"compaction" tests in residuum

[vollEngineer]

I understand that compaction tests are impractical for residual soils. Does anyone know of a paper or reference speaking to this issue?

 

[GeoPaveTraffic]

I'm not understanding your question.

A compaction test is just a density determination compared to a standard. Therefore, they can be conducted on any material.

If your question relates to making judgements about "natural" soils by comparing a density measurement to a standard, then I would say that is meaningless not just for residual soils but for all soils.

 

[vollEngineer]

My question is about the necessity for compaction testing in residuum. You don't compact residual soils, you leave them in place as long as the exploration indicated that they can support the bearing pressure indicated. A compaction test on residual soils will probably indicate a low failing percentage. But that percentage is not a true indicator of the capabilities of that residual soil. I'm hoping there is some paper out there that can help me get more clear about it.

 

[cvg]

assuming by residuum, you really mean top soil. Of course, most projects are constructed on deep layers of soil and depending on the depth, testing of the density may be unnecessary. However for the top layer, depending on the type of project, we either
a) strip the top soil and replace with compacted fill
b) compact the top soil in place
c) test the top soil for in-situ density and evaluate
d) allow for the settlement in the design

I would not say the tests are impractical and we often require minimum density for the subgrade, even if it is topsoil.

 

[vollEngineer]

What I am talking about is soil that has been weathered from rock into its structure and has not been disturbed by man. This soil extends into its parent rock.

 

[Dirtguy4587]

Are you using this material as backfill or borrow? If not, I would think that its in-situ state would be dense enough not to require any compaction - this could be verified with some test holes and investigation (SPT).

 

[vollEngineer]

That's closer to what I'm talking about, Dirty. It is residuum being used in its in-situ state. We already performed SPT and stated that the residumm soils indicated in our boring logs would be adequate yadaya. But the spec writer put in 95% of standard. And you may know that this residual soil will not meet (probably) 95% of a Proctor value. But it has structure and has carried mountains and can carry this slab. I'm having difficulty discussing this theory with my client and had hoped to find a paper that just puts it to bed.

 

[cvg]

residual soils can be in-place weathered rock, but more normally are deposited by various process such as wind and water. The soil structure and density of these soils can be highly variable and may or may not be acceptable as subgrade, without some compaction. Without testing the in-situ density, how can you make a reasonable determination of the strength? Therefore, how can you recommend this to be acceptable? I'm speaking as the devils advocate - you are not likely to find a "paper" that will back you up. You may have to provide actual data to substantiate your claims.

 

[vollEngineer]

I hear ya. These soils are from the in-place weathering of rock and we have performed some testing (including SPT of course). I will just use a geology textbook to explain soil structure for this type of condition, I guess. Seems like there's a paper on everything else, though! Shucks.

Thanks, everyone, for trying!

 

[Ron]

Not always a good idea to assume that undisturbed in-situ materials are dense enough to accept load without compaction settlement. I would assume that most of your load is being applied in the upper 5 feet of soil. That soil has little overburden and could be at a density that is significantly less than a compacted density of the same soil.

At the least I would require proofrolling for uniformity. It only costs a little more to check and achieve reasonable compaction (95% of Modified Proctor)if the soil conditions are good. Then you'll be able to sleep a bit better and you will have met your professional standard of care to fend off the lawyers if something does go wrong.

 

[GeoPaveTraffic]

In my work experience, residum is soil that is weathered from parent rock. cvg, I differentiate residum from alluvial,loess, or lacustrian soil.

vollEngineer, Assuming that the soil is clay or silt, i.e. cohesive; I would say that density is not a valid measure of soil strength or allowable bearing capacity. Soil structure has a lot to due with the strength of the soil in these conditions. This seems to be your opinion as well.

As for your client, tell them that it is not necessary to excavate and recompact the soil, but if that is what they want to do and they have the money. I say go for it. You got nothing to loose and the client is the one who is making a bad choice and costing him/her self the money.

Clients are like horses, you can tell them what they should do, but it is there choice to drink.

 

[BigH]

Residuum? You had it right in the first place - residual soils. The trouble with spec writers is that they do "what has always been done before" - and he might have worked, say on a highway project where, sadly, I see a desire to compact the foundation to a percentage of some standard - which might actually be lower than in situ for the sake of "what"?? In embankments, the "depth of compaction" of any natural subgrade soil will be small and not really influence settlements or strengths to any important degree. If, though, the soil is within, say, 1 m of the final pavement grade - I can see doing it - but as Ron indicated, it is more for the idea of finding soft zones or pockets of topsoil/peat that might be present - not in actually 'compacting'. For foundations, as you indicated in your few few posts, you develop a bearing capacity based on the materials' in situ characteristics. Would one compact a firm clay when putting on a footing (as GeoPaveTraffic indicated?) - No. Would I compact to a standard a sandy/gravelly soil? - No unless the footing is the minimum width of 600mm or so - and then only compact to build out any disturbance - but not to a "compaction level". Obviously the spec writer likes numbers . . . In the end - if you are satisfied that the soil is good for what you say, tell him that you prefer not to compact - as it might have an adverse affect - and you will confirm conformance with your design parameters by other means if they want - - dynamic cone test, etc.

 

[fattdad]

I'm not sure what I'm going to write is unique to "residual soils," but it would certainly relate to them. I HATE construction specifications that require "compaction" of undisturbed earth to some degree of "relative compaction" (i.e., 95 percent Standard Proctor). A properly executed geotechnical engineering study should have properly characterized the engineering strength/behavoir of the undisturbed natural soils and the resulting recommendations should have been consistent with these findings.

Here's where the body is buried: Let's say the natural soils have some favorable undrained shear strength (e.g., some residual micaceous silt or clay) and let's say the moisture content is at 95 percent saturation. Let's say you "check" the compaction and it's at 90 percent. You (incorrectly) run the compaction equipment over the subgrade and because the natural soil is not at a favorable moisture content for compaction, you stir up a subgrade of mud. WHY? Do you blame the site conditions" Do you blame the contractor? Should the owner pay? There's no good reason (if the original geotechnical study was complete), it's not a changed site condition, the contractor was following "spec," but the owner should not pay (but likely will).

If the OP relates to cut-to-fill operations, then sure use a nuke gauge (for the fill placement). If the residuum has mica, do a few lab mositure contents to make sure you are not being affected by the mica (or field burn offs).

If you have a natural subgrade that's become disturbed by construction, it needs to be repaired. Dead reckoning will tell you that (or a proofroll) and any repairs will likely entail fill placement, which requires compliance to the earthwork spec.

Sorry for the long post.

f-d

¡papá gordo ain’t no madre flaca!

 

[vollEngineer]

This is a highly-visible project and everything must be by the book. Literally. So I have requested the architect to re-interpret our fill recommendations to exclude residuum in this project site and change the book.

I think one of the lessons learned, here, can be that the geotechnical engineer should have the opportunity (and take it!) to review the drawings and specs during design to ensure these erroneous interpretations of fill recommendations are kept out of the specs during construction. If we had reviewed the specs during writing, we might have helped the architect avoid this confusion. It's much more difficult for architect, contractor, owner, building official, and inspector to change them now that we're under construction.

 

[msucog]

i'd get the wording changed if it were me. we typicaly do not perform inplace density testing on residual soils for the purposes of evaluating the strength. the numbers are rather bogus in my opinion if you're trying to compare it to a standard/modified proctor curve. if it's recompacted, then sure. residual soils gain their strength from residual stresses. fill gets its strength from the particles being rearranged (ie. compacted). if you have a residual soil that is disturbed, then the soil's "strength fabric" has been broken and should be compacted as a fill. for residual soils, i rely on spt, dcp, lab testing, poaky stick (aka probe rod), etc etc to assess the strength and/or stability of the materials.

the writer of the specs likely hasn't updated the specs in 20 years and probably doesn't understand half the things in there. i'd dare to say they don't even understand the difference in standard and modified proctors much less the difficult topics such as foundation support. assuming you're the geotech, do your client a favor and get it fixed. also, if you ever end up in court, the lawyers don't care what is appropriate...if it's left in the specs (ie. CONTRACT documents), then it could be a sticking point for someone to try and kick you...simply because you didn't follow the contract. (not saying it would work but you've still got to spend money/time defending the ridiculousness that the opposing lawyers come up with).

 

[oldestguy]

Then comes the question of "Why 95 percent?" "Why 95 of Modified?"

I got quite a setting back once when the contractor was not getting 95% in a clay. He asked us to test the cut material as it sat before being excavated. It had an unconfined compression value of a few tons per s.f. I'ts "percent compaction" was 82.

Contractor's question was why we allowed him to build on that undisturbed clay at its condition and wanted 95 in the fill?

Why dry the stuff out to get that spec. when in a few years it would revert to its former moisture content.

Then, another job comes along where our inspector enforced the 95 percent in a highly plastic clay. Later the buildings (plural) were all distorted as the clay took on water reverting to its former condition. A very embarrassing situation I tell ya.

The subject is not that clear cut for the average job.

 

[BigH]

oldestguy said in fewer words what a number of us said more verbosely. If you are compacting fill - as in his penultimate paragraph, I would have hoped that the compaction was on the wet side of optimum - less swelling potential.

 

[theCorkster]

Embedded in the opinion is the understanding that the "residium" as it is inplace will perform as encountered over the life of the structure.

Residual soil/completely weathered bedrock in our local has shown from post-construction distress in roadways and structures that the new improvements change drainage conditions. Where water previously did not accumulate in the soil, it did after the construction. Over time pavement subgrades that were resistant to deformation, and difficult to cut, saw that strength degrade with moisture development. Completely weathered siltstones/claystones in some cases behaved well, and in other cases expanded and lifted improvements.

The question that I bleieve needs to be addressed with these materials is how will the material's behavior will change to the likely changes associated with changes in the residiums moisture contente with time. The answer to this question is your guide.

 

[ncarolinageo]

I think there is a very easy way to summarize most of what has been said here - compaction testing in residual soils (which are indeed soils that come from the in-place weathering of the parent rock material) is not the proper way to evaluate their strength or ability to carry building loads - as these soils gain much of their strength from their natural structure. I am located in a geology that has a lot of soil types that when unsidturbed will yeild compaction test results ion the mid 80% range - but when evaluated by SPT or laboratpry testing yield much higher strength parameters than one would think based on the compaction test results.

I would also second your opinion that the geotechnical engineer be retained to review the plans and specs. I cannot tell you how many times I have run into this - or similar issues - that could have been resolved by properly educating the architect, owner, and/or local byuilding officials about what we do.

 

[darthsoilsguy2]

we test footings in residual soils for percent compaction all the time....or at least according to the guy taking the scheduling calls.