Recommendations with Settlement in Mind 4

[MRM]

As with most geotech firms, we typically provide foundation/subgrade preparation recommendations for various projects so that the final structure will not experience large settlements or other problems. Those well-known "acceptable" settlements are usually 1" total and 1/2" differential.

In working on soft ground sites or marginal sites, it seems that providing recommendations that could result in settlements of 2 or 3", even when recommending a stiffened foundation, mat foundation, grade beams, or partially weight compensated foundation to help reduce differential settlements is taboo. I'm not even talking about an 8 story combined orphanage/condominium/retirement home/school/church structure either. I'm talking about mini storage unit buildings, pole barns, small cottages, and warehouses for example. It seems as though geotechs no longer consider the structure usage or the needs of the occupants anymore.

I've read papers from academians comfortably seated in their offices, reassuring us that 'unless differential settlement approaches 1.5" in 30', we won't have a structural problem in most cases.' I'll bet they're generally right too. In practice, however, we don't typically follow these guidelines. When questioned, any structural engineer you ask is likely to tell you, "we can't have more than 1" total and 1/2" differential." Those criteria are far-reaching in our profession. One exception: we once had to provide soil and foundation recommendations for a large steel frame cement plant baghouse structure that absolutely could not settle more than 1/2" total or 0.1" differentially! I'm not making that up. The column pads were 20 to 30 feet apart. That's a slope between column pads of about 0.00033. I had a tough time with that one since I couldn't absolutely guarantee that the isostatic rebound of our recently glaciated state (10,000years ago) wouldn't exceed that figure in 100 years! Expansion and contraction in the steel members will generate more internal stress than these settlement criteria. I thought those criteria had to be typos. They weren't. By the way, the column loads on that building were far from uniform. Some columns were loaded to 20k while others were loaded to 400k...

I'd like to hear your opinions on the philosophy of providing more economical recommendations to clients by allowing for more settlement than normal (say 1"-3"), by utilizing a beefier foundation that can transmit differential stresses more uniformly.

Do you feel that this line of thinking is something a clear-thinking engineering firm should ever even consider? I know that most everything we do and recommend needs to have a healthy dose of CYA, but sometimes, it gets ridiculous. Will we always need to recommend 80 foot piles every time we're worried that the foundation settlement might approach 1.5" for a steel frame building that the road commission will use to park trucks?

 

[GeoPaveTraffic]

MRM,

Good post, in short you are correct that discussing the amount of allowable settlement with the owner AND the structural engineer is the way to go. There are many structures that can take more than 1/2" differential settlement. Once the owner hears the options, and the costs associated with each, they are likely to drag even a very conservative structural engineer along.

Now for my soap box:

This problem points out what is wrong with geotechnical engineering today. Many, not all, geotechnical engineers have allowed themselves to be treated as a small inconvience that must be delt with before the project can be completed. They often do not deal with the owner of the project, they reguarly bid their work, they do less field and laboratory work than they should, and in the end they write a report that they could have written before the field work was even started.

Until geotechnical engineers stop bidding against themselves and start comunicating with the ownners of projects, this is not going to change. This current state of the practice is why I got out of geotechnical engineering (I just couldn't take it anymore) and into project management.

Sorry for the tirade, but it just kills me that we are the ones doing it to ourselves.

 

[dgillette]

Would seem to me that discussing the settlement criteria (and their effect$ on the bottom line) with the owner and structural engineer is good client service, and may be remembered when you are seeking future work (at least if it isn't to be awarded by low bid). There may be structural alternatives that can survive 1/2" of diff settlement cheaper than you can build a foundation to prevent it. Likewise, things like flexible utility connections could be cheaper than preventing 1" total settlement.

I've been very fortunate in my current job and to a large extent in my previous jobs to have geotechs generally driving the bus and dealing directly with owners (dam owners, miners, land developers, and even, believe it or not, one semi-rural county highway department!!) who are able to see the big picture.

 

[Ron]

GPT...good tirade. I agree and we need a larger audience!!

As for the settlement issue, there are many reasons you don't want to stretch it out too far, not the least of which is you can't count on the construction being what you expect and, if you remember your last liability seminar, you need to be careful about extrapolating data or even interpolating between borings.

We are working on land these days that may have been reclaimed, filled poorly and inconsistently, or was rejected as a building site back 20 years ago there was more land in now crowded areas on which to build.

 

[MRM]

GeoPave and dgillette,
I see what you mean when you're describing discussions with the owner and other project team members about different foundation options with varying potential settlement figures. I agree that would be important. However, where I'm coming from is that in our firm on "marginal soil" sites, we would generally recommend rather deep piles to hardpan, bedrock, or dense sands or gravels, or talk about extensive site preparation options like deep compaction or others, and skip any discussion on other options that could result in, heaven forbid, 2” of settlement. We generally wouldn’t even present these additional options involving settlements over the magic 1" figure. We essentially make a business decision by deciding for the client that the alternate options are not options for him. Don’t get me wrong though; piles and vibrocompaction and other extensive site preparation techniques have their place.

I also understand that this level of conservatism may be good business practice in this field though. As Ron said, there may be significant variation. Perhaps I’d feel differently if it was my name on the company’s logo.

Sometimes I wonder where the true value in geotechnical engineering comes into play, if, like GPT mentioned, a report can usually be written before the borings are done. Maybe the value we could potentially provide to a project could be drastically increased if we became more innovative and presented better thought out options to the client. This would require us to actually look at the soil borings we ordered, watch them drill the borings in the field, look at other existing structures, consider the building type, consider the client category, run a consolidation test or two on some of the soils, roll some clay in our hands, and run some LL’s and moisture contents to understand the soils we’re dealing with. I believe that when you understand the types of soils you have, you don’t need to fear them as much. Innovative options with just the right amount of conservatism would result.

Now to get management to go along with this…

 

[Ron]

MRM...I could get on an endless soapbox about how we engineers have screwed ourselves and our colleagues by degrading the profession for the sake of fast, misgotten, profit. I am convinced that when the Supreme Court struck down the provision against bidding in our Code of Ethics, it created the downward slide in our profession.

Professional services selection should ALWAYS be first based on qualification before price. Contractors and owners invariably take the opposite view..."just give me a piece of paper"..."if you're willing to sign it, I'm willing to rely on it"....so then many engineers will do a half-ass job (insufficient borings, fewer lab tests..as GPT stated) and provide a report loaded with qualifications (if they've been to a liability seminar) or not ("risk" mismanagement!).

Every time I see a Geotechnical firm come in with a price for almost any geotech investigation of less than $1500, I cringe. It just shouldn't be done.

If a mechanic working on a car can charge $80 an hour or more, why can't a geotechnical engineer have the balls to charge for his services in a professional and appropriate manner? If the geotechnical engineer is good, knows what he's doing and can write a decent report, he should be charging over half of that $1500 for his engineering expertise and report. That only leaves less than $750 for field and lab work. That leaves maybe a couple of 20-ft borings (if the site is close to your office) and no lab work....is that the way to progress the profession? NOPE!

...and it's not just the geotechs...structural, mechanical, civil....the same.

OK...I'll stop...the blood pressure is rising.

 

[MRM]

I agree with you wholeheartedly. It's very disappointing what we've done to ourselves. One thing that stood out at me is when you said, "If the geotechnical engineer is good, knows what he's doing and can write a decent report, he should be charging..."

You're, of course, correct although it seems that that's just it...in the profession that we've created for ourselves, the geotechnical engineer doesn't need to be good, doesn't need to know what he's doing, or even be capable of writing a decent report in order to practice. Since we've gone down this road, we can no longer give ourselves the budget in a project to actually look at and understand the soils and make real, project specific recommendations. I know I'm preaching to the choir here.

Here's another point of frustration for me; I believe that there are those who don't understand the basics of soil mechanics that have filled in within some companies as the "geotechnical engineer." I think that it's common to recruit the geologist to do this work when it needs to be checked off the list. They've gone from totally ignoring the pesky "unconsolidated debris/sediments" that exist from the surface of the earth to bedrock, and now give recommendations about what kind of footing would be appropriate for these "unconsolidated sediments."

I think that environmental scientists and/or engineers also tend to fill in as the geotechnical engineer. Heck, they're always drilling holes to install wells anyways. They must know something about putting a structural load on soil right?

Before the geologists and environmentals jump on me here, let me say that there are probably some that do actually understand some soil mechanics, but the vast majority do not. I'll be the first to admit that although I do know rock mechanics, I have very little knowledge of structural geology, for example. If it were up to me to find oil or gas in some Devonian rock formation, I couldn't. I don't know syncline from a sine curve. I don't know a B-tex from a Rid-x either.

How would a very talented geologist or environmental scientist, while not a geotechnical engineer, deal with the behavior of the "unconsolidated sediments?" Piles to bedrock of course! Although sometimes, maybe if you have nothing but mineral soils, sands and gravels, with really high numbers with the "hammer test," and no groundwater observed, you can recommend a shallow foundation with a bearing pressure of 2,000psf...provided the structure is a single story office building.

Now if you observe the "hammer test" go into the soil under its own weight, then it's piles until you hit something hard. Never mind that you're in competent fine to medium sands and the lack of resistance was caused by a driller who refused to keep a head of water on the augers to prevent heave or small amounts of localized liquefaction while driving the sampling spoon. Why let useful details about the soil types and drilling get in the way of the foundation recommendations?

Clients appear to be willing to accept these overly conservative "cookie cutter" recommendations, and companies are happy to provide them at a very very competitive fee. They need to check that box off the project list, remember.

By the way, to those practicing structural engineering, have you noticed any similar trends going on in your end of things?

 

[oldestguy]

Hey, great stuff here. No easy solutions.

My only comment, having been around a long time, been on rigs,got many dirty boots, etc., is that it is a good reputation that keeps the jobs coming in, in spite of bidding.

One thing that has been helpful is to get contractors on your side where you do a lot of work. If they see you are practical and don't give thse CYA recommendations all the time for no good reason, they put the word out. A reputation for adding job costs for no reason can be for the others, but not me.

Where the contractor has a good reputation and gets work on that basis, he usually will recommend you for the work, even it it not the chapest. They can tell the difference between being practical and not.

There is a lot more to geotechnical engineering than foundation settlement concerns. Fixing bad sites is only one.

 

[GeoPaveTraffic]

MRM,

Each company has to make a decision about how many and what options to include in each report. You can provide so many options that the client/owner just put the hands up and give up, that doesn't help anyone. When you have a marginal site, I would suggest going and visiting the client/owner; sit down and go over the site conditions, state your understanding of the structure, then talk about the possible foundation options, their costs and the associated risks. If the client/owner knows what they want based on the meeting, great, if not you have planted the seeds for them to start thinking about it.

Either way, prepare a thorough report. If the client knows what they want based on the meeting, in the report say we met, discussed options, and option xxx was choosen by the client. If no disision was made, then list all of the options with a discussion of the pros and cons in the report with general (not final) recommedations asking the client to contact you with the options that the would like final recommendations for. That way you are solving the problem as a team, not us and them.

oldestguy;

I agree with you and your comments about contractors. They were some of my best clients while working as a consultant. However, if you work in a mid to larger sized firm, you cannot get enough work from contractors alone to stay in business. Most of us have had clients that we truely enjoyed working for. They called, told us the project, we came up with a plan, they said go do it. However, unless you are a one or two person shop, those clients don't seem to produce enough work to keep you going and you end up working for a lot of clients that you would really rather not know. And there are many many more that fall into the later catagory than the first.

 

[oldestguy]

I went from a 120 person consulting business to myself alone. It had been a multi-disciplined operation, but geotech was a big part.

Both businesses had somme of these good clients, but you are right, to keep many persons working, some problems of how to get jobs came up. However, for some reason, in the larger business, we never were hit severely by competition, I think mainly due to the reputation of honesty and professionalism required by management. I wonder if our membership and activity in professional organizations helped? This was pushed by management.

Perhaps there now are more of these problems then when I was with the larger firm.

Of course, as a single person I could do some choosing and it also was more fun.

For those stuck in a competitive in house situation might consider the individual consultant business, but be sure those good clients like your work. And be prepared for long hours. Never had a client bitch about the cost either. Oh, not so; an attorney didn't want to pay at all, until I sued her.

Good luck all.

 

[eric1037]

I almost always discuss the tolerable settlements with the structural engineer prior to issuing a report. I have been frustrated many times when they say, "We always design for 1" total and 1/2" differential."

They don't seem to want to take the time to consider if the proposed construction can handle any more settlement.

Conversely, I have been involved in several projects that wanted to limit differential settlement to 1/4". We actually had to perform settlement analyses on each column and adjust the recommended bearing pressure. We also had to pepper the site with borings and couldn't just use a generalized profile.

Obviously, the costs are higher in the short term, but significant construction and maintenance costs can be realized.

I find that many times the problem is that some of the up-front activities like geotech and environmental come out-of-pocket before financing is in place. I have had some success by doing preliminary evaluations for estimating purposes, then following up with more in-depth evaluations once our client gets financing.