Any circumstance where you'd design without a geotech report? 5

[thejonster]

I have an owner who seems to think that helical piers just need to come to torque and voila no geotech report needed. This is an area I've spent too much time thinking about, despite previous replies seen to this same question (see bot of post), I'd like to know what y'all think with the info I've dug up.
For liability in design it seems most common to require a geotech report no matter what. This is despite the fact that the IBC allows to design without a geotech report, and the language only encourages an investigation when 'in doubt', which bugs me, and some guides I'm seeing seem to allow for this as a possibility. I'm wondering how common this is

My reasoning for always requiring the report is that the presumptive bearing values don't take settlement into account, and the owners who pinch pennies the hardest are probably also the ones who like to sue. BUT owners can do what they want and threaten to walk away, and some engineers choose to manage their risk and go for it on smaller projects.

Question:
Are there any circumstances where you design without a geotech report?
i.e.
Shallow retaining walls, low wood decks (low DL, spread out, settlement less important), accessory dwelling units, etc?
Or... Helical piers where you can (poorly) estimate bearing capacity with installation torque?

The code seems to convey that the building official should only require an investigation "where the classification, strength, or compressibility of the soil is in doubt"

Some helical manuf recommend an alternative soil investigation, see pg 24 (2-5)
HELI-PILE® SIMPLIFIED DESIGN AND INSPECTION GUIDE

http://helipile.com/userfiles/1210/files/DesGdMarch2020.pdf

Deep Foundations Institute (DFI), Helical Pile Foundation Design Guide

Helical Piles A Practical Guide to Design and Installation Howard A. Perko

Load test can exceed a geotech report, could it replace?

https://up.codes/viewer/colorado/ibc-2015/chapter/18/soils-and-foundations#1810.3.3.1.2

...
Included as background:

My summary of responses to this same question
[how to assume drilled pier skin friction and lateral capacity for patio column. no geotech. - Eng-Tips](

https://www.eng-tips.com/viewthread.cfm?qid=414545)

You can't. You need to know the type and characteristics of the subsoil.
[Drilled piers and load testing - Eng-Tips](

https://www.eng-tips.com/viewthread.cfm?qid=147441)

you are barking up a dangerous tree without any geotechnical input before you start your design.
I would be very very careful in showing a deep foundation system without a geotechnical report. I would also be very careful of an Owner/Client that is too cheap to pay for a soils report for a structure requiring deep foundations; this may not be a good client on a number of levels.
[Frost Adfreeze for Screw Pile Design - Foundation engineering - Eng-Tips](

https://www.eng-tips.com/viewthread.cfm?qid=436260)

i would still be very concerned without a geotech report.
Suggest to at least get some soil boring data of the site based on your concerns. It will help you also to choose the size of the helical piles. Also, I guess that the contractor has historical torque-resistance data for similar soil conditions you are dealing with, otherwise the torque - relationship you are planning to use may be useless.

 

[LBengineer]

Great question and I'll be interested to see some other responses to this question.
I design mostly retaining walls, shallow foundations and highway sign drilled shafts. As a general rule I always do a subsurface investigation. In a majority of these cases it does not mean a full Geotech report, but my company is pillaged to own our own drill and to have operators to go punch whatever holes we need. So for cases like the Drilled Shaft highway signs I send out 2 operators and they give me samples and boring logs. I would never feel good or put my name on something without knowing what's in the ground.

 

[phamENG]

I do it all the time, though I don't love it, for houses. All shallow footings. I list the presumptive value that I used and require the contractor to hire a geotechnical engineer to verify the allowable bearing pressures. If they fail to do so, that's on them.

Piles is another story altogether. There are plenty of...less than reputable companies out there that promise they can anchor the moon with their torque equation. Trouble is...they can't. All that torque equation tells you is the density of the soil the main helix is in. Oftentimes that's enough. But sometimes it isn't. Around here, we can get lenses of marine deposits at 20ft or so. So a helical hits that and torque reaches the spec and they call it good...but a 40ft boring would have shown you that 6 inches below the bearing helix the marine deposits end and you have prehistoric river muck for 15ft. So you load up the pile, and it blows through the 6" of stiff stuff and fails.

I require either a) a geotechnical report with load capacities and test pile recommendations (preferably dynamic measurements like CAPWAP). If they insist on not getting a geotech report, I might do some 'rule of thumb' pile designs using conservative methods for minor structures and then require load tests...but the fee would have to be uncommonly high with lots of liability releases. (One exception is beach houses - there are some acceptable design practices for small beach houses 'on stilts' that I'm comfortable using, but 'small beach house' isn't a common term down here anymore.)

 

[dik]

pham... all the literature I've read about screw piles is that for a given soil, there is a close relationship between the torque and the capacity. This varies with different soils.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

dik - that's true, if the soil column is relatively uniform or you have thick soil layers. That's not always the case. You don't know what's down there unless you sample it.

 

[Brad805]

My interest in doing projects without a geotech has waned greatly over the years. In the case of screw piles I am not sure if you can delegate the design or not. That is what is happening most of the time in our area. The good suppliers I work with have engineers with an advanced degree in screw pile design. Those engineers give the project thought, others, not so much. Over the years I have grown a distaste for screw piles. Not because of the pile design, but the suppliers you encounter on the smaller projects can be dreadful. In our area, it seems any ding dong that can afford a bobcat and torque head thinks they are a screw pile company. The certainty in correlations between torque and axial capacity some suggest is mind boggling. I have read a lot of geotech reports over the years and I have never come across one that states with absolute certainty that if I have X torque I get Y axial capacity. I see ranges and lots of different variables.

I will share a story about my past experience about using assumed parameters. My former boss had his MSc in geotech. He was a very practical, smart civil engineer. I worked under him for 10yrs or more. In that time we did a lot of pile design for our buildings. In some cases we had an independent geotech and others we used assumed parameters he would confirm on site. I was trained by him on site. Fast forward 10 or so more years. I was randomly selected by my professional association for a practice review. Great fun. One project was selected for a technical review and I had assumed the soil parameters for the CIP pile design. We had been on site many times during the pile installation and it was far better soil than we assumed. This was a small (5000t2) cold storage building at a commercial property. We had worked at the property before, and did have previous geotech information. Buildings like this are typically governed by the minimum pile spec geotechs always have in their report. I am registered as a structural engineer with the governing body, and it was their opinion we should have engaged a geotech for a project like this. Nothing more than a recommendation, but I was annoyed at the lack of consideration of the facts.

For residential jobs it always seems there is geotech that services those clients. Those characters usually seem reasonable for the project scopes, and not hiring them seems like a risk for me. Recently I had two small commercial projects where they wanted to avoid a geotech because the building code did not require it. One I walked away from, and the other client did get a very good geotech I have worked with many times.

If interested, there is a screw pile course being put on in the near future TLNT Screw Pile Course. It is a Canadian company, but I here soils South of the border have some similarities. I enrolled since I still get stuck dealing with them and it is good CPD hours.

 

[skeletron]

Shallow foundations ---> owner's are generally not interested in paying >$2000 for site investigation + report from geotech especially for a renovation or SFH. So I am forced to design to presumptive minimum values in Part 9 (similar to residential code) for Part 9 structures only. It's kind of hokey too because the code accepts a picket test as a reasonable method to get from soft to stiff soil.

Deep foundations ---> get a geotech or a specialist involved

Retaining walls ---> geotech mandatory

Part 4 buildings ---> geotech mandatory

Screw piles ---> provide the loads and request a ______ report. Usually the sealed report indicates the design and installation to achieve the torque. I only have worked with one company that does this, and had a chat with their engineer before undertaking the work.

*Not entirely my professional opinion and more of a hot take but...I'd almost rather the contractor spend money digging a deeper, wider hole and spend more time compacting and preparing the subgrade properly than the other options. Only applicable to urban areas, low-rise, residential, shallow footings etc. with a list of exceptions longer than a baby's arm...but that's the reality of it. Personally, I love reading geotechnical reports.

 

[Brad805]

"Shallow foundations ---> owner's are generally not interested in paying >$2000 for site investigation" I get that for small renos, but I have a hard time with that if the client is dropping $600k+ for a house.

 

[phamENG]

client is dropping $600k+ for a house

But they aren't. The bank is. And the lot, down payment, and the rest of the design team are already at about 175% of what they thought they had to pay out of pocket.

Sad reality that they can't see through it, but it's the way it is around here. That's one of the reasons I put the geotech on the contractor if the owner won't do it upfront. At least that way it gets lost in the $600k financing.

 

[skeletron]

Oh yeah, I have a hard time understanding the cost tradeoff argument too.

 

[phamENG]

I also have to fight the fact that 90% of the other guys doing residential around here don't require them, and most of the architects will happily tell homeowners they don't really need it since the code gives me values to use. Ever tried explaining bearing capacity vs allowable bearing pressure and settlement to a non civil engineer? Not a good way to make friends...

 

[Lomarandil]

Just to clarify my understanding a bit... There have been a few statements suggesting that prescriptive soil values (e.g. IBC) do not account for settlement. Is that entirely true?

I understand concerns about particularly sensitive structures, or about regions that geologically are likely to have varied stratigraphy. But I was under the impression (and practice) that prescriptive values do include some consideration for settlement -- appropriate for a structure with a continuous shallow footing and known soils within the influence depth.

When comparing a calculated strength capacity against the prescriptive value, there's often a huge variation. Are they accounting for some factor other than settlement?

----
just call me Lo.

 

[WinelandV]

Lo,

I believe the table in the IBC does account for settlement. But the question is, what soil type is it? I'm sure there's lots of pressure (especially in residential construction) to assume a more competent soil so that the foundation size is reduced.

My company will use the values in the table frequently - but we ALWAYS assume that the soil is the worst, which puts the GBP at 1500psf**. We tell our client right away - if you don't get a geotech report, we're going to assume the worst soil type and won't budge from that. If they want something better, then they need to get a geotech report done. Granted, I don't do residential, so who knows what pipe dreams the architects and contractors are telling the client about foundation sizes.

**A quick digression - assuming the worst soil profile doesn't always guarantee a conservative design. I had a site in Kentucky that had a geotech report done, and the allowable GBP was reported as 1200psf!

Please note that is a "v" (as in Violin) not a "y".

 

[thejonster]

Thank you guys for the thoughtful responses, great discussion

I asked my lawyer about putting the liability on the owner/contractor and he asked what would a judge or jury say when you proceeded without an investigation? I know we feel stuck here, but there are lawyers who only deal with design professionals, he said "how much is a soil investigation?? you don't want the kind of clients that skip that"
I'm pushing pretty hard on the scale towards avoiding this type of work, FWIW you can get 30 years experience here paying with a half hour phone call
[URL unfurl="true"]https://theaiatrust.com/legal/find-a-lawyer/[/url] and search by state

Has anyone heard differently from their lawyer? I'd love to hear a second opinion on this from a lawyer who deals with this often

The same owners who don't want a geotech report are the same ones who will happily sue your @$$ when they see cracks in the drywall representing their life's work. I get it though, that those reports aren't paid for by the loan, it's out of pocket for the owner

 

[thejonster]

Regarding the Heli-pile Simplified Design And Inspection Guide

they're suggesting a test probe, I'm guessing that's a lot cheaper and can tell you more of what's below bearing depth but I don't know exactly what it is, and if it is enough to keep us out of trouble. It sure wouldn't give a recommendation for settlement. Has anyone gone with something like that and regretted it? Is an ADU low enough risk to manage without geotech recommendation?

 

[thejonster]

I found a similar table with presumptive capacities in Title 24 HUD that states that settlement is NOT considered. I'm assuming that IBC didn't consider settlement either

[URL unfurl="true"]https://www.law.cornell.edu/cfr/text/24/3285.202[/url]

 

[Greenalleycat]

My take, with 7 years in structural but 12 years doing geotechnical investigations

Councils around here usually require geotech by default so take the choice out of my hands regardless, but let's pretend they didn't put that pressure on

For the vast majority of structures I would always get geotech of some kind
However, for typical work I much prefer structural engineers to undertake that work i.e. stick down a couple of hand augers and some DCP
I consistently get ridiculous crap back from geotech engineers and I'm getting pretty sick of it

My logic is, I have seen plenty of odd geotechnical conditions in 12yrs of doing this
Very few sites can you safely assume properties - it always needs to be verified by someone, at some point
But equally we don't want to go over the top with that - a geotech will charge you 2.5k and write a 12 report full of irrelevant rubbish that could have been dealt with for half the cost and a 2 page report

I also dislike the separation of geotech from structural - the building doesn't perform that way, so why do we separate the engineering?
The reality is that most geotechs only know soil and know nothing about structure, whereas engineers know structure and know something about soil
It makes more sense for us to undertake the investigations and then design to suit
I always leverage off this in my geotech work - if I cannot figure out my foundation design recommendation before leaving site then I haven't done my job. But I can only do that because I know how to design those solutions, what will work, what's economic, what can be built - most geotechs don't know that at all


Additionally, most of the haziness or conservatism in geotechnical work can be easily covered by slight tweaks in structural design
It's not hard to widen a foundation 50mm or whatever to cover some soft spots - but I prefer that the structural engineer has all that information, not the geotech, as then the designer can make a decision based off all the information, not just what's in a report

tl;dr I dislike most geotech engineers but think you need some form of geotech on most projects