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.

 

[JLNJ]

When I’m asked to help write the geotech RFQ I try to give as much info as I can on the loading and my expectations for the type of foundation (i.e. shallow or deep), all the while giving the geotech the latitude to design and recommend the best and most applicable system.

I DO put together a boring plan to give all the bidders a starting point on even footing.

 

[dik]

Something like:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1644540700/tips/Geotech_Survey_lmc1ll.docx[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1644540700/tips/Geotech_Survey_gdnxtz.pdf[/url]

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

-Dik

 

[GeoPaveTraffic]

While I know this was posted in the structural form to get the input of other structural engineers, I would like to provide some insight from the prospective of the geotechnical engineer. Remember it is not just the number of borings. Other items that are equally important are the depth of the boring, how the boring is drilled, sampling frequency, type of sampling, laboratory testing, and quality of the engineer(s) doing the analysis.

Ideally, the structural, civil, and geotechnical engineer would talk about the project and jointly develop the geotechnical scope of work to provide the needed information based on the project specifics. This almost never happens. Lacking a meeting, the geotechnical engineer should develop the scope based on the following information provided by the rest of the team:

1. Site plan showing cuts/fills, retaining walls, building footprint, and parking.
2. Estimated structural loads.
3. Estimated elevation of the lowest floor(s) of the structure(s).
4. Allowable settlements.
5. Overall risk tolerance of the project team and the project owner.

Based on the above info and the geotech's experience in the area an appropriate scope of work can be determined.

If the geotechnical engineer is not asking for the above information, then they really aren't preparing a scope of work for the specific project. Instead they are developing a scope based on their perception of the project, which may or may not be even close to correct.

Lastly, you need to consider how the geotechnical engineer is being selected. All too often the choice is made based on price. If you are telling the geotech that the choice will be made on price, then you will get the lowest price that the engineer thinks he can possibly seal without getting in too much trouble. That does not make for good engineering, it makes for very conservative engineering.

 

[Aesur]

Thank you for the input @GeoPaveTraffic! I agree with everything you said and hate that the Geotech is typically picked based on price. There are a few geotechs in my area that we talk with often and many more than I never hear a peep from, even when I call to talk with them. If it were up to me, I would rotate projects between 3 great geotechnical engineers in my local area and skip the rest.

 

[MacGruber22]

There are many, many reasons, both logically and anecdotally to avoid this as SEOR. As an example, you wouldn't want to be caught in a lawsuit if, say, an underground stream is later discovered within the footprint of a building recently constructed, in an area where no borings were taken. Do everything you can to assist the geotech as GeoPave mentions, but don't provide opinion regarding borings.



-Mac

 

[theCorkster]

Geotechs tend to develop a lot of understanding over years of work that rely on an understanding of a site's geologic conditions that should guide their selection of an appropriate scope of field work (aka borings).

When I worked with a regional firm, I have client's from another area give me grief over the scope of subsurface exploration because their experience was molded on the knowledge of subsurface conditions in their local area. When these developers would ask for a reduction in cost due to field exploration, I'd tell them that a reduced scope would result in pile foundations embedded into bedrock, and the construction cost would make them choke. As well stated by GeoPaveTraffic, when we understand what the load configurations and locations area, we can then, based on our understanding of the site geologic conditions, prepare a rationale field investigation plan.

With regard to risk, I will tell clients, that own the property and there is a risk-cost continuum we help them navigate. Higher field investigation costs generally help reduce risk, while lesser field investigations increase risk - and the risk is theirs. Geotechnical engineers did not create the site conditions, and short of removing the site soils with a teaspoon, we will never completely illuminate every risk. I walk from client's that think otherwise - in my opinion, they or their risk managers, are generally trying to unload their risk on someone else.

Finally, with respect to the Guidelines for Geotechnical Investigations, they are guidelines. I recently prepared a scope of work for a project on a hillside comprised of several feet of colluvial soil over geologically old, stable indurated sedimentary rock. Not familiar with the area, the non-local structural engineer wanted to add soil borings because that was what he was familiar with in geotechnical reports in his locale. Regardless of several conversations with him, he was adamant about adding the borings. I explained to him and the prospective client why the borings were useless with respect to the information they would provide. So sometimes the structural engineer's demands exceed what is rationally appropriate.