CBR PROCTOR and others

[Paologh]

I am studing the soil compaction procedures and some questions came me in mind.

First of all say CBR 10 is equal to say PROCTOR modifyed 10?

is there a connection between the elastic modulus E and CBR?

thanks to all who might be so kind to answer to my doubts

 

[fattdad]

Q1: No
Q2: If you find one, I wouldn't trust it.

f-d

¡papá gordo ain’t no madre flaca!

 

[Paologh]

Thank you f-d.

I have one site where, the company that is working told me, "we can compact the backfill till to reach Proctor modifyed value of 90". What is 90? The determination of the proctor index is same we do to have the CBR?
paolo

 

[fattdad]

Typically, the CBR test is performed at 95 percent relative compaction (we use Standard Proctor) as that typically corresponds to the specified level of compaction. If the field density testing confirms this level of compaction (and the soil type is consistent), then the CBR in the field should be about as shown in the laboratory.

If a contractor indicated that they can compact to acheive a "Proctor modified value of 90%", then the contractor is indicating that the dry density of the compacted soil in the field will be more than the maximum dry density as determined in the laboratory using ASTM D-1557 (Modified Proctor). This is very likely, but (not being a trusting soul) I'd confirm with field density testing.

What type of project is this? How does the CBR value relate to the project outcome (i.e., is this some DOT road project or is this some industrial pavement application)? Then again, you may be using CBR to correlate to the modulus of subgrade reaction (i.e., not soil modulus) for industrial slab on grade subgrade confirmation.

f-d


¡papá gordo ain’t no madre flaca!

 

[Paologh]

Thank you f-d

My project consists of foundations for telscopes. For these foundations (massive and big concrete foundations) we need a very hight stiffness. The horizontal stiffness is given by the concrete itself and by the soil that is all arround the foundation (coppacted at proctor 90%). In our Finite Element Programme we must enter a stiffness modulus for the soil that is all around the foundation.

Is there a way to measure the elastic modulus on site?

Thanks
Paolo

 

[civilperson]

Yes, measure the deflection of a circular plate with a given load.

 

[fattdad]

The dilatometer is a great tool for the direct measurement of soil modulus. Where are you located (i.e., are you in the U.S.?). If so, you would likely be able to find an in-situ testing firm to do a few dilatometer tests - they're not too expensive.

Regarding the metal plate method, that's more suited for the modulus of subgrade reaction, which is different from soil modulus. I'd think for a finite element evaluation, you'd more likely need the soil modulus, then again I didn't do too well in that class, so many years ago - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Hello Paolo.

I have to wonder whether you can really achieve "very high stiffness" with 90 percent of Modified Proctor. Maybe you need 95% of Modified or possibly more? This is NOT a routine fill for a highway embankment, and extra compaction cost is probably small "in the big picture," even if you need thin lifts (15-20 cm) and large rollers.

Regards,
DRG

 

[Ron]

Paolo....the elastic modulus of the soil would be better determined by cyclic triaxial testing. The plate load test as noted by civilperson will work for the field value, as long at you use cyclic loading.

I would not specify less than 95% of the modified proctor for compaction. That's only going to help the top of the soil though.

As for the dilatometer...it's great for subsurface, but for near surface with no overburden, I wouldn't rely on it.

 

[iandig]

Going back to the original post, and taking comments direct from DRMB IAN 73/06, there is the following equation given for assessing the sub-grade stiffness modulus:

Where the DPT is utilised the results will be used to
determine the surface stiffness modulus using the following equation:
E = [2 * (1 – ?²) * R * P]
D
where: E = Surface Modulus (MPa)
? = Poisson’s Ratio (default = 0.35)
R = Plate Radius (= 150mm)
P = Contact Pressure (kPa)
D = Deflection (microns)

The equivalent CBR can then be determined from the
following equation [as defined within IAN 73/06]

E = 17.6 (CBR)^0.64 MPa

The above equation can be re-written as:

CBR = 0.0113 x E^1.5625
Not sure of this helps, and as has been discussed above, not sure how much credence I would give to the relationship HOWEVER we are now assessing the sub-grade stiffness using the Dynamic plate/Falling Weight Deflectometer and as such less emphasis is placed on the CBR than ever before.

 

[Paologh]

Thank you so much to all of you. Now ideas are more clear.

The backfilling is made with broken rock compacted to modified proctor 90%. The thikness of the backfilling is around 3 meters or more and for some foundations it has been already completed.

I need to know the E modulus of the soil compacted over all the thickness. As far I have understood the dilatometer and DMT in-situ test can help me to check the grade of stiffness over all the thickness of backfill (for the foundations already completed).

Another solution could be, the foundations not completed, make the CBR test during the compaction of the layers of backfill and use it as ianding says.

Am I wrong?
Paolo

 

[fattdad]

For foundation design, the elastic modulus to depth is much more critical then what is being called "surface modulus", which I would guess is the same thing as the modulus of subgrade reaction. Structural engineers use the modulus of subgrade reaction to model the reaction of a concrete slab to a given point load. If you are designing a foundation system, elastic theory will show the attenuation of stresses with depth. You may have a soil interval between the depths of 5 and 10 ft, which would have little influence on slab-on-grade design, but a much larger influence on foundation design.

Bear in mind that if the subgrade soils are saturated and prone to compression, you should really assessing the one-dimensional consolidation coefficients, rather than the elastic behavoir of the soil. If the soil is not saturated and not prone to consolidation, then I'd model the subsurface using soil modulus as that is more typical for finite element models.

Please recognize I'm not challenging the advice given on determining the modulus of subgrade reaction, I'm just not sure that it's relavent to your stated problem.

f-d

¡papá gordo ain’t no madre flaca!

 

[psock]

Sort of a related question. I have a project where there is no specification on the under-slab compacted stone (ABC) density. The geotech report says "design slabs using MSR of 150 pci". Can this be related to a specific percent compaction of crushed granular material, or should I just use 95-100% to be safe?

 

[Ron]

The MSR is not directly related to compaction, but is affected by it. The 150 pci requirement is not that difficult to achieve for most any granular material. I would specify 95 percent compaction (as compared to the Modified Proctor) for any subgrade material, at least to achieve uniformity.