Forensic Evaluation of Engineered Fills
Forensic Evaluation of Engineered Fills
(OP)
Reading ASTM E141 it seems to me that when evaluating existing fills the standard practices of our industry are falling short. E141 indicates that location of and evaluation of the units being examined (dry density of the engineered fill) should be performed in exactly the same way as done for the sample (i.e. nuc's or sand cones during construction). That is ... using SPT unit weights (or shelby, or whatever) to evaluate the relative compaction of an engineered fill is bunk. Am I reading E141 right? Seems like the degree of precision and accuracy needed for unit weight determination for geotechnical analysis would fall short of the accuracy and precision bar necessary for verification of an engineered fill.





RE: Forensic Evaluation of Engineered Fills
SPT is nothing but an index test, like liquid limit or consumer price index. Plain old Shelby samples in engineered fill probably would probably be too disturbed for good density numbers.
*Inexperienced lawyer generally costs more than a top-shelf engineer with gray beard and a long list of projects and publications. How many lawyer hours does it take to equal the cost of two days of a small back hoe and operator, plus an engineer and a technician to do the testing, and a little bit of lab testing like density, gradations, water content, and such? Not many.
Q. What do you call it when a lawyer falls into the test pit?
A. Free backfill.
RE: Forensic Evaluation of Engineered Fills
I've looked exhaustively for studies that support or refute 'gray beards' methods but haven't had any luck. You have any ideas there?
(E141 is ASTM's Standard for "Acceptance of Evidence Based on the Results of Probability Sampling" ... Seems like our forensic/expert engineers should be held to some standard.)
Thanks for you input.
RE: Forensic Evaluation of Engineered Fills
You would have to try pretty hard to place any kind of nonhydraulic fill at only 70% of maximum with motorized equipment!
RE: Forensic Evaluation of Engineered Fills
I'm with you. I've been watching earthwork for over 30 years. If any of my tech's came in and said 70% I'd tell them they were using the nuc upside down. You and I both know 70 is rediculously low, but finding a study or reference that states that... good luck.
RE: Forensic Evaluation of Engineered Fills
An SPT N value would be in the order of 70% according to these sources.
I once saw a lab manager fired when the client asked him what the specific gravity of the project sand was - and he said 2.65, approximately. The client jumped up and down and screamed "How could it be - it is only about 1.56 g/cc (about 15.5 kN/m3). Of course, the client was mixing up specific gravity and maximum dry density. Perhaps your grey beard is having a "senior moment".
RE: Forensic Evaluation of Engineered Fills
As for correlation between SPT "N" value and the modified Proctor...not good. As for the Shelby tube...depends on the material whether the densities will be significantly disturbed. It would be similar to a drive sleeve method which is certainly valid in materials passing a No. 8 sieve or so.
Opining that compaction in "engineered" fill is at 70% of the modified Proctor is patently absurd. As BigH and others have noted, you can get 70% from just dumping and wetting.
As for determination of density, any direct method should prevail...sand cone, shelby tube or drive sleeve. The nuke would be a second tier choice, but not a bad one.
As for the gray beard...age doesn't necessarily beget knowledge or wisdom. Does he have 30 years of experience or 1 year of experience 30 times?
RE: Forensic Evaluation of Engineered Fills
My handle would have been, "I hate idiot engineers who should never have had a stamp and ought to be tossing pizzas for a living." But it wouldn't fit (and everyone hates lawyers - so I remain anonymous), and unfortunately there are times in my professional career that I haven't wondered if I just tossed a pizza!
RE: Forensic Evaluation of Engineered Fills
RE: Forensic Evaluation of Engineered Fills
RE: Forensic Evaluation of Engineered Fills
Now if the investigation is for assessing the fill as placed against the project specification - that is different and I agree that his method is highly suspect - especially if one is in gravelly fills or fills with gravel sizes - the split spoon is inadequate for this due to the gravel size to opening aspect.
RE: Forensic Evaluation of Engineered Fills
Thanks for your input everyone. If any of you run across any references in your travels that discuss the variability of UW test data from boring sampling methods, please keep me in mind. Happy Holidays!
RE: Forensic Evaluation of Engineered Fills
Back in the mid-80s there was some work performed by the Bureau of Reclamation in Denver in the Division of Research that as I recall involved placing and compacting sand at a target density into a large mold (say 5' by 5' by 10' high). I believe that SPTs were taken as well as sand cone tests. Unfortunately, I don't recall who was the principal investigator. However, I don't believe sufficient testing was performed to develop precision and bias between the methods to measure density.
I'd stand by the case that selecting material from cuttings and then comparing it to a small "undisturbed" sample from a specific interval is quite a leap, even from a geotechnical perspective. Unless the fill is a processed material without much gravel or uniform fine-grained soil, the comparison is based on a guess. If I was dealing with that testimony, I would definitely want test pits to visually confirm the viability of this approach. Photographs with physical property tests showing a high degree of variability might sway a judge/jury.
If the fill contains sand and gravel, it is possible the coarse material can break down as a result of drilling and laboratory processing, including compaction, resulting in an erroneously high laboratory maximum density.
I noted that you did not respond to BigH's comment regarding compaction records during construction. If that is the case then I warrant a higher degree of evaluation as dgilette described is appropriate since the fill's content is really not known.
RE: Forensic Evaluation of Engineered Fills
Thanks for your input. It's kind of funny, seems like we're all pretty much of the same opinion but undoing what has been done is difficult without the benefit of a publication. It just looks like an engineering stand-off to those in position to decide if there is or isn't an issue.
RE: Forensic Evaluation of Engineered Fills
In this case it seems like a good lawyer is exactly what is needed. A contractor signs a contract to place and compact fill to the requirements of the contract, which include compaction tests by nuclear gauge or otherwise, but not usually SPTs. So it seems like a good lawyer could stop any legal action by others right there. Similarly, a testing agency is contracted to test by those same methods. The results of any other test are irrelevant.
I'm probably just stating the obvious. And, I suppose that if law were actually practiced to the requirements that it meet the rules of common sense, we wouldn't need any lawyers.
RE: Forensic Evaluation of Engineered Fills
Can I get that tatoo'd without a copyright infringement?
Your position that results of any other tests are irrelevant is in step with ASTM E 141 ... but unfortunately in my neck of the woods ... we haven't reached that level of refinement. It's whoever tests last, tests best (i.e. they have the most credibility) and validity of the methods don't enter into the equation.
Thanks for the input. :D
RE: Forensic Evaluation of Engineered Fills
This can be a pain in the butt for the expert, but why not go that route and look for holes in it.
Ask for his reference material. After all, this can't be just something he made up from thin air. What is the "standard of the industry" that he meets with his criteria for data to be used for such a conclusion? Then, show any calculations he makes. A good lawyer needs a little help on this, since they can make black lok white if they are smart.
All this stuff usually can be found with interrogatories if they have not already limited the listing of such witnesses.
On this subject, I have OK'd many a random dumped fill for use as a building site using SPT, but never tried to tie that into percent compaction. The words "loose, firm, dense, very dense, etc." as used by drillers don't have numbers attached as I recall.
On the other hand, I have been set back by a contractor or two when our guys on the job were asked to test "percent of Modified Proctor density" of nearby natural ground where we have OK'd the placement of foundations and the number was far below what we specked for the fill, like 82 percent for a clay as an example. Then we scrambled with testing unconfined compression, etc. on the fill as well. That brings up the question I sometimes ask "Why 95%?". Not many can explain that other than to say "Then we know we got it".
RE: Forensic Evaluation of Engineered Fills
RE: Forensic Evaluation of Engineered Fills
That's part of the overall issue I have with our industry's approach to forensic engineering. Brand x comes through post-construction and gets 89% compaction with their testing program in lieu of the 90% specified - and they mandate to remove it all and do it over again. I have not seen a forensic engineer (locally) temper their findings with documented precision and bias between samples or variations in post construction testing methods - For instance a sand cone in the bottom of a test pit is at best equal to a sand cone on a flat pad being compacted and more likely the sand cone is compromised to at least to a limited degree ... and depending on the care of the one performing the test ... possibly to a great degree. And then ... I have never seen locally where a forensic engineer evaluates the potential strain associated with their 89%.
Maybe I need to move??? Anyway ... thanks everyone for your input. Maybe my 'counter part' is a reader of posts and will begin to rethink his approach.
RE: Forensic Evaluation of Engineered Fills
RE: Forensic Evaluation of Engineered Fills
A new college campus on an area of high-shrink swell clay had several buildings built on a few feet of compacted clay fill, dug from proposed ponds. The conscientious engineer demanded and got 95 percent. Subsequent feeding of roof drain water, fed via sand filled plumbing trenches, caused major heaving of floors, damaged interior partitions, etc.(fortunately footings were much lower in natural ground). We were darned lucky the insurance coverage was there or we would be broke.
In clay ground, I have been very careful not to do that again. (It was not me, but a nameless fellow engineer ).
RE: Forensic Evaluation of Engineered Fills