Geotechnical Forum

[chess5329]

Hi members, I hope you are interested in participate, so we can clarify and identify doubts regarding geotechnical problems.

I would like to start with this one

Thanks

11.9 A concrete pile 60 ft long has a cross section 15 in x 15 in is fully embedded in a Sat clay layer. D sat= 122 ld/ft3, soil friction = 0, and Cu 1450 lb/ft2.

Assume that the grounwater table lies below the tip of the pile. Determine the allowable load that the pile can carry. (Let FS=3). Use the (lambda) method to estimate the skin resistance.

 

[Momen]

We need good Moderator for Geo technical. Some one need come forward to join here

 

[Phalanx]

Hi members, I hope you are interested in participate, so we can clarify and identify doubts regarding geotechnical problems.
I would like to start with this one

Thanks

11.9 A concrete pile 60 ft long has a cross section 15 in x 15 in is fully embedded in a Sat clay layer. D sat= 122 ld/ft3, soil friction = 0, and Cu 1450 lb/ft2.

Assume that the grounwater table lies below the tip of the pile. Determine the allowable load that the pile can carry. (Let FS=3). Use the (lambda) method to estimate the skin resistance.

I'm certainly not a geotech guy, but know enough to get into trouble. I don't know the lamba method but could post a solution using the alpha method if that would be helpful.

 

[Badger]

Here is my solution, let me know if this what you guys get.

11.9

Given: Concrete pile 60 ft long, cross-section 15-in x 15-in, fully embedded in saturated clay layer. Assume that the groundwater table is below the pile tip, internal angle of friction = 0, sat = 122 pcf, cu = 1450 psf.

Find: The allowable pile capacity for a factor of safety, FS = 3. Use the (lambda), method to determine skin friction.

Qult= Qp+Qf

Lamda method: f =Lamda(Pva + 2cu) Qf = Lamda (Pvm+ 2cu) As

Cu = 1450 lb/ft2, the mean undrained shear strength for the depth of pile.

Pvm = mean effective vertical pressure for the embedded length.

Pvm = Y'(L/2) = (122 pcf) (60 ft/2) = 3660 psf

As = perimeter x length = 4 (15 in) (1 ft/12 in) (60 ft) = 300 ft^2

From Figure 13-34 on Page 484, Essentials of Soil Mechanics (McCarthy 5th ed.), for cm = 1450 psf,Lamda is about 0.17.

Qf = Lamda (Pvm + 2cu) As =0.17 (3660 psf + (2)1450 psf) (300 ft^2) = 334,560 lbs.

Qp = Atip cNc (Nc ≈ 9) equation 38.10 P. 38-2 CERM

Qp = (15 in/12 in/ft)^2 (1450 psf) (9) = 20,390.6 lbs

Qult= Qp+ Qf = 20,390.6 lbs + 334,560 lbs = 354,950.6 lbs

Qall = Qult /FS = (354,950.6 lbs)/3 = 118,316.9 lbs or ≈ 118 kips

 

[maximus808]

Sorry guys, I'm not taking the Geo Depth, but I thought I was decent in Geo until I saw this problem.

Badger, is there an area in the CERM that explains this method. I can't find the Lambda equation method? Wow, would something like this pop up in the morning portion for Geo you think? It seems pretty in depth for a breadth question. Thanks.

 

[PE_2009]

Sorry guys, I'm not taking the Geo Depth, but I thought I was decent in Geo until I saw this problem.
Badger, is there an area in the CERM that explains this method. I can't find the Lambda equation method? Wow, would something like this pop up in the morning portion for Geo you think? It seems pretty in depth for a breadth question. Thanks.

This kind of question won't be even asked in depth section. The best way to prepare for exam is to focus on topics given in NCEES specifications. There is no point wasting time for this kind of questions.

 

[Badger]

I doubt if you will run into a pile question using the Lamda method. It took to long to solve.

The Lamda method is explained on p.687 of Foundation Engineering Handbook by Hsai-Yang Fang, I do not have this book, but you can google it and probably see the equation.

It is highly probable you will solve a couple of pile problems involving end bearing or skin friction, or both.

So you will need to be familar with alpha and beta methods in the CERM. I would also be familar with pile groups, too

So you will probably see something requiring these equations (see Six Minute Solution Geotech #74)

Qult = Qp + Qf; Qp =Ap x c x Nc, where Nc = 9 (eqn 38.10 CERM 8th), and Qf = pL(fs), where p = perimeter, L=length, and fs = alpha x cohesion.

There are only 4 questions in deep foundations, but a pile question could come up under Other Topics. I had a pile question in the breath section, I think it was in the construction section, it was something I had not seen before, I think I got it right. I would not have the first time I took the test.

I would definitely read the pile section and work some of the problems in the CERM.

 

[Phalanx]

This is wrong.

This kind of question won't be even asked in depth section.

Deep foundations are listed on the NCEES outline for the geotech and structural depth exams. I agree with Badger in knowing how to solve these problems using the alpha and beta methods. These methods are pretty straight forward and don't take too much time.

 

[Badger]

Somebody said on anold thread something like: if you are in doubt how to solve a geotechnical problem, is to default to the methods used in the CERM, unless the problem specifies a different method.

I think that was helpful to me.

I would add on the geotechnical subjects not very well addressed in the CERM, I would default to Das' Principles of Foundation Engineering (I used the 5th edition). Things like geotextile, sheet pile, surchares, and temporary structures.

I didn't think CERM chapter 37 Retaining Walls had enough problems, only 3 that seemed pretty hard until I did more problems form Six Minute Solutions and Das. I would recommend doing the 3 CERM problems a few times.

I don't like using charts, so I had a hard time understanding Boussinesq solutions. I would learn that inside and out, also CERM chapter 40 Special Soil Topics.

 

[maximus808]

I had a hard time understanding the shallow foundation and pile questions from the CERM. I ordered this book:

http://www.amazon.com/Civil-Engineering-Al...e/dp/0071502653

All in one PE Review Guide. I hope this book explains those concepts better. I do have the 6 min. solutions for Geo which I will do. I'm decent at classification and phase relationships but shallow and deep foundations is my weakness in geotech. I didn't pass this recent PE exam and I struggled in the geotech portion of the breadth (62%.) Any recommendations on how to get better at these topics or what books are best to brush up on these. Thanks guys.

 

[Badger]

I had a hard time understanding the shallow foundation and pile questions from the CERM. I ordered this book:
http://www.amazon.com/Civil-Engineering-Al...e/dp/0071502653

All in one PE Review Guide. I hope this book explains those concepts better. I do have the 6 min. solutions for Geo which I will do. I'm decent at classification and phase relationships but shallow and deep foundations is my weakness in geotech. I didn't pass this recent PE exam and I struggled in the geotech portion of the breadth (62%.) Any recommendations on how to get better at these topics or what books are best to brush up on these. Thanks guys.

I purchased that book too, before I found out I passed. For $67 it is a good book. Probably pretty good for the breath section. Not enough to pass the geotechnical afternoon alone.

As to the morning geotechnical breath I would do all the geotechnical afternoon questions in the NCEES 2008 sample exam. Of the four morning questions one of those might show up. I would also do the afternoon structural problem 520, Construction afternoon problems 501, 512,514,517, 519, and 520.

Actually I would do all the afternoon geotechnical, construction,transportation, Water resources and environmental problems. They really helped me for the morning session.

If you are doing geotechnical in the afternoon, I would do all the Six Minute Solutions Questions.

I would also make sure I did well in the morning. The first time I took the test I was not well prepared, passed the morning struggled in the afternoon. This last time the morning was harder, but I was better prepared. Also with crossover type questions in construction and structural, the morning was about 12 geotechnical questions, and that helped me.

In the afternoon it was all I could do to finish 33 questions, and then quickly finished one more. I was pretty confident with the ones I answered. But again there were questions I had never seen before.

Good Luck

Good Luck

 

[maximus808]

Thanks Badger for the advice. Right now, I'm finishing up the 6 min. solutions for Transportation and doing Chelapati's problems for Trans as that's my depth and I know I can improve on those areas.

As for Geo, I have the 6 min. solutions for the breadth and depth and will attempt to do those after I finish the 6 min. solution for trans. For some reason, I like the way the 6 min. solutions are laid out with breadth problems and then depth problems. I have the Lindeburg practice problem book but it's immense and sometimes I feel overwhelmed by the vast number of problems in it. I'm signed up for the Passing Zone so hopefully there are problems assigned from that book. In college we used Das's Foundation Book but I don't remember much from it. Did you find that book useful?

Since this is my second attempt, I have my CERM and manuals tabbed and now it' about doing as much problems as possible. I will focus on Trans, Geo, and Water. Construction and Econ I feel that I'm ok and the review classes should help me brush up on those morning questions. Structural is probably my weakest subject but I kind of have a feel of what type of questions to focus on.

Thanks again.

 

[Phalanx]

As for Geo, I have the 6 min. solutions for the breadth and depth and will attempt to do those after I finish the 6 min. solution for trans. For some reason, I like the way the 6 min. solutions are laid out with breadth problems and then depth problems. I have the Lindeburg practice problem book but it's immense and sometimes I feel overwhelmed by the vast number of problems in it. I'm signed up for the Passing Zone so hopefully there are problems assigned from that book. In college we used Das's Foundation Book but I don't remember much from it. Did you find that book useful?
Thanks again.

I agree that the Lindeburg practice problems book can be intimidating at first glance. Think about it this way. That book covers 88 chapters worth of material, but several chapters can be skipped depending on your depth module. For example, I took the structural depth, and here is a list of chapters that I skipped all-together (no reading, no practice problems).

1-13, 22-34, 70-77. Out of the 88 chapters, I completely skipped 34. In addition to that, I barely worked any of the structural problems since that was my area of expertise. I figured that if I could do the afternoon problems, the morning problems would be pretty easy.

As for the Das book, I have one too, but I didn't use it when studying. I only used the CERM which covers all of the geotech material on the morning exam.

 

[maximus808]

Would you consider the lindeburg practice problems useful? I'm doing the 6 min solution problems now but I want to do some of those lindburg ones too. It basically comes down to taking the time to do the problems. I have stop worrying about what book to use but just do it. I remember telling myself that I didn't want to overwhelm myself with material but after the exam I wish i did more than bank on the ncees practice problems. Don't get me wrong, some of those were helpful but I shouldn't have relied on them. Thanks everyone for your advice. I hope to pass this this time

 

[Ambrug20]

Would you consider the lindeburg practice problems useful? I'm doing the 6 min solution problems now but I want to do some of those lindburg ones too. It basically comes down to taking the time to do the problems. I have stop worrying about what book to use but just do it. I remember telling myself that I didn't want to overwhelm myself with material but after the exam I wish i did more than bank on the ncees practice problems. Don't get me wrong, some of those were helpful but I shouldn't have relied on them. Thanks everyone for your advice. I hope to pass this this time

I consider that the Lindeburg Practice Problems are very useful. Some of the problems are hard and required a lot of info and 3-5 steps to finish the problem. Each step could be a separate question on the exam. In this case you learn a lot and it also teach you simple math to use. (Many of us know the formula, but don’t know how to use it in the problem). These problems give you good practice and help you to review a lot of material.

 

[maximus808]

I agree Tanya. I'm taking the trans depth again and realized how good the practice problems are. To me they are more realistic to the exam than the ncees problems. The ncees practice problems to me were not as helpful as they were too straight forward. I felt my stength in trans was geometric design but realized i wasnt as good as i thought. This time it will be different i hope