Six minutes solution confusion problems

Dear colleagues, I need your professional help as I have concern for the followings problems in six minutes solutions book for after noon geo exam:-

• Problem 17 :- why use Rankin although there is friction angle value , also why neglect c term - (2cKa) ^.5 although there is value for C.
  
• PROBLEM 16 : why change from allowable to ultimate
  
• Problem 100 : it should mention that at midpoint of 6 ft layer.
  
• Problem 49 : the coefficient factors not matching with all values mentioned in reference books like Das book (Nq values depth, surcharge, 000 ETC)
  
• PROBLEM 83 :- NOT CORRECT , if you make summation for horizontal force will not equal zero (not equilibrium system ).
  
• Problem 81 : L at top should not include mobilized length for pressure.
  
• L at bottom level should not include the un mobilized length.
  

The answer is using the summation of un mobilized length fully with mobilized length fully which not realistic , which level this will happen?

-problem 42 :- as the drain from one side (due to rock underneath ) so the drainage thickness should be H/2.

Problem 38 : Nq=9.70 , it should be 8.34.

-problem 62 :- effective stress should be fixed at 15 D not increase more beyond the depth .

Problem 68:- alpha value not correct it should be 0.50 .

Problem 70: why using actual rather than block one ?

Problem 71 :- why use full depth although the rock under Neath , it should use half of thickness .

Problem 81 : - N (gamma) = 9.70 , not as per the das and other reference book , it should be 8.40 .

You should probably list the edition and printing that you have. Have you checked the PPI errata page? http://emars.ppi2pass.com/errata/ There seems to be a lot of errata for this book (looking at E1, P1 anyway).

Hi ptatohead,

yes i have check all ERRATA , not including those problems .

First of all which version of this book you have? I have the one I bought about a year ago when studying for my exam

16 – why are you saying ultimate? The load is divided by area and compared against allowable capacity. So what is wrong here? Unless I am missing something

17-i am assuming when you say “there is friction angle value” you refer to the one given at the bottom of the wall as the other one for lateral pressure is internal friction angle and would not cause to choose ranking or coulomb. Based on that assumption, I can tell you that using rankine is just for calculating the lateral component here. And since the calculation of lateral component is straightforward, and has nothing to do with the friction angle at the bottom (remember you are calculating only the lateral “push” force. ) as far as why we didn’t use c, the question states that it is a cohesionless soil and for those soils you do not use that component. Why? Because it does not crack like a clay soil which causes that component (the one you wrote by formula) to exist at the top portion (please also read about this from a textbook, to understand better about why this is so)

38-in my question 38 there is no Nq. So I think we have different book versions unless you wrote the question number incorrect.

42-what you say is only used for time of consolidation calculations, not for settlement, which is asked here, unless your question asked time of consolidation. And also, on the contrary of what you said, when there is impervious rock, you take the whole thickness anyway, because to whole layer must drain from one side only, so even for that calculation it should be H, not H/2

49-there are slight differences in some formulas on this because these were obtained with experimentation only.

62-i think they take all layers and calculate it one by one and not think about 15D etc… because there are different layers and the question wants you to calculate that capacity

68- I don’t know why they used 0.83 either, it should be 0.5-0.7 as far as I know but I am not able to check right now… but remember this is not precise as everything is based on experiments

70-didn’t understand your question, may be different question. My #70 is a pile group settling

71-again based on your previous comment and without even looking at the question, I think you need to study consolidation. If you have rock underneath, it means an impervious layer. So water can not escape from there. So you take full length. Only one drainage path, and the longest path water travgels is the full height

81- again N values can vary due to experimentation

83-did you convert the force per area or foot to force? If so, what I can tell without checking the solution in detail is that you solve these by allowing “only enough” passive force will be generated at the bottom, to withstand other forces. Does it check out that way?

100-volumetric strain is given so you must take the whole volume into consideration. The mid point you refer to is used when calculating the effective stress there already