Using "Peck's Method" for design pressure 1

[civilsid]

I'm studying for my Civil P.E. and ran across a question that said to use Peck's method to find the maximum design pressure. None of my reference material says anything about this method and an internet search has also been fruitless. Any good references would be helpful.

Also looking for any good recommendations on passing the exam in April. Are most of the questions math based equation questions or are there a lot of "non-quantitative" type questions? (I will take water resources in the afternoon.)

 

[civilperson]

My guess would be the design of footings on sand/silt using corrected blow counts, (N values). The smallest average value of N is used to determine the allowable soil pressure.

 

[cvg]

Three books including two textbooks both classics in the field of geotechnical engineering, over 300,000 copies sold (per 1999)

With K. Terzaghi and G. Mesri, Soil Mechanics in Engineering Practice, 3rd Edition

With W.E. Hanson and T.H. Thornburn, Foundation Engineering

With A. Casagrande, L. Bjerrum and A.W. Skempton, From Theory to Practice in Soil Mechanics

http://www.ngi.no/english/files/publ207_edb_-_redigert.pdf

 

[cvg]

http://www.dca.ca.gov/pels/e_geo_refs_06.pdf

this is the reference list for the California exam which includes Peck's text

 

[civilperson]

cvg,
So what is "Peck's method" of finding maximum design soil pressure? I read those texts and do not remember any method with that label.

 

[PEinc]

Design pressure for what? Footings? Braced or tiedback retaining walls? Please be more specific.

 

[oldestguy]

Next question. Assuming it refers to the design of footings, does any one use it?

For the hundreds of jobs I have done, and knowing others in the field,it never is used.

In my view settlement usually governs over shear failure. More often than not, some form of presumptive bearing pressure as from a building code is used. On rare occasion, pressure meter allows a higher number for design.

But for some exam, probably written by a college professor, he had better know it.

 

[civilsid]

Thanks for the many replies. Here is the actual question:

A braced excavation is cut 12 ft deep in a clay with a unit weight of 125 lbf/ft3 and a cohesion of 3000 lbf/ft2. Using Peck’s method, the maximum design pressure is most nearly...

I guess that is a good point to note from "oldestguy". Some stuff is just very unlikely to be applicable in the real world. I know I'm not going to study and learn everything. I have made lots of notes in my CERM for esoteric stuff but if it is that far out there, I guess I will have to learn to let some of it go and not worry about it.

 

[mdhshanwil]

Looks like this is referring to Apparent Earth Pressure diagrams, orig developed by Terzaghi & Peck (mostly the latter, if I know the story correctly) based on field studies of instrumented braced excavations. They're semi-empirical and conservative. See the old standby: T&P (1967) or, in my PHT (1974) the discussion starts on ~pp461.

Note major modifications to the AEPDs (as used today) by Henkel (1971) and FHWA (1998).

Google "apparent earth pressure" and you'll find more...

Etc Etc

 

[oldestguy]

Looks like the question writer maybe is talking about designing the bracing system, not vertical bearing capacity; but who knows. You may have to spell out your interpretation of a vague question. Assuming the hole is dug for something to sit there, again, who knows.

In the lateral pressure case, Peck did some measuring of lateral earth pressures for systems like this in Chicago and he probably is asking for, not the average, but the highest unit lateral design pressure, not necessarily saying he wants it located with respect to vertical.

In that case, I would assume the trapezoid shown on page 347 and 350 of "Soil Mechanics in Engineering Practice" by Terzaghi and Peck, (My editon is copyrighted 1948) is what he is after. That I have used.

Then again, look at page 148 and you will see that there is a formula for an unsupported cut height. But is that a Peck method? Probably not.

An unconfined compressive strength of 3 tons per sq. ft., is a pretty stiff clay and maybe it will stand OK for 12 feet, unsupported. Chances are it has experienced shrinkage in the past and now you have a rather stiff clay with lots of cracks in it.

I once was almost buried in a 14 ft. test pit trench in that sort of stuff, so I'd not trust any stiff clay standing by itself unsuported.

Gets pretty sticky doesn't it?

I'd stay with the trapezoid, but given no phi value, you have to assume one.

 

[BigH]

Sounds to me as if it is the pressure diagram for a braced/strutted/anchored excavation - and the question writer was too lazy to include the other "names" saying Peck instead of Peck, et. al. These diagrams are in nearly any text book that discusses the subject. Usually the pressure diagram is "rectangular" but with some truncation in the upper or upper and lower zones depending on certain conditions (for cohesive soils) - for sands it is based on something like 0.65x... (Sorry, I didn't run to the shelf to get the exact equation!). The question writer should have been more precise in his referencing. (You realize that there are probably many new geotechs who haven't even heard of Peck???!!!) - Bowles, Das, perhaps, but Peck?? How many really know the contributions that Bjerrum made other than the vane correction factor???

 

[mdhshanwil]

BigH--

<Thread_drift>

I agree with you about the loss of our history as a profession. Especially sad considering how young we really are. As an example, I recently received a scanned copy of firsthand notes of Terzaghi's 1955 Applied Soil Mechanics course and his 1959 Engineering Geology course. Nobody seems to know what to do with them, or want to deal with them.

</Thread_drift>

 

[PEinc]

mdhshanwil,

Can you share the notes here with us? I'm sure Terzaghi won't mind.

 

[jdonville]

mdhshanwil,

SlideRuleEra has generously agreed to host interesting materials on his website

http://www.slideruleera.net

for EngTips folks in the past.

SRE, would you be intersted in this?!

Jeff

 

[mdhshanwil]

I'm with you as far as disseminating the knowledge. As I alluded to above, this is the sort of thing that needs to be archived and distributed.

The files are somewhat large, and would need to be hosted somewhere. 3 PDFs altogether, totaling 20 MB.

I'm sure Terzaghi won't mind, but I'll need to check with the note-taker or his proxy. I expect he won't either, but I need to make the inquiry.

Is there a way to email folks here without spamming the boards?

 

[oldestguy]

Anybody interested in Terzaghi'a soil mechanics text in German?

I once had a copy and gave it away, but could retrieve it if necessary.

Don't know the printing date, but got it from former employer who apparently was in his class about 1930ish. at MIT or Harvard.

Would be a nice thing if translated and reprinted I suppose.

 

[jdonville]

mdhshanwil,

Please the send the notes to me (if permitted) at jtdonvilleAThotmailDAWTcom (change uppercase words to appropriate symbols). I can forward to SRE or another if they agree.

Jeff

 

[fattdad]

Calculate the active earth pressure for the braced excavation depth, add 30 percent and make it a rectangle. That's it in a nutshell. If I recall correctly, there is a trapazoid configuration for sand and a rectangle for clay (or the other way around. I'm certain that the problem statement is referring to the classic work shown in the T&P textbook (as others have already stated).

Good luck on your exam.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

I'd love to get a hand on them, too. I'm a bit of a pack-rat. I know what you mean. I once saw original drawings of some of the important bridge in one Canadian province in the trash bin. I pulled out the trash and paid for it. Unbelievable. Why? The young turks didn't understand nor care about the history. Go back and read some of the early Rankine Lectures. Also go to the earlier ICSMFE conferences and read the introductions in the initial volume. Quite enlightening.
to all - who want to keep geotechnical history alive!!

 

[fattdad]

I too would welcome any/all notes from the lectures:

fatt-dad AT comcast DOT net

¡papá gordo ain’t no madre flaca!