Presumptive bearing pressure wording....a survey. 3

[WARose]

More and more I am seeing on drawings presumptive (allowable) bearing pressures and foundations designs based on them. So how is everyone wording this on your drawings?

What I am getting at with this is: what I am seeing (in many cases).......the contractor wouldn't know if he's reached competent soil or not. For example I saw one the other day that said: presumptive pressure [X ksf], contractor to excavate to competent soil, compact to 95% proctor, yadda, yadda.... In many cases I don't see any test required at all.

So what are you doing on your drawings? Thanks.

 

[BAretired]

A soil test carried out by a geotechnical firm is really the only good way to know where competent soil lies. Anything less than that means you are simply guessing.

BA

 

[skeletron]

In my scope of work (residential) no one wants to pay for the testing and report. I state that I use minimum code values "in lieu of available geotechnical information" and then include a list of provisions to make certain that the bearing subgrade is at least competent (ie. no organics, no wet soil, etc.). Then you leave it up to the risk of the contractor during construction. I'll go out to inspect the rebar and do a pocket pen on the soil for my own piece of mind...but in this type of work no one wants to pay the money for the actual data. There's an argument to be said that geotechnical reports are pretty boilerplate too, but there are some good ones out there that will give you a heads up (based on experience in the area) before conducting a testing program.

 

[WARose]

....and then include a list of provisions to make certain that the bearing subgrade is at least competent (ie. no organics, no wet soil, etc.).

But what I am after here is: what (specifically) do you say? I ask because I am thinking about modifying what I say on my own drawings.

Also, I had a contractor say to me that for the testing I am mandating (even with the presumptive route) it's just about as much cost as a soils report. So I need some ideas here to satisfy both worlds (i.e. good soil and cost control).

 

[BAretired]

In my neck of the woods, no one wants to pay for a soil test either, and in some situations, based on foreknowledge of the soil conditions in the area, it may be justifiable to dispense with soil testing.

With no knowledge of soil conditions, the engineer of record is taking some risk, I believe undue risk, by assuming an allowable soil bearing pressure. Some engineers assume a value of 1500 psf, which is probably overly conservative in most cases, but may not be attainable in some cases. Without some idea of soil conditions, a soil report should be mandatory, even if it is nothing more than a site visit by a geotech when the excavation has reached foundation bearing level.

BA

 

[JSPenney]

I generally specify on the drawing (for these small projects): Allowable bearing capacity min. 1500 psf to be confirmed by a geotechnical engineer. There's a few geotechnical engineers here that do a visual inspection in an open trench during construction for a low cost.

 

[milkshakelake]

In my area (NYC), subgrade inspection by a special inspector is required. That alleviates most of the problems. Besides that, I say something like "Foundations shall bear on 3 TSF soil" in the footing schedule. If I don't make that statement, DOB comes in hard and questions everything, which is annoying to say the least.

The best way is to have a geotechnical report and base foundation design off of that. But many areas don't use it, especially in NYC. At that point, things like allowable bearing pressure become guesswork and the liability for bearing pressure goes onto you. I have a trick to solve this, though it's far from accurate or noted in any textbook. Take the boring blow counts (usually diagonal addition) and multiply it by 2/10. That's the ksf. So if the diagonal added blow count is 22, the allowable bearing pressure is 4.4 ksf. Round it down to 4 ksf. You will be liable for this so it's better to have a geotechnical report, but it's impossible in some areas.

In response to skeletron, I'm sure he's done wayyy more projects than me and I respect him a lot. But I can't help but say that a statement like "in lieu of available geotechnical information" will never pass in the building department here, especially in an audit.

 

[phamENG]

I recently started doing the following:

"Shallow footings have been designed for a presumptive bearing pressure of 1500PSF based on Table R401.1 of the 2015 Virginia Residential Code. These values do not consider the potential for settlement. Contractor shall engage a geotechnical engineer to evaluate subsurface soils for potential foundation settlement. Proceeding without such an evaluation is at the contractor's sole risk and the contractor thereby assumes all responsibility and liability for adverse settlement (uniform, differential, and/or other)."

The odds of having a strength failure under the footing is pretty slim on a house. Settlement, on the other hand...well that happens daily around here.

 

[WARose]

I've been seeing that a lot too phamENG. That goes back to what I was talking about in terms of cost (a full blown geotech vs. a site visit to inspect just a couple of footings).

 

[MTNClimber]

I generally specify on the drawing (for these small projects): Allowable bearing capacity min. 1500 psf to be confirmed by a geotechnical engineer. There's a few geotechnical engineers here that do a visual inspection in an open trench during construction for a low cost.

We state the same thing for retaining wall projects when we're designing for the contractor. Sometimes geotechs don't have a completed site plan before the subsurface explorations and therefore there are zero borings near the wall alignment. I'm not taking responsibly for that. We state the max bearing pressure (applied) for each wall in the elevation view, we state the soil parameters used for the calcs on the General Notes sheet, and then state that a geotechnical engineer has to confirm that our assumptions are adequate before and during construction. For the foundation section on the General Notes sheet we state:

FOUNDATION SOILS SHALL BE OBSERVED BY THE OWNER'S GEOTECHNICAL ENGINEER OR ENGINEER'S REPRESENTATIVE WHO SHALL CONFIRM THAT THE SITE HAS BEEN PROPERLY PREPARED AND THE DESIGN PARAMETERS PRESENTED HEREIN ARE APPROPRIATE PRIOR TO FILL PLACEMENT. IF ACTUAL SOIL CONDITIONS DIFFER FROM DESIGN ASSUMPTIONS, THE WALL DESIGN MUST BE REISSUED AND MAY SUBSTANTIALLY CHANGE.

I'm pretty sure there are sites that no one is going out to check the foundation subgrade, but now they own that.

I know this is hard on small projects but it's just how we choose to do business. In my geographic areas of practice, it's not costly to hire a small geotech firm to go out, have the contractor dig a test pit to check for anything suspect, and write a letter saying that the conservative values used appear to be adequate. If the site is within 1 hour of the office and it's actually a small project, I've seen it cost $1,000 or less. In my opinion, if the contractor/owner doesn't like it they can go somewhere else for the design.

Edit: PhamENG's is perfect. Shed that liability. It's never worth it.

 

[phamENG]

WARose - the trouble is, just looking at it for a couple of footings won't tell them anything. Unless you have a very consistent soil profile in your region or you're rocky enough that it doesn't really matter, there are too many variables that can only be seen by taking a boring to the bottom of the influence depth. I've seen borings in my area with WOH material almost 100 feet deep after you get through a "crust" of about 6 feet with 2-5 blows per foot. Two blocks away, you have a 2' lense of WOH material a feet down but reasonable through the rest of the column. For the first one, the footing "inspection" probably would have looked okay, only to have serious settlement problems later. The second one might have caught it requiring a little over-excavation and then all's well.

I've also found that most contractors (and most engineers) aren't aware that the presumptive soil bearing pressures have nothing to do with settlement. I've had a handful of geotech reports come back around here recommending 1000psf for settlement concerns, or requiring piles. I had to put up with the contractors complaining about the piles because "this dirt looks great!" And it did. But if they dug down another 2 feet, they'd sing a different tune.

 

[Agent666]

In my opinion/experience, by not insisting on a proper level of geotechnical input, you're inheriting all of the risk if/when things go wrong. I wonder what your insurance would have to say about the matter? Geotechnical engineers have (in my opinion) good insurance for a reason and they are generally very risk aware, with some being very risk adverse.

https://engineervsheep.com

 

[manstrom]

xxxx psf soil bearing capacity to be confirmed by a geotechnical engineer or representative.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[skeletron]

A couple of things with some of these well-crafted statements that shed liability:
1. Is anyone actually reading your general notes?
2. Are your general notes starting to span multiple pages?
3. Is the language hiding in legalese? That is, are you grooming the language in such a way that the hammer-swinging-high-school-educated (no disrespect) contractor actually can't find out what you are saying?
4. Are you mentioning any of this stuff (settlement, risk of no geotech) in writing/verbal conversation to the client/contractor? Or are you hiding it in the general notes?
5. Is there a succinct way to: (a) say what you designed for, (b) the assumptions that you have made, and (c) the risk that still exists based on the assumptions.


I agree that my basic statement is...basic. But it is within the prescriptive values of the code for the low-scale projects I work on. Retaining wall? Go get a geotech. Anything in an area known for maybe being quirky? Go get a geotech. Anything multi-storey? Go get a geotech. Anything that's not on shallow foundations? Go get a geotech. But at some point you need to actually jump into the design and see if it is feasible based on some pretty standard assumptions. Then step back and see where your assumption stands. It can be mayhem out here with some of the jurisdictions, so I'm not surprised that NY or VA would be a little more restricted.

 

[phamENG]

skeletron - I always recommend a geotechnical report and try to explain the risks - including the fact that I'm not a geotechnical engineer and my insurance isn't going to cover a geotechnical issue. The majority of my work where this matters is with architects, and the builder doesn't come along until after the design is done. So if I can't convince the architect to convince the owner (I rarely get to talk to the owner) to pay for borings up front, my notes are sort of a last ditch effort to get it done - an extra $3k added on to a $450k construction budget to be financed may be more palatable than paying out of pocket up front. The good contractors are reading the general notes. Not all of them do, but there's not much I can do about that.

 

[Rabbit12]

I do a lot of work for industrial clients and there is no way they are paying for geotech report for small projects. Most of the time the structures can tolerate settlements without sacrificing performance and are lightly loaded. Even on those, I'll write a letter to the client/owner and communicate the risk of settlement and communicate they are taking the risk to save a couple grand.

 

[Enhineyero]

I use the wording "structural engineer have designed the foundations for the XXX assuming an allowable bearing capacity YYY on ZZZ soil, these assumptions shall be confirmed by a geotechnical engineer to be engaged by the builder prior to any works". I only do this for small projects. Like really small jobs say a timber deck or veranda. My recommendation to my clients is to get a geotech done, its cost next to nothing compared to the overall cost of building a house or a significant extension, plus savings on foundation sizes.

 

[phamENG]

savings on foundation sizes

This is a problem for me. Because everyone (in the industry) around here knows that a geotech is not going to save anything on the footings. With a few very rare exceptions, a geotech will either confirm code minimum or tell you that you need piles. I think I've seen two or three reports locally that have gone above 1500psf.

 

[Enhineyero]

This is a problem for me. Because everyone (in the industry) around here knows that a geotech is not going to save anything on the footings. With a few very rare exceptions, a geotech will either confirm code minimum or tell you that you need piles. I think I've seen two or three reports locally that have gone above 1500psf.

Good point, I realise now its dependent on your location. I practice in an area where soil varies a bit, so it does make a difference to get a Geotech especially at known reactive clay sites.