retaining walls coulomb vs rankine formula

[ketanco]

When calculating the active or passive earth pressure at retaining walls, if there is friction between wall and soil, and that friction angle is given , then we use coulomb formula (that long one) even if there is no incination of backfill or wall correct?

if so, in question 76 of six minute solutions of geotech, why did they use the simple rankine formula when calculating Ka - active earth pressure coefficient?

it is just a simple rectangular retaining wall, with no inclined geometry or no inclined backfill but the question gives you a friction angle between soil and the wall. i used that long coulomb formula for calculating Ka because of that reason. but they used rankine and it makes a little difference of course. i didnt understand why they used rankine, although the question gave the angle of friction between the wall and soil.

 

[John QPE]

My limited understanding of retaining walls is that Rankine is a simplified formula that ignores the friction between the wall and backfill because this is usually negligible. Coloumb, I believe, is a more accurate approach.

 

[desantmf]

Ketanco, your approach is correct. if you assume friction between the wall and soil you should default to use the coulomb formula to be more accurate. During the test it will most likely tell you which method to use, if not, your approach is correct.