Soil Stiffness versus Proctor-Any correlation method?
Soil Stiffness versus Proctor-Any correlation method?
(OP)
Pals
I have designed an important slab on grade(heavy warehouse loading) and as usual have specified a compacted base to 95% proctor.
The geotech has surprised me by submitting to me values of The Elastic Modulus of Soil,Es, as measured after compaction of some layers of the base.
Some "googling" has confirmed that The Modulus method is now favored to the old density(proctor) measurements, being simple to use and producing more meaningful data. But I have ended up with no more practical info.
Any correlation between Es and Proctor?. Am I supposed to use Es to find deflection for the thickness of the base compacted? Any formula for that?Should I see texts on soil mechanics?
Thank you in advance
respects
IJR
I have designed an important slab on grade(heavy warehouse loading) and as usual have specified a compacted base to 95% proctor.
The geotech has surprised me by submitting to me values of The Elastic Modulus of Soil,Es, as measured after compaction of some layers of the base.
Some "googling" has confirmed that The Modulus method is now favored to the old density(proctor) measurements, being simple to use and producing more meaningful data. But I have ended up with no more practical info.
Any correlation between Es and Proctor?. Am I supposed to use Es to find deflection for the thickness of the base compacted? Any formula for that?Should I see texts on soil mechanics?
Thank you in advance
respects
IJR





RE: Soil Stiffness versus Proctor-Any correlation method?
The resilient modulus can be correlated to a CBR value (Mr=1500xCBR), but that give you a stability parameter not a compaction parameter. You can achieve compaction without achieving stability; however, you must achieve a certain level of compaction to achieve the DESIRED stability.
Stick with the Proctor and have the geotech give you a correlation curve for the two if he has done any field testing or lab testing. He should be able to give you a modulus vs. compaction curve that you can use for field QA work....Proctor curve still needs to be done.
If you want to use the modulus value, you can use it in an elastic layer analysis of the pavement. You can find a reasonable treatment of this analysis in Pavement Analysis and Design by Yang H. Huang or Principles of Pavement Design by Yoder and Witczak.
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
I am in the construction stage. What I usually do, is do thickness design for the slab using specs like ACI360, assume a reasonable Soil springs and ensure that I will have that spring by specifying a good base and compaction level.
Then during construction I have a geotech do lab work on the compacted base, and see if any improvement is necessary. Usually none. But then I always have had my proctor value.
respects
ijr
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
Here's the way I do this in practice.
Do a soil exploration
Get representative bulk samples
Classify the soil.
Do a Proctor (i.e., moisture-density relations)
Prepare a sample for CBR testing (targeting 95 percent relative compaction)
Obtain the 4-day soaked CBR value.
Correlate CBR to modulus of subgrade reaction.
Provide a design subgrade reaction value for the slab-on-grade design.
Provide enhancement to the design subgrade reaction value for varying thicknesses of subbase material (per ACI 360).
During construction, for cut areas, we would proofroll the native soil to make sure that the subgrade did not have pockets of disturbance. For fill areas, we'd have proper documentation that the contractor acheived the requisite compaction - but I'd also proofroll to make sure that surface areas of disturbance were removed, if present
Document the placement and compaction of the subbase stone.
Test the concrete being delivered.
Just my cookbook explanation of thoughts. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Soil Stiffness versus Proctor-Any correlation method?
Please correct me if I am wrong.
From your response, I sensed that the slab was designed utilizing a specific/preliminary Ks. As in the construction, you somehow asked your geotech to verify the onsite modulus after compacted to 95% proctor. Thus your geotech submitted the freshly measured Ks for comparasion. Is this the case?
RE: Soil Stiffness versus Proctor-Any correlation method?
As for the relationship between 95% proctor and Es, there may well be one but I would expect it to be different for different soils, in the same way as MC vs CBR and Density vs CBR will be different. In general for clay soils I tend to find 95% of the 2.5 kg rammer will give between 2 and 3% CBR on the wet side, and around 5% for 100% of the 2.5kg MDD. This is a rule of thumb which is useful for assessing results from the lab but needs to be proved for each material, its sometimes more but when assumed it always seems to turn out less [the 'butter side down rule']. Where there is an increase in the granular content, this changes significantly.
RE: Soil Stiffness versus Proctor-Any correlation method?
Not that way
I dont really assume spring stiffness. I base spring stiffness on the subbase, ie the layers underlying the compacted layers.
When a the compacted layers are brought up to densities that match 95% Proctor, that ensures that the compacted layers are very strong and you do not need to worry about. It is only when they are poorly compacted that you worry on that.
To share with you my little experience, I once placed a huge stadium stand on a thick backfill(10ft or 3metres), and specified both the material and the proctor number, and they did it exactly like that. No problems have so far been reported.
This is the practical procedure I usually use.
respects
ijr
RE: Soil Stiffness versus Proctor-Any correlation method?
Thanks for the clarification and the "share" :)
I am still puzzled why the geotech provided you the Ks rather than the proctor you would require, and was looking for. (I kind of playing doctor here :)
Are you sure the Ks was for your site, it colud belongs to others, submitted by mistake.
RE: Soil Stiffness versus Proctor-Any correlation method?
A silt with find sand (i.e., ML) may be properly compacted to 95 percent compaction, but have a CBR of 3, and corresponding k-value of 100 psi/in. A fine to medium sandy lean clay (i.e., CL) at 95 percent compaction may have a CBR value of 10 and a corresponding k-value of 200 psi/in. If placed on a 4-in thick layer of dense-graded aggregate you could use a design k-value of 110 for the former and 220 for the latter.
The use of soil modulus in industrial slab design is a separate topic. If you have an industrial slab that's 40,000 sf and has a big racking system and designed for 500 psf, you need to use the k-values to design the structural interaction between the rack, slab and subbase. Soil modulus will foretell whether you will get settlement from the deep-seated layers. Let's say you have a soil layer of soft clay that's slightly overconsolidated and not saturated (like maybe some residual soil). Your new areal load may well extend into this soft clay layer and as a result you may realize several inches of settlement at the center of the building versus the corners or edges. You need to understand soil modulus for this case.
For my practice, I'd look at the soil logs to identify whether this is a relavent concern, before I'd even worry about the k-value. After all who cares if the interaction between the rack legs, the concrete slab and the subbase are good if the entire building settles 4 inches in the middle and 1 inch at the corner?
ACI 360 has some folks that are advocating some "long-term" k-value to address what is really properly described by soil modulus and it is infuriating! We already have a method to look at soil compression - it's called soil modulus and load attenuation. Some hybrid "k" value is misdirection!
End of rant.
f-d
¡papá gordo ain't no madre flaca!
RE: Soil Stiffness versus Proctor-Any correlation method?
RE: Soil Stiffness versus Proctor-Any correlation method?
Es = Soil elastic modulus
Mr = Soil resilient modulus
= 1500*CBR
These are not the same, soil resilient modulus is a measure of the soil stiffness under rapidly applied loads (like traffic loading). Es is the value used in settlement analyses, not Mr.
ks = Modulus of subgrade reaction, for which I have seen relationships based on CBR values.
In any case, the influenced soil for a mat foundation (roughly 2*width of mat) will extend much further beneath the upper lifts of compacted fill.
Hope that helps.
RE: Soil Stiffness versus Proctor-Any correlation method?