Different ways to compute settlements of footings

[oldestguy]

Hi Friends:

After viewing thread 256-19870 today I am wondering if it might not be interesting to compare different methods for predicting settlements of footings on two types of soil: sands and fine grained materials.

The reason I wonder is that I have used a method I seldom hear about. I've used it since 1959 when it was publicized. My experience has found that its use has been rather close to field measurements (where that could be done). It is from an August 1959 ASCE paper of the SM division Number 2135 by B.K. Hough. It had many discussion contributions, including Ralph Peck (who criticized it). However at least one discussor compared this method to actual settlement measurements and his results show it came out quite close.

If two typical cases could be compared, by different methods, perhaps tied to some actual on-site soils and measured settlement data, it would be interesting to see the results.

I'm thinking the work in steel mills by D'Apolonia (sp) might be some data to use. I do not recall where that was published, but seem to recall the Hough method compared well with his findings.

The Hough method makes some generalized assumptions that simplify the computations, but they seem to work for me. It is included also in his second edition of his text "Basic Soils Engineering", Ronald Press Co.

It is easily set up under a spread sheet program or it can be used in a dedicated computer program with the simple 60 degree approximation for load spread. I also use it in a compiled Basic program that runs on some computers that can run DOS.

 

[BigH]

oldestguy: First let's celebrate that Cornell beat Colgate in their homecoming. They are up and down this season - losing by a big margin to Yale, winning big against Georgetown, losing big to Harvard, winning now against Colgate - seems like they like a every other game win - . . .

For settlement, I am not sure that I have Hough's method at hand. Perhaps you can ask SRE to post it in our page he has on his site.

For sands I usually use Schmertman's method (the first one, not the modified one as the first is easier to use). I check this against elastic settlement as given in most texts. And, of course, I look at the charts - N values vs footing widths. For the most part, I look to see if I have problems, not trying to find exact or near exact solutions. For the most this is sufficient - for most structures what difference if the settlment is 20 mm or 23 mm? or 15 mm? If we start getting more than 30 mm then we would want to look at things more closely. Years ago, in Ground Engineering, there was a paper that looked closely at some 15 different methods of settlement computations. Their conclusion at the end is to look at some three or four methods and take an average of the computed values as being a "reasonable" estimate of the value - I know that McCoy might not agree statistically and all that since these methods are coming more and more into the fore - but again, most of the reliability and other methods seem overkill, in my view, for typical foundations on typical soils (not offshore platforms, etc.).

As for fine grained soil settlements, of course, we would normally use the e-log p' or better yet strain-log p' methods although in some countries, they like the mv methods. The hardest part of using this method, as you can imagine, is the choice of preconsolidation or quasi-preconsolidation pressures - as it affects the percentage of new load taken in recompression or in vigin compression. There are other methods - such as the stress path methods. Som, in his geo book (Theory and Practice of Foundation Desing) has an example of the computations of fine grained settlement. The consolidation settlement was found to be 62 mm; with the Skempton-Bjerrum correction was 48 mm and with the stress path method 31 mm. One can see that there are wide variations - up to 30% of the average of computed settlements. I still think that if one is within 25% of the actual settlement in consolidation, you've had a pretty good day.

In the end, settlement computations ring bells as to whether you have a situation that you can live with - or not or have a situation that requires hard thought - as to amount of risk you are willing to take, the consequences of exceeding your predictions, etc.

Years ago, Dames and Moore were sued when a large diameter oil tank settled some 40 inches when it was estimated that only 30 inches would occur. The client tore down the tank and installed piles and then rebuilt the tank. D&M argued that even though the tank settled more, the tank was still operational and its service use was not impeded. Hence, the risk aversion of the client was not correct. ENR didn't give the final verdict - at least that I saw - but it always go me thinking that regardless of the settlements you "compute" it is only if they indicate a situation that the structure/use is affected is of real importance in the long run.

One interesting point is that of compaction levels and settlement - it is in a number of threads in the structural section - this is a topic that perhaps one or more of us can explore for a paper in our ETJPE ! Look for an announcement for papers soon!!!

[Cheers] to all - enjoy the baseball playoffs (Go TRIBE!!!)

 

[Mccoy]

I know that McCoy might not agree statistically and all that since these methods are coming more and more into the fore

BigH,
I realize that all methods proposed in the literature are approximate, often data values themselves are approximate, or sparse, or values are derived by uncertain correlations. So, the evaluation of settlements probably remains an art more than a science. Science may guide as, but, as you rightly point out, the final decision is taken more by the inches or half-inches scale, wether a structure may or may not tolerate that load. Many times we come across multi-layuered problems and, even if we apply the proper elastic modulus or edometric or consolidation parameter, we ignore that the pressure propagation is affected by changes in layers rigidity. If layers are two, OK, Poulos & Davis offer solutions, 2+ layers starts being akward. To this regard Bowles gives misleading indications, Fang is more precise but admits the approximation.
Teh most realistic is Tomlinson, he says straight out that calculating settlements should more properly be called an estimate

Oldestguy, I confess I never heard about Hough's method

 

[fattdad]

I was trained on using the Schmertman method for "sands" and one-dimensional consolidation theory for clays. That said, when dealing with pre-consolidated clays, the stress-strain relationship is often similar to a soil modulus value. So, I often assign a soil modulus (or hire out for dilatometer testing) for preconsolidated clays, silt/clays or silt/clay/sand soils.

When using a modulus value to characterize the stress-strain relationship, I then look at the Boussinesq stress increase beneath the footing (I typically take the center) and then integrate stress increase divided by modulus value with respect to depth. Essentially, it would be Z*deltaSigmaV/EsubS. Just remember to get everything into consistent units.

I've compared this to Schmertman and it's too close to say there's a difference.

f-d

¡papá gordo ain’t no madre flaca!

 

[emmgjld]

"Basic Soils Engineering" The well worn text I give to a new Geologist. Things are well explained, even if somewhat dated. That I consider this text to be very good probably explains why the binding is about wore out and in need of 'duct tape'.

I would be very interested in not only a comparison of computed settlement vs realized settlement But also real soil/structure heave compared to computed.

 

[vulcanhammer]

One document that does some comparison of the various settlement methods is EM 1110-1-2908, which can be found at

http://www.vulcanhammer.net/download/foundations.php

 

[Panars]

oldestguy-

You might not have seen the Hough method mentioned much in the past, but it's use should become more prevalent in the USA. The AASHTO LRFD specification has adopted the Hough method for estimating settlement of non-cohesive soils.

 

[vulcanhammer]

My document number is wrong; it should bt EM 1110-1-1904. Page link is correct. Sorry.

 

[BigH]

Can we get the Hough Method from somewhere?

 

[Panars]

You can find in the following FHWA publication.

Geotechnical Engineering Circular No. 6
Shallow Foundations
FHWA-IF-02-054

http://isddc.dot.gov/OLPFiles/FHWA/010943.pdf

 

[Mccoy]

I remember an interesting comparative research is cited in Poulos' Buchanan lcture on foundation settlements:

http://ceprofs.tamu.edu/briaud/buchanan web/Lectures/Eighth Buchanan Lecture.pdf

 

[BigH]

Peter: Many thanks for the reference. I'll look it up although I have probably already downloaded it before.

I find it interesting and ironic, though, that AASHTO would be putting forth a method of settlement calculation that seems to be at odds with nearly any and almost all tomes on the subject of geotechnical and foundation engineering. I am not trying to discredit Hough - it might be a very good method - but, as seen in my bibliography below, it is never mentioned as a method for settlements on sands/gravels. So, how did it get into the AASHTO specification? Would be interesting to know who the proponent of its inclusion was.

WSDOT in Chapter 8 of their (bridge?) manual talks about the Hough Method and how it overestimates settlement by a factor of 1.8 to 2.0. See comments below:

[blue]Based on experience (see also Kimmerling, 2002), the Hough method tends to overestimate settlement of dense sands, and underestimate settlement of very loose silty sands and silts. Kimmerling (2002) reports the results of full scale studies where on average the Hough Method (Hough, 1959) overestimated settlement by an average factor of 1.8 to 2.0, though some of the specific cases were close to 1.0. This does not mean that estimated settlements by this method can be reduced by a factor of 2.0. However, based on successful WSDOT experience, for footings on sands and gravels with N160 of 20 blows/ft or more, or sands and gravels that are otherwise known to be overconsolidated (e.g., sands subjected to preloading or deep compaction), reduction of the estimated Hough settlement by up to a factor of 1.5 may be considered, provided the geotechnical designer has not used aggressive soil parameters to account for the Hough method’s observed conservatism.[/blue]

I have also gone through the geotechnical hardbacks that I have with me here in Indonesia. Included are the CDN FDN Manual 4th Edition which is touted as the "up to date" tome with LRFD etc and the Handbook of Foundation Engineering by Day that is being pushed by ASCE as the new "bible". A listing of the books I have perused is below. None of them talk about Hough's method for settlement on sands and gravels (one has a reference to an estimate of Cc with eo value for clays - Murthy, I believe). I have Sowers on eFile and a few others, but I think the list speaks for itself.

[red]Day (2005) Fdn Engr Handbook (ASCE Press, Int’l Code
Council endorsement)[/red]
[green]Cdn Fdn Manual (2006) (4th Edition)[/green]
[red]Tomlinson (1995)Fdn Dsgn and Constrution (6th Ed)[/red]
[green]Som (2003) Theory and Pracitice of Fdn Dsgn[/green]
[red]Khan (1998) A Textbook of Geo Engr[/red]
[green]Murthy Principles of Soil Mechanics and Fdn Engr
(5th Ed)[/green]
[red]Krynine and Judd (1957) Principles of Engr Geology and
Geotechnics[/red]
[green]Kaniraj (1988) Dsgn Aids in Soil Mechanics and Fdn Engr[/green]
[red]Terzaghi Peck and Mesri (1996) Soil Mechanics in Engr
Practice (3rd Ed)[/red]
[green]Bowles (1996) Fdn Analysis and Dsgn (5th Ed)[/green]
[red]Conduto (1999) Geotechnical Engr Principles and Practice[/red]
[green]Fang (1991) Fdn Engr Handbook (2nd Ed)[/green]
[red]Lambe & Whitman Soil Mechanics[/red]
[green]Craig (1997) Soil Mechanics (6th Edition)[/green]
[red]Varghese (2005) Fdn Engr[/red]
[green]USACE EM-1110-1-1904 (1990) Settlement Analysis[/green]

 

[Panars]

The Hough Method is recommended in the FHWA publication I referenced above. The reason given is that does not under-predict settlement (see pages 88-93). I know this doesn't agree with the WSDOT text your quote above. I am just reporting what it says. The recommendation is based primarily on a study by Gifford (FHWA-RD-86-185).

The primary author for the FHWA publication is from Seattle, Washington, so I would hazard a guess that Tony Allen from WSDOT had some input. Also Jerry DiMaggio of FHWA was a technical consultant.

Mr. Allen is the vice-chair of the AASHTO technical committee T-15, which is responsible for the foundation sections of the AASHTO specifications (Sections 10 & 11 of LRFD). Mr. DiMaggio is also a member of T-15. So that would explain how the Hough Method came to be in AASHTO.

 

[BigH]

Peter: thanks for the info. It still amazes me, though, that a "obscure" (relatively) method has been proposed. Interesting the way things work.

 

[oldestguy]

Panars: thanks much.

I looked at the link you provided and it does pretty much copy the original paper by Hough. However, in his second edition text, "Basic Soil Engineering" (I have given my copy away) he mentiones this method for the first time, but there he cut the Bearing Capacity Factors about in half for some reason. That would explain the over estimating situation perhaps. When I have used the method, I used the chart as we see on fig 5-19, which was the original chart he provided for finding the B value in his ASCE paper. I also have run lab consolidation type tests and have developed the B value that way for silts and silty sands. These have varied some from that chart.

There also is a chart on Figure C1-4 that is like one have used on rare occasions, but I do calcs for many equal settlement lines. Then the chart is given to a structural engineer so he then can design for one settlement, say one inch, and then size all his footings for that settlement amount. If he wishes to save some on concrete, he uses a greater settlement (uniform) amount for all footings. This means the bearing pressures vary, but the idea is to keep the differential settlements to zero, hopefully. Of course we are looking at all footings at same elevation, etc.
Unfortunately, I have not followed up on what happened on these actual few cases. Maybe it all was fine and "as planned". No complaints however.

Perhaps the over-estimating result mentioned in these comments was the reason.

B.K. Hough, a MIT grad and Terzaghi student, a former Cornell professor,between about 1945 and 1958, died about 10 years ago at age of 80 something. His consulting practice was predominately in the north-east part of the States and it took, place from 1949 to about 1985. Prior to Cornell he was with Corps of Engineers.

I'm still waiting to see if some compilation of varous methods and actual measured settlements might have been done, but I do not have means to search.

 

[Panars]

I have scanned the Gifford report into a PDF file. Is there somewhere I can post it? It is about 7 meg in size. If you would like me to email it to you, send me an email. You can find one of my e-mail addresses by Googling "Peter Narsavage". At present, the first five links are all to me.

 

[vulcanhammer]

The Gifford report can be posted on vulcanhammer.net. Just visit the contact page and send me an email and I'll get back on where to send it.

 

[Panars]

vulcanhammer-
I got an "Mail delivery failed: returning message to sender" from my attempt to contact you through the vulcanhammer.net website. It says the paludavia address is invalid.

 

[vulcanhammer]

I have fixed the problem. Sorry for the inconvenience. You can also reach me through the contact form on vulcanhammer.info.

 

[oldestguy]

Hi Panars:

Nice job of finding the info and posting on Vulcan site. It pretty well answers my question.