Standard Practice- Subsurface Exploration
Standard Practice- Subsurface Exploration
(OP)
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?.
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?.





RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
So quick and easy answer is, "No, these practices are not consistant with accepted engineering practices."
RE: Standard Practice- Subsurface Exploration
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
RE: Standard Practice- Subsurface Exploration
This is why I do laboratory testing. To protect the owner - NOT for CYA.
¡papá gordo ain’t no madre flaca!
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
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.
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
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....
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
One question to all who are still interested, if you were purchasing the building, would you rely on the report and its findings?
RE: Standard Practice- Subsurface Exploration
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).
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
RE: Standard Practice- Subsurface Exploration
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!
RE: Standard Practice- Subsurface Exploration
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.
RE: Standard Practice- Subsurface Exploration
¡papá gordo ain’t no madre flaca!
RE: Standard Practice- Subsurface Exploration
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
RE: Standard Practice- Subsurface Exploration
It is worrying when one sees the comments/opinions made by structural engineers and compares them against geotechnical engineers. I think that the chains of communication between structural and geotechnical engineers are important and one can never get enough information from the structural guys and their requirements. The cost of getting a proper geotechnical investigation is a mere fraction compared to the costs of should it go wrong. Something that most clients do not consider - the cheaper geotech always wins but sometimes it is not always the best advice one gets!
As a competent and professional person, I would certainly get another geotech opinion - it is not the question of what may happen to this relatively light structure, but what worries me is the situation on his next job where he does the geotechnical investigation for a multi-storey building?
You mentioned that he has geotech experience - what level may I ask? Using a mere pocket penetrometer to classify augered (disturbed) soils I find worrying. The thought comes to mind "one gets what one pays for".