NCEES Geotechnical ultimate pullout resistance & Peck earth pressure diagram

[icreep]

Questions regarding book "PE Civil: Geotechnical Sample Questions and Solutions" (April 2013 2nd printing)

Question 528. What is the source for the formula used in the solution, P = 2*L*sigma_v*C_i*tan(phi)?

Question 529. Where did the equation sigma_a come from?

Please see picture URL for clarification.

http://imageupper.com/g/?S020001001F13822470112424190

Thank you for your help.

 

[ketanco]

Q 528) This is a formula for calculating the Tension in a geogrid. it means, you are developing resisting force along the faces of your geogrid. area of grid PER FOOT = 1 foot x length of grid... you multiply this by 2 faces of grid x tan (friction angle) x interaction coefficiant x effective stress at the depth ..... (think of interaction coefficient as a friction factor) to obtain your resisting force

Q 529) That is the equation for calculating Peck pressure distribution, when there are braces in the excavation. The key thinking here is you can NOT assume a regular active pressure (triangular) distribution like in other retaining wall problems, where there are no braces. Here active pressures can NOT develop. So a different distribution is needed, and this formula gives you a rectangular distribution along the height. so it means, whichever strut you are looking at you have the same pressure,. so you must take the whole H = 25 feet , not the H=21 feet to the strut

 

[icreep]

It's more clear now. Thanks for your input.