Standard Practice- Subsurface Exploration 3

[Mazzman]

I received a copy of a subsurface exploration report prepared a licensed engineer with geotechnical experience. The project is a 8000-square-foot, wood-frame commercial building, and the work was conducted for a local developer.

The report states that the field sampling was done using a Giddings ATV rig. Apparently, this is a small pull-behind trailer with a power auger mounted on it (see attached photo from Giddings website). I'm told the engineer runs solid stem augers and conducts the field work himself.

No Standard Penetration Tests are conducted; however, the report of geotechnical exploration reports “Estimated N-Value” on the boring logs.

The logs in the report include “Cohesive Strength (psf)” values. The engineer told me he runs the augers down, and then lifts the augers exposing some soil within the auger flights. He then uses a hand-held penetrometer on the soil exposed on the augers to measure strength. He says the value he reports is equivalent to undrained shear strength.

I asked how he gets N-values if he does not run the SPT. He said the “Estimated N-Value” reported on the boring logs is based on some correlation he has between hand-held penetrometer value and Standard Penetration Test value.

The logs include USCS classification, but there is no indication in the report that soil classification tests, or any tests, for that matter, are conducted.

There are statements in the report that do not seem to be supported by data:

The report states “Penetrometer tests were performed on representative soil samples to evaluate soil shear strength, compressibility and consistency in-situ”. Based on the description of the field testing, it does not appear the sampling was conducted in-situ. Also, hand held penetrometers are not usually considered useful to predict compressibility.

The report says “This report…has been prepared in accordance with generally accepted geotechnical engineering practices.”

My question to you: Are the practices I've described here consistent with local geotechnical engineering practice for a similar project in your area?.

 

[cvg]

I have yet to see a geotechnical report without any lab testing. Usually at least a sieve analysis and possibly atterberg limits and proctor etc. However, most of my work is public works, not for developers.

 

[erich]

While I have seen reports without lab testing as in when conditions are excellent or for projects with very little load, the concept of using re-molded samples to somehow correlate to in-situ N-values really scares the bejeesus out of me.
So quick and easy answer is, "No, these practices are not consistant with accepted engineering practices."

 

[jdonville]

Mazzman,

You get what you pay for.

A former consultant - now in design-build - I see alot of ho-hum geotech reports. It's obvious which ones the owner decided to spend a few more shekels on. In my experience, most developers look at the geotech report as a necessary expense - which they try to minimize.

In my opinion, you have the option of telling your developer client that you will have to produce a more expensive (to construct) design to compensate for the uncertainties in the geotech report. Offer to perform or hire out a new investigation to your own specifications.

Jeff

 

[fattdad]

Here's where the body is burried: Just how exposed is the owner to changed condition claims by the contractor during construction? I would think that the geotechnical engineer would be able to get enough information to make a reasonable engineering judgement on whether the foundations should be sized for 2,000 psf or 3,000 psf. I would think that the soil samples would show whether the soil types are suited for use during earthwork (i.e., as controlled fill, etc.). However, during construction all it will take is a contractor to take a soil sample get a lab to run a few tests and show that the logs are not representative of the field conditions, or the location of the sample is notably different from the logs - and ask for a change order.

This is why I do laboratory testing. To protect the owner - NOT for CYA.

¡papá gordo ain’t no madre flaca!

 

[msucog]

if the exploration was cheap, then it sounds like he gave somebody exactly what they paid for. i've had jobs where the only real option that the owner wanted to pay for was a few hand augers and dcp. the owner takes lots of risk and we do the best we can at the time and recommend additional exploration later. sometimes, the owner will have us come back later to perform drilling w/ spt. sometimes, the owner doesn't come back to us and may or may not have someone else do testing. when something goes wrong, the owner is on his own. however, with the way codes are now, that stance is quickly being harder for owners to get by with some cheapo exploration that provides on minimal information. so owners are being forced to have "legit" explorations performed. "legit" is different depending on who you ask. in my opinion, the one you describe is lacking substance to say the least based on what i can see through my dell computer screen.

 

[GeoPaveTraffic]

My question to you: Are the practices I've described here consistent with local geotechnical engineering practice for a similar project in your area?.

In a word NO, at least not in the mid-west of the US.

If all this guy ever does is simple wood frame construction, he may make it. But I sure hope he never does an important building like school or hospital.

 

[Mickney]

From the information provided, I would call this report sub-par at best. It may even warrant calling your state board for their review of his engineering practices. What geologic area was the work done?

 

[Mazzman]

The comments that you have provided are in line with my opinion and that of others in my office. In the last few months we have been provided with three reports prepared by the consultant. Each report uses the practices I've described. These practices are in no way representative of the "standard of care" practiced by geotechnical consultants in this area of residual soils overlying limestone bedrock. The practices I've described take the practice of geotechnical engineering to a new low, and we have considered alerting the state board.

 

[DRC1]

If the soil is consistent and you aref amiliar with tit, you can tell a lot from a handfull of dirt. A good field engineer can clasify soils very closely to the sieves and atterburgs, however it is good to run the tests to check. He is definately overselling the information he is getting out of his pocket pen, or else he is being way to conservative. However in limestone/residual soils, it is hard enough to be sure you have not missed a change when workig with standard drilling equipment, let alone this set up. The sad part is is that according to the web page, you can get an STP set up, rotary drill for tubes, a penetrometer and a GPS set up. So with a few more bucks (they are called bussiness loans) he could have a truely sweet rig.

 

[DarthSoilsGuy]

i would not expect the state board to act based on this information, unless he already has a record. i don't suppose it wouldn't hurt to ask if there has been any disciplinary action(s) against his license. in my state (Vermont, USA), you can find this out easily on the internet. don't forget your border states.

i am weary of the pocket penetrometer as a tool. But, it is used (although i've never seen a contractor use one) for trench stability. one could say "It's better than not having it". Another could say "It is directly used for the highest purpose in Civil Engineering... protection of life". i would be in the 1st category. anyway, it's marketed for testing for cohesive soils.

but, who knows. i don't trust it so i don't use it. maybe, if i still did drilling field observations, i could buy one and build up my own level of confidence testing the multiple portions of the inside of the Split Spoon samples and "relatively undisturbed" auger cuttings. Maybe, i would find out that it has too many problems. maybe, if i did like it i would consider doing this myself instead of sending a technician out with a hand-auger and DCP to perform a "limited subsurface investigation" (they are all limited anyway). all this being said, i wonder what his Plan B is? What if the soils are not cohesive enough to pull out "in-situ" like chunks. Does he then whip out the solid stem auger and manually DCP it?

There are two direct correlations he's making.
1. Handheld penetrometer verses split-spoon
2. Solid-stem cuttings verses undisturbed sample/in-situ
3. (indirect correlation) all the indirect correlations on top of it (that is geotech engineering) concerning split-spoon.

if i were to look at this method seriously, the 1st challenge i would have to answer is:
Are the intact cutting representative of the in-situ conditions? What about all the cuttings you can't test because they have been damaged? how do you factor in these soils which may have been damaged because they are weaker than the chunk that came up, or may have just been in the wrong place at the wrong time?

there are some philosophical questions too..
What if the egr makes twice as many borings than he would with a full operation? What if 3x? what if he has bored hundreds of holes already in the vicinity, with "standard equipment"? i could keep going on different themes here.

i would prefer an old dog with dodgy equipment and common sense than a newbie with great equipment. But, i would also prefer an old dog with great equipment over both... And, i would definately not want some hip-shooting clueless jacka$$ with dodgy equipment. unfortunately, i don't have enough info to say who did the work, but i cut the list of suspects in half.

post too long, must stop....

 

[casimmons]

Let's not get too excited here. Obvoiusly what this guy did is below the normal standard of care. But consider what's being built- a one story wood frame building. In my area those are built all the time without any geotechnical. And if the soils are typical residual soils the geotechnical would have recommended minimum width footings anyway. What this guy did was probably enough to determine if there was something obviously bad about the site- very soft soils, trash fill, etc. He should not try to pass it off as a geotechnical report that meets industry standards but it may be sufficient for the proposed construction.

 

[Mickney]

Hmmmm, interesting point casimmons.

One question to all who are still interested, if you were purchasing the building, would you rely on the report and its findings?

 

[Mazzman]

From Mickney: One question to all who are still interested, if you were purchasing the building, would you rely on the report and its findings?

I wouldn't, but then I know something about these things. The owner who will take possession of the building from the developer won't know any difference. The developer probably doesn't know the difference, although I have since explained it to him. He got his "geotec" (and likely got it cheap) and checked the box on the lender's requirements form, so he doesn't really care, as long as there are no problems with the building before he turns it over.

Sure, it's a small, relatively insignificant structure and the chances of problems due to poor geotechnical practices are probably small. But, who's to say the same practices won't be applied to larger, significant structures? Do we as knowledgeable professionals have the responsibility to report shoddy work when public safety could be affected? I don't mean to get overly dramatic about this, but I've honestly never seen such blatantly poor practice. We won't even talk about what the engineer's E&O carrier might say about this (if he carries professional liability insurance).

 

[civilperson]

If a complete and thorough site exploration was implemented and the recomendations were the same as the minimum geotech report, would you say that the complete report was excessive and a waste of money? I think that a criticism without any established faults of the report recommendations is poor judgement and unjustified. What you call poor practice could also be called cost effective if the results are the same.

 

[casimmons]

Mazzman- do you really think "public safety" is at stake here? My guess is that a poor geotech could result in excessive settlement and building cracks, but not some kind of collapse that would endanger public safety. Civilperson has a good point, what recommendations or judgments of the report do you disagree with? If there are real issues then you can let the client know that you can provide a higher level of service. I have some clients who hire the "cheap guy" when he thinks the site is fairly simple, and hires me when he thinks there will be problems and requires more expertise. Of course you don't always know which sites will be problematic before doing any exploration. In those cases I have to "clean up" the mess that the cheap guy started!

 

[fattdad]

I'm with casimmons on this. Site conditions for one story wood framed buildings can often be sufficiently characterized using hand auger borings and rebar penetrations (if you are familiar with the setting). As an engineer if these data allow you to formulate your engineering recommendations you'd then be in a position to make recommendations and properly seal the drawing/report.

If I were to purchase the development, I'd likely do a few hand auger borings in the landscaped areas of the development, just to make sure that what the original report called silty sand wasn't what I typically call sandy fat clay. Whether this matters or not may be moot or at some point in the future, may become relavent. Just doing my own due diligence, I guess. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[Mazzman]

To civilperson: There is no basis to determine if there are faults with the report because the report recommendations are based on suspect field data. If I had any confidence in the data on which the conclusions are based, I would agree with you.

To casimmons: As I said, my statement of "public safety" is probably overly dramatic and overstated. Again, I have no basis to disagree with the conclusions because the field data on which the conclusions are based is of no value. I saw the site and there is about 7 feet of fill in one corner that reportedly was placed about 5 years ago. The report does not address it, makes no mention of old fill on the boring log in the area...etc.

To fatdad: I disagree with you. Rebar penetrations? I fight that battle here. We have a 260 pound technician and a 150 pound technician. Do you think they lean on the rebar with the same force? How do you decide if it's "good for 3000 psf"? Do you use a No. 4 bar or a No. 8 bar? Don't put me on the witness stand if I have to defend the use of a rebar to determine soil strength and allowable bearing pressure.

I'm finished with this thread, and thanks for the discussion. You've questioned my conclusions enough that I'll take it no further, but I still believe the engineer in question does a disservice to the practice of geotechnical engineering.

 

[fattdad]

My goal was never to be considered cavalier. Sorry if I wrankled any ire by suggesting an empirical approach to foundaiton design for wood-frame construction. There are many things better than rebar penetrations and hand auger borings, but the geotechnical profession does have precident in using 1/2 in rebar as a qualitative method to characterize density, just as it has qualitative methods to usinging a pocket penetrometer to characterize consistency.

¡papá gordo ain’t no madre flaca!

 

[dirtygeo]

Certainly the level of detail and testing in a site geotech investigation can vary as a function of the type of construction (to a point). However, the issue I would be concerned with is a certain level of work being represented beyond its value in this case. Estimating N-values and providing specific comments on soil strength from pocket penetrometers in auger cuttings seems, to me, to be overreaching. Auger cuttings are completely disturbed samples and cannot be relied on for accurate penetrometer results. It may not be an issue for a lightweight structure where the actual contact pressure will be 2,000 to 3,000 psf or less, but it still represents poor practice.

As a practicing geotechnical engineer in southern Ontario I am well familiar with the soil conditions and geology of the area where I work. Based on this experience it is possible to conduct a limited investigation for a lightweight structure (wood frame as described here) without the need for detailed lab testing and relying more on qualitative evaluations of the soil, i.e. native stiff silty clay can support a conservative design bearing pressure of 2,000psf. But the basis of the recommendation needs to be clear and not oversold. If you provide more information and detail than what can actually be backed up with hard data you are setting yourself (and the owner, the guy who pays the bill) for trouble. It is a recipe for extras during construction, unhappy owners/client, and potential lawsuits.

What happens if a sand deposit is encountered? Can you tell from auger cuttings if the sand is loose, compact, or dense? enough to state an N-value? I don't think so. The level of fieldwork and lab testing needs to match the level of development and cater to the site conditions as they are uncovered. The report needs to properly present the work done and the basis for any recommendations.

If you do a limited amount of work, simply say so and qualify the results/recommendations. Our goal should not be to provide a cadilac for the price of chev, but rather educate the client on the value of buying the cadilac in the first place. Remember too that sometimes a chev is just fine to get you around the block, as long as you know you are just driving a chev.

Just my two cents.
(Sorry if the last offended any chev owners)

Dirtygeo