Density-Strength Relationship for Various Soils w.r.t. Compaction Standard 3

[ScarpShooter]

I'm looking for some data pertaining to the influence of the compaction standard on peak shear strength for a given soil. The situation is the construction of embankments/pads and choosing a compaction standard for the project specifications. The main objective is adequate shear strength, settlement is less critical in this situation. This seems to be one of those things in geotechnical engineering that originally had a solid rationale but has since fallen into the pit of "we've always done it this way or used this value..."

Obviously compaction and peak shear strength are related but the main question I am trying to answer is: Is there an appreciable increase in peak shear strength for a specimen compacted to 95% modified Proctor vs a specimen compacted to 92% standard Proctor? I'm assuming the answer is dependent on the soil type but have always wondered about the exact sensitivity? Owners, contractors, often complain or begrudgingly oblige compaction standards and I've always wished there was a better answer than that is what is in the specifications. Obviously a specification is as much a contracting issue as anything but I'd love to have some data/paper in the back pocket that discusses the effect of compaction and shear strength at failure to know for myself that the difference is either significant or insignificant.

My final thought on this is that the effort applied during compaction essentially sets up the initial void ratio in the embankment at an arbitrary point. To the extent that the stress applied during compaction is higher than the final in-situ stress once the embankment is constructed and loaded, the soil will be over-consolidated and likely exhibit dilatant behavior during shearing. If the embankment is high, or the surcharge loads great, then the arbitrary point in the embankment could be considered as normally consolidated. Given a choice the former would be preferable. I know that the standard and modified Proctors have a force (12 400 ft-lbf/ft3, 56,000 ft-lbf/ft3 respectively) associated with the test methods. Does anyone have any insight on the physical significance of the two values w.r.t. the expected void ratio at the end of the test?

 

[geomane]

You are right. It will depend on soil type, gradation, stress history, etc.

Check out the attached pdf (Soils and Foundations - Vol I). Look at page 5-79. It discusses shear strength increase based on relative compaction and relative density.

 

[geomane]

Oops.. I uploaded the wrong attachment in my last post.

Correct attachment this time.

 

[fattdad]

for a given soil, the factors that influence drained behavior are governed by critical state soil mechanics. So, you got that right! If you are below the critical void ratio, the soil will dilate, produce negative pore pressures and lead to increased strength with shearing. . . to a point!

The other factor is the compaction moisture content. Two identical samples - one compacted to 95 percent on the dry side and the other compacted to 95 percent on the wet side will also display different friction angles and cohesion intercepts.

Then we have the simple truth that mother nature is not always consistent!

My approach is to take the borrow, get the proctor, test the sample for strength and use that value with a safety factor. I'd use a minimum population of 3. That said, I don't use a safety factor on settlement.

good luck, it's important to establish an approach that makes sense, so carry on!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

To provide some illustration on compaction effort and peak friction angle i put the figures from Bowles 1997 and Holtz and Kovacs 1981 together. The relative min and max density was probably determined using the shake table procedure.

As fatdad mentioned critical state soil mechanics govern the behavior with void ratios related to the critical state line. Been and Jeffries book on soil liquefaction provides a good reference on how to determine the critical state line if you so choose. It is quite expensive but it will determine if the material sampled is anticipated to dilate or contract during failure. Generally speaking though if you compact the material at the bottom of a large embankment and go back and drill CPT's or SPT's will the result be loose or very loose to indicate contractive behavior is the real question.


The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[ScarpShooter]

Thanks for the information guys, this provides insight into the sensitivity of effective friction angle w.r.t. initial relative density (~11-17 degrees). Unfortunately in the Appalachian plateau we rarely deal with the cohesionless soils referenced in the figures. I would expect that the effective friction angle of primarily clay soils/clay, sand, gravel mixes is not as sensitive to relative density as suggested in the plots. OCR is perhaps the more appropriate term w.r.t. categorizing the initial void ratio of the clays. For clays I would expect the level of compaction to primarily influence the effective cohesion. More effective compaction -> higher OCR -> higher effective cohesion -> higher shear strength. Since OCR will vary throughout the embankment/pad even if the exact same compactive effort is applied at all locations (OCR = pre-consolidation pressure [applied by compactor]/effective stress), I may be chasing ghosts with my original query regarding a simple answer to the effect of compaction standard on realized in-situ shear strength.

Often we will run drained direct shear tests at relatively low compaction levels (specimens prepared to 90% standard Proctor density at +3% of optimum and sheared under 0.5,1,2 tsf effective confining stresses). One would think that under these conditions the results would be relatively conservative. Surprisingly, the lab results often suggest a high cohesion intercept that is typically above the "Typical Strength Characteristics" from the USBR tests and in the widely published tables of typical strength characteristics. In that sense, the data, and how to incorporate it in a slope stability evaluation is often somewhat of a head scratcher.

 

[fattdad]

regarding compacted cohesive soils. I'll accept your premise that increasing compaction may (will?) increase the effective cohesion. I'll offer a caution; however. Effective cohesion may (will) attenuate in the upper 10 ft or so, owing to wetting/drying, freezing/thawing, etc. I'll use the term, "Softening."

The method I'd use to evaluate the consequence of softening is to take the sample, push it through the No. 40 sieve and normally consolidate in the odometer to the specified normal load. That's correct, run all samples without ANY compaction!

When doing this approach, you'll get a peak strength, but you'd consider this the, "Fully-softened" strength. You'll also get a, "Residual" strength, which would be the same residual strength you'd get from an overconsolidated sample.

You get to decide whether this is conservative, liberal or whether it makes sense!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

I agree with f-d yes that compacted clays will show a softening behavior increasing water content but also to consider for high clay fraction soils is particle reorientation at high displacements as shown in geotechnical engineering of dams by fell.

Also as you mentioned you are using direct shear tests I would caution you that to determine the residual strength from this test has problems. firstly repeated reversal is necessary and this may destroy the alignment of particles on the shear plane. Secondly repeated reversal can lead to soil squeezing out the two halves of the box or tilting. But the key thing to consider is to determine the correct residual strength as shown in figure 6.19.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[ScarpShooter]

Thanks fattdad, I'll start by saying that I've always appreciated your value to this eng-tips geotechnical community, so thanks for that. With respect to my original question regarding the compaction specification, what I interpret from your post is that the compaction specification is perhaps not that significant because the effective cohesion increase associated with it is perhaps a temporary phenomenon because it can be undone by environmental effects. Obviously this is limited to a zone of environmental influence near the surface. There may be a secondary benefit to compaction in that good compaction will inhibit infiltration and decrease the size of this zone of influence to some extent. This gave me an "aha" moment because I once heard a well educated and respected geotech engineer from a international pipeline company spout something out about as long as a slope made it through the first year or so he considered it stable. I thought this a bold statement at the time and have been thinking about it ever since. I think your comment provides insight into the time dependent phenomenon, the "softening" that if a slope can survive that then it is probably in some sort of equilibrium. Of course you will see some failures in previously "stable" slopes during extreme conditions such as the record setting rain we've had here in 2018. I like your laboratory evaluation technique for the fully softened/residual strength conditions. It makes a lot of sense. I think for the evaluation of strength conditions outside of this potentially fully softened zone the typical techniques with preparing specimens at the desired specification Proctor conditions is the best bet and in terms of understanding the sensitivity of strength to the Proctor MDD it is a matter of running more tests.

 

[fattdad]

"show a softening behavior increasing water content"

I never said this.

Softening is not related to increasing water content. Water content varies during the year. Softening developed from changing water content and the resulting changes in water pressures.

To the OP, CGPR did a workshop on clay strength. Google Virginia Tech, CGPR (Center for Geotechnical Practice and Research) and check out their publication list for #67 and #80. Also, check out the work of Tim Stark, Steve Wright, Tom Brandon.

f-d

ípapß gordo ainÆt no madre flaca!