mm lifts, it's up to the contractor to select means and methods to comply with the specification requirements. Thinner lifts and hand equipment may be appropriate. It's not appropriate to say the contractor is obligated to 300 mm lifts and the jumpin' jack won't get it done. Use thinner lifts and a jumpin' jack will work just fine.

Flowable fill is a good product. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[karma134]

1. @Ron I really like the idea of blending the cement. I will follow that up as an alternate proposal. So question is can I use OPC43 and what should be its blend(Cement:infill material).Are there any guidelines?
2. @BigH: a) You were right to assume that the bottom of the fill material is RCC and also that the abutment is rock surface.
b) I apologize for the confused term I used here. I was wrong to use CL here. What the designers stipulated were a free draining material with the following gradation limits:
Sieve size(mm) % passing
0.075 <5
4.75 50-80
100 80-100
c) Dimensions: L= 25 mtrs: Ht: 7 mtrs : Width: 8 mtrs.
d) Yes I had also relayed the same suggestion to go with RCC instead of backfill to my seniors but you know how things go un-listened.I even suggested a low grade concrete a M15/C15 grade PCC.
e) What would be the implication of confining a yeilding material in a non yeilding material? Because I think as long as you compact properly and the area is confined within a solid structure the development of settlement due to consolodition will be negligible.
e)I like your suggestion of grouting the fill material.


So in conclusion I got 2 winning suggestions: 1. from @Ron 2. @Bigh
Thank You all for the advice.I hope my concern is really valid.

KARMA

 

[cvg]

so it is not clay, but sand instead. Moisture control during compaction is critical. 1 or 2 sacks of cement per cubic yard could be used to stabilize it and make it flowable. No compaction would be necessary. Google "Controlled Low Strength Material (CLSM)" to find typical mix design for flowable fill.

http://www.concreteconstruction.net/images/Controlled Low-Strength Material_tcm45-342124.pdf

 

[Hoaokapohaku]

Karma...Are you sure those are the correct gradation specs? Besides being wide enough to drive a very large truck through, those look more like specs for a dense-graded granular material typical of structural backfill or subbase specs, not for a free draining material. I can't speak for Ron or BigH, but I suspect their "winning suggestions" might have been different if you had given the gradation specs before they responded.

I'll await your response before I say more, except that if the spec gradation range you gave is correct, the potential top size aggregate and lift thickness makes compaction with small equipment virtually impossible. If the intent is truly a free draining material, and compaction is with small equipment, the spec should be for something no larger than an ASTM #4 or #5. ASTM #67 would probably be ideal. Anything down to a #8 would work fine.

 

[dirtsqueezer]

Grout it, man. Sand goes in at eighty percent. That's before you even compact it. Also, your optimum compaction percentage should be double your current moisture content. If there's a slab holding your trash rack, it will crack with settlement using your current eighty percent. With your confined space and especially filling on concrete, your numbers should jump with just a spray of the hose. Either way, what's a few more hours labor compared to a job you can be confident of? Best of luck.

 

[fattdad]

Sand does not go in at 80 percent relative density. It may go in at 80 percent relative compaction.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

I doubt it would go in at either 80% of RD or 80% of SPD without adding water and compaction.

 

[dirtsqueezer]

I took the language to mean that he was measuring relative compaction as they filled. How on earth would you measure relative density?

 

[aeoliantexan]

80% relative density is hard to achieve; you will need lots of water and lots of energy.

Once watered and compacted to spec, more water should have little effect.

Once compacted, the material should have a high modulus, but the contrast with concrete leads to stresses that may crack the RCC.

Adding cement and compacting or using flowable fill are good ideas, but the fill will still be more compressible than the concrete. Hard to know whether it will perform adequately without a pretty sophisticated analysis.

Relative density of sand can be measured with sand cone and vibrating table, but your material will be hard to test if it really contains the coarse material the spec allows.

I would consider putting a few drilled piers through the backfill to support the RCC. Spacing depends on the thickness of the RCC.

 

[fattdad]

considering that the notion of zero percent relative compaction doesn't exist there is some practical value of minimum relative compaction. What is the minimum relative compaction at a relative density of zero percent? It's about 80 percent. In other words when you consider soil at it's maximum void ratio (minimum density) you are usually close to 80 percent relative compaction.

I don't make this stuff up. I actually had to research this for a nuclear project I worked on several decades ago. I got the file out for this thread.

I'm just thinking a spec for 80 percent relative density would be very similar to a spec for 95 percent relative compaction (there is a basis for this claim). As such, if the soil can produce a proctor curve, I'd convert the spec to relative compaction. I really was sure of my thought when the OP was talking about CL!!

f-d

¡papá gordo ain’t no madre flaca!

 

[dirtsqueezer]

I was just assuming you were talking 'maximum dry density', which is a standard or modified proctor, measurable with a troxler, ect. Got a little confused there.
How do you measure the density you're talking about in the field without a drill rig or something similiar? I'm used to working with gt engineers writing out compaction reports when we're placing fill. Isn't that other stuff all pre-construction?
Kind of off topic. Just curious.