Number of Geotechnical Boreholes for a Building 7

[Aesur]

I debated in my mind whether to post this question here or geotechnical and ultimately decided this is the best location because I am more wanting input from other structural engineers.

I have started seeing more and more questions from the geotechnical engineers asking how many boreholes and the locations of these boreholes for the preparation of their geotechnical reports. I am curious if any of you have any recommendations or resources that provide more information on determining minimum number of boreholes, etc.

As I understand it, two triggers for more boreholes would be varying soil conditions and higher seismic zones. As I am not a geotech, I am not as familiar as they would be with the soil conditions in most locations off the top of my head so it would seem to me that they would know better on number of and locating bore holes on a site to provide the most accurate results.

Is this a common question you see? As it hasn't been common in my area for as long as I can remember (other engineers in my areas also noted that they are seeing this request more and more frequently), my pessimistic side suggests there may be an ulterior motive, such as shedding more liability and being able to point more fingers at the structural engineer should something happen. This gets into the discussion of if the geotech report is part of the construction documents, which more often than not, regardless of what the report says in regards to this is made part of the construction documents by referencing and submitting the document to the jurisdiction and it being a condition of the permit.

 

[EZBuilding]

This is a common question I see come in - we are often asked for boring plans.

We generally try to push back on providing this type of information. We try and be helpful in identifying the locations of the building we anticipate will have the highest demand of load - but steer clear of identifying what the level of study should be required for the geotechnical study. The geotechnical Engineer should be the party that identifies the number and depth of borings required to provide the guidelines for their geotechnical report.

 

[Brad805]

The question is about time on site and time in the lab. When firms are price shopping that makes a difference. Whomever is paying for the geotech should give the geotech engineer some latitude when they are finally on site to determine what is needed and agree to pay if additional work is necessary. I agree with EZ that it is best to steer clear.

 

[phamENG]

I don't have a problem identifying the minimum number of bore holes and where I want to know about it (heaviest column, etc.), but I include the caveat that says the actual number and location of the bore holes is up to the geotechnical engineer such that they are confident in their recommendations for the foundation design of the building.

I don't usually get the question from the geotechs, though. It's rare that there's much communication before the report is written. It's often done before I'm involved and it's given to me when I write my proposal so I know what kind of foundation I'll be designing. It's usually from owners or architects who haven't worked with geotechs before and don't know what to ask for.

 

[KootK]

The best advice that I ever received on this topic came from JAE circa 2008: Link.

I'd cut off a thumb to have that guy back in action here on the forum. Having him in lurker mode is like having Aaron Rodgers as the waterboy.

 

[slickdeals]

I try to push back as much as possible to the Geotech, specifically referencing IBC 2018 1803.3.1.
I have used the attached document as a reference in the past.

 

[Celt83]

We had a local Geotech give us a presentation a few years ago and he indicated that we should never be locating or setting a quantity of borings as we simply don't have access to the local historic soil data that they do. Had a job early in my career where they followed the sq. ft. guidance in JAE's post and then during construction the site was constantly flooded, someone did some sleuthing and found a historic landplot and it turned out that the borings that were done ended up on each side of an old creek, missing the body of water entirely.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[OH_PA_AISC]

All;

I bring a different background to this discuss preforming trenching and excavation projects. In Ohio the Glacial till is a big factor. The Wisconsinan glacial stopped just pass the Lake Erie shoreline. The Illinoian Glacial made its way down to Kentucky where it forms Ground Moraine (ridges). I'm always surprised by the constant change in ground conditions in the Cincinnati area. Local Geotech rules.

Thanks for reading

 

[phamENG]

KootK - thanks for posting that link. What did happen to JAE? I've been wondering that for a while.

 

[KootK]

KootK - thanks for posting that link. What did happen to JAE? I've been wondering that for a while.

That's a story that I'm not sure I'm authorized to tell here in its entirety. Here's what I can say without speaking out of turn or engaging in speculation:

1) As far as I know, all is well with JAE in terms of health, career, and general well being.

2) JAE visits this forum often. You can see that for yourself by visiting his profile periodically.

3) JAE has not contributed anything here in some time, obviously.

4) JAE is a member of an alternate structural engineering forum that doesn't receive much traffic. He's more active there than he is here but, still, we're talking monthly contributions rather than daily, at best.

I miss JAE's contributions here massively. He was a big part of the KootK mentorship program for a long time, even if he didn't realize it (or enjoy it). I'm not as dependent on his mentorship as I once was, however:

5) I hate to see others deprived of JAE's mentorship. Prior to the rise of phamENG, I considered JAE to really set the standard for the particular combination of helpful advice & spot on bedside manner.

6) For me, the forum's just a lesser thing without JAE around. As much as I clearly love strutting my stuff, for me, a big part of the draw here has always been my interactions with folks like JAE whom I consider to be my betters. As you know, there are a number of reasons why I was MIA for parts of 2021. It's no exaggeration to say that JAE's absence was one of those reasons. I'm not yet finished beating him down on the minutia of every possible technical topic. And further to that, I hope that I'm not part of problem with respect to JAE's absence here. I don't believe that to be the case but I don't know that for sure and it's not out of the realm of possibility.

For some time now, I've been considering starting a petition thread here that would, effectively, be a love letter to JAE to try to get him back. I'd state my desire to have him back on side and ask for a short comment & bump from everyone who feels the same, even if it's just a #METOO#. I've no doubt that would wind up being a triple digit thread. On the other hand, petitions aren't really what we do here so I'm not sure that's appropriate. And, for all I know, perhaps JAE would not relish the attention.

 

[phamENG]

Got it. Glad to hear he's okay physically. I think just about every thread I've ever archived has a considerable contribution from him, so I'm right there with you.

Petitions may not be what we do, but maybe that's just because we haven't had a worthy cause before? Getting JAE back would certainly be that cause if ever they may be one.

 

[KootK]

Who knows, maybe JAE just managed to break the addiction and is happier for it.

 

[Aesur]

Thank you everyone for all the feedback, lots of great information here.

Going forward I plan on providing minimum number of boreholes I want to see for the building, specially where loading is higher, etc. Additionally I plan on noting that this is a recommendation and that the geotechnical engineer shall determine if additional locations are required and the owner shall include a contingency as needed for this. Hopefully in this way, it I'm being a team player but making it clear I'm not a Geotech and don't have the final say.

 

[jittles]

I think this should be 100% by the geotech. Whatever they need to do to provide you enough information. They're the experts/professionals on the subject.

 

[dauwerda]

JAE once told me that JAE stood for "Just Another Engineer". If that's the case, I have a long way to go to become just another engineer. I will certainly jump on the bandwagon of saying that I miss his contributions around here, but I am thankful for all of the archived information that there is with his thumbprint on it.

Prior to the rise of phamENG, I considered JAE to really set the standard for the particular combination of helpful advice & spot on bedside manner.

How many times have I read phamENG's posts and thought to myself, "damn I wish I could be half that eloquent."

 

[dauwerda]

In industrial facilities with lots of large equipment, large (and small) non-building structures, and a PEMB building or two we would typically put together site plans showing where we knew we wanted borings (under specific large equipment and such) and then add wording that the geotech should select any other locations required per their experience/expertise (similar to what phamENG noted above). We would also provide typical foundation reaction forces for said equipment and structures.

 

[Lomarandil]

For non-building structures, I've also used the NAVFAC DM7-1 guidance as a starting point (agree with the others about letting this be informed by a locally experienced geotechnical professional).

I've modernized the formatting (and converted to metric), attached here.

----
just call me Lo.

 

[dik]

I generally let the geotekkie determine the number of boreholes. If the soil is uniform throughout the site, generally 3 or 4. If the site condition is wildly variable, then more as the geotekkie determines.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

I completely missed your comment about me in there KootK. Thank you. I hardly think myself worthy of a comparison to JAE (or you, or half the members on this forum). I still consider myself a blind squirrel in a forest with an abundant nut supply. And thank you, dauwerda - any eloquence I may have can be credited to my grandmother. She's 100 years old now, but still sharp as a tack and won't hesitate to correct me if I misuse lay and lie...

Back on topic - I agree with everyone who's saying that we need to leave it up to the geotech. It is their bailiwick and, let's face it, our insurance probably doesn't cover it. But I also think that the collaboration between the geotech and structural has often been underplayed - especially in my neck of the woods. Even on mid-sized projects it seems that things go like this:
- Site survey, concept architectural, geotech report
- Structural engineer enters the pictures and the building is designed
- Geotech comes back to check compaction and the lab collects samples, does material testing, etc.

Notice the lack of "structural engineer supplies geotech with actual building loads for settlement verification" or any other real communication. Don't know if it's us afraid of initiating billable hours that somebody else will make us pay for, owners refusing to approve the billable time for that collaboration, or if we just forget that the geotech's report is usually preliminary with big warnings to confirm the design with them since their calculations had to assume loads and foundation sizes. So the geotech can't do it 100% on their own - we're all members of a design team and we have to work together to make sure the end result is better than a bunch of sheets of paper stapled together.

 

[MTNClimber]

I've had colleagues say they like it when someone else provides the boring locations because they believe it releases them from some amount or all liability for differing site condition claims.

That is not my preference. I believe some engineers need to remember we are a team to help the owner.

At the very least, please leave it open ended and collaborative between the geotech, structural, and civil. Sometimes borings or test pits are justified for other items too (i.e. retaining walls, temporary shoring, large site cuts, razed structures, ect.) or, as previously mentioned, geologic items (shallow rock, shallow groundwater, ect.) which is usually the geotech's job to look for clues for while reviewing the site conditions and existing documentation.