Foundations

[chess5329]

I need help to undestand when to applied correction for overburden, this is in shallow foundations:

The problem is asking find "allowable bearing capacity"

This is my thinking: If the problem ask "ignored correction for overburden" then we used: qall= qult/FS, but if the problem don't say nothing should we automatically find qnet and

then use the formula qall=qnet/FS having qall=qult-Yd/FS where qnet =qult-YD

I'm using problem 53 six-min solutions.

Thanks for your replies.

 

[chess5329]

Anybody!

 

[MA_PE]

chess:

you can't assume that someone has the reference handy to see the actual problem details that you are referring to.

Post the complete problem statement up and maybe someone can provide some insight.

Otherwise I just guess and say the answer is b.)

 

[Badger]

I need help to undestand when to applied correction for overburden, this is in shallow foundations:
The problem is asking find "allowable bearing capacity"

This is my thinking: If the problem ask "ignored correction for overburden" then we used: qall= qult/FS, but if the problem don't say nothing should we automatically find qnet and

then use the formula qall=qnet/FS having qall=qult-Yd/FS where qnet =qult-YD

I'm using problem 53 six-min solutions.

Thanks for your replies.

Hi,

This also confused me to, since most problems I have looked at, outside of the CERM, require correction for overburden.

I beleive 6 min solution question 53 says to ignore corection for overburden. which makes the answer b. 1380. Hopefully in the exam they will state whether or not to consider correction for overburden. It would be tough to answer, at least in 6 min sol'n question 15, the answer would be b either way.

 

[chess5329]

I need help to undestand when to applied correction for overburden, this is in shallow foundations:
The problem is asking find "allowable bearing capacity"

This is my thinking: If the problem ask "ignored correction for overburden" then we used: qall= qult/FS, but if the problem don't say nothing should we automatically find qnet and

then use the formula qall=qnet/FS having qall=qult-Yd/FS where qnet =qult-YD

I'm using problem 53 six-min solutions.

Thanks for your replies.

Hi,

This also confused me to, since most problems I have looked at, outside of the CERM, require correction for overburden.

I beleive 6 min solution question 53 says to ignore corection for overburden. which makes the answer b. 1380. Hopefully in the exam they will state whether or not to consider correction for overburden. It would be tough to answer, at least in 6 min sol'n question 15, the answer would be b either way.

Hi Badger,

I hope also in the exam; the problem will make a clarification regarding this.

Thanks

 

[Polysloman]

NCEES Geotech # 518. Is that why they took the overburden pressure at el. -20ft. for 2x2 square precast pile. Asking for the Allowable Tensile capacity.

p. 166, Geotechnical Solution 518:

Replace line 6 with the following:

Overburden pressure at el. -20 ft, P 5(120) 30 (115) =2,325 psf

ERRATA for

Civil PE Sample Questions and Solutions

Copyright 2008

ISBN: 978-1-932613-31-5

Errata posted 08-26-2009

Page 8 of 17

Is that one of the rule of thumb for piles?

 

[Badger]

NCEES Geotech # 518. Is that why they took the overburden pressure at el. -20ft. for 2x2 square precast pile. Asking for the Allowable Tensile capacity.
p. 166, Geotechnical Solution 518:

Replace line 6 with the following:

Overburden pressure at el. -20 ft, P 5(120) 30 (115) =2,325 psf

ERRATA for

Civil PE Sample Questions and Solutions

Copyright 2008

ISBN: 978-1-932613-31-5

Errata posted 08-26-2009

Page 8 of 17

Is that one of the rule of thumb for piles?

Thanks for reminding me, I was going to redo this problem.

The original question says to ignore the resistance of the soft clay. So height used for skin resistance is only 30 ft, the amount the pile is embedded in the sand. So you wanted the average overburden pressure along the pile, which is equal to the overburden pressure at elv -20.

Overburden pressure at -20' = 5(120) + (30/2)(115) = 2325

The orignal solution was correct, Avg overburden pressure Po =[5(120) +5(120)+30(115)]/2 =2325

I couldn't figure out where the extra (5)(120) term came from.

It should of been Po =5(120)+ (30/2)(115) =2325

Soft clay at the top probably doesn't add much to twensile capacity, but it does add to the avg Po along the pile surface in the sand layer.

I don't know if this answers your question, but looking at this problem again was helpful to me.

So thanks.

 

[chess5329]

NCEES Geotech # 518. Is that why they took the overburden pressure at el. -20ft. for 2x2 square precast pile. Asking for the Allowable Tensile capacity.
p. 166, Geotechnical Solution 518:

Replace line 6 with the following:

Overburden pressure at el. -20 ft, P 5(120) 30 (115) =2,325 psf

ERRATA for

Civil PE Sample Questions and Solutions

Copyright 2008

ISBN: 978-1-932613-31-5

Errata posted 08-26-2009

Page 8 of 17

Is that one of the rule of thumb for piles?

Thanks for reminding me, I was going to redo this problem.

The original question says to ignore the resistance of the soft clay. So height used for skin resistance is only 30 ft, the amount the pile is embedded in the sand. So you wanted the average overburden pressure along the pile, which is equal to the overburden pressure at elv -20.

Overburden pressure at -20' = 5(120) + (30/2)(115) = 2325

The orignal solution was correct, Avg overburden pressure Po =[5(120) +5(120)+30(115)]/2 =2325

I couldn't figure out where the extra (5)(120) term came from.

It should of been Po =5(120)+ (30/2)(115) =2325

Soft clay at the top probably doesn't add much to twensile capacity, but it does add to the avg Po along the pile surface in the sand layer.

I don't know if this answers your question, but looking at this problem again was helpful to me.

So thanks.

Badger,

Could you explain why do we have to add the overburden pressure (5x120) to the avg. overburden pressure for sand (30x115)/2, if the problem specifies ignore the resistance of the soft clay?

Thanks

 

[Badger]

NCEES Geotech # 518. Is that why they took the overburden pressure at el. -20ft. for 2x2 square precast pile. Asking for the Allowable Tensile capacity.
p. 166, Geotechnical Solution 518:

Replace line 6 with the following:

Overburden pressure at el. -20 ft, P 5(120) 30 (115) =2,325 psf

ERRATA for

Civil PE Sample Questions and Solutions

Copyright 2008

ISBN: 978-1-932613-31-5

Errata posted 08-26-2009

Page 8 of 17

Is that one of the rule of thumb for piles?

Thanks for reminding me, I was going to redo this problem.

The original question says to ignore the resistance of the soft clay. So height used for skin resistance is only 30 ft, the amount the pile is embedded in the sand. So you wanted the average overburden pressure along the pile, which is equal to the overburden pressure at elv -20.

Overburden pressure at -20' = 5(120) + (30/2)(115) = 2325

The orignal solution was correct, Avg overburden pressure Po =[5(120) +5(120)+30(115)]/2 =2325

I couldn't figure out where the extra (5)(120) term came from.

It should of been Po =5(120)+ (30/2)(115) =2325

Soft clay at the top probably doesn't add much to twensile capacity, but it does add to the avg Po along the pile surface in the sand layer.

I don't know if this answers your question, but looking at this problem again was helpful to me.

So thanks.

Badger,

Could you explain why do we have to add the overburden pressure (5x120) to the avg. overburden pressure for sand (30x115)/2, if the problem specifies ignore the resistance of the soft clay?

Thanks

As I understand the question, it says to ignore the the friction or skin resistance of the soft clay.

The equation calls for the overburden pressure, i.e. the effective vertical stress which would include the clay layer. Po = Y1(H1) + Y2(H2). For the average vert stress for the midpoint of the pile embedment in the sand layer( from elv -5' to -35.)', if you assume that is at elv -20' the vert stress there is 5(120) + 15(115) = 2325.

I guess in this case the 5' of soft clay is kind of like a surcharge.

I have been struggling trying answer your question, I believe that this is it, after looking at eratta answer and drawing the pressure diagram.

FYI, I found the equation they used in The Principles of Foundation Engineering, Das 5 ed., p. 505, Eq'n 11.43. I think this is good reference.

I hope I have answered this correctly, let me know if this helps.

Thanks,

Also it has a Figure 11.19 for the eart pressure coefficient.

 

[chess5329]

Thanks Badger and yes, it does help. You are right regarding ignored skin resistance for the soft clay, and of course we have to add the initial vertical stress from top soil.

I have that clear, thanks again.

Now that you mentioned the DAS book, I have that book also and I'm planning to use it during both sessions, may I ask you what other books for Geos (afternoon) you planning to take?

 

[Badger]

Thanks Badger and yes, it does help. You are right regarding ignored skin resistance for the soft clay, and of course we have to add the initial vertical stress from top soil.I have that clear, thanks again.

Now that you mentioned the DAS book, I have that book also and I'm planning to use it during both sessions, may I ask you what other books for Geos (afternoon) you planning to take?

Hi,

The books I am planning on taking:

1) Principles of Foundation Engineering, DAS 5th Ed

2) Foundamental of Geotechnical Engineering, DAS 2nd Ed

3) Six Minute Solutions Geotechnical Problems Wolle 2nd Ed

4) Essentials of Soil Mechanics and Foundations, McCarthy 6th Ed

5) Soil Mech. ansd Foundation Design 201 Solved Problems, Affi

6) Traffic and Highway Engineering, Garber 1986

7) 2000 and 2008 NCEES CE Sample Questions and Solutions

8) Civil engineering Reference Manual and Practice Problems, Lineburg 8th ed

9) Civil PE Sample Examination, Lindeburg, 2nd 2008

10) Kaplan Civil Engineering Sample Exam 2008

11) 3 binder from a PE Review course a friend took

12) 3 ring binder with worked out geotech afternoon problems

13) 3 ring binder with worked out breath problems

14) fluid mechanics text,

15) a statics/strength of materials book

I expect I will use 1,3,8, and 12 for the afternoon and 6, 8,13, and 15 for the morning.

Now that listed them it seems like a lot of books, but if I can get 4-7 answers out of the extra 8-9 books it will be worth it.

I bought collapsible wheeled crate from Office depot that somebody recommended, it will hold most of the books. I will carry the CERM, calculators, and couple of books in a book bag, and it all fits.

The CERM, Das, and Six Minute solutions books have seemed to be the best references. i have found some answers to obscure stuff in the Highway Engineering Book and the McCarthy book.

Good Luck, less 48 hrs to the test.