Doubt on Monobe Okabe Coefficients.

[ishvaaag]

I have ported the Monobe-Okabe expressions in a text to one Mathcad sheet, and I have replaced active where passive coefficient and viceversa because the labeled as active push was giving a higher value (say 2.29) than the labeled passive (say 0.6)...yet another text and AASHTO both confirm my original reference against my change.

Yet what the Monobe Okabe equations maybe saying is that once disengaged the wedge and downwards the earthquake push is bigger than when the wall in the displaceable building or retaining wall moves against the soil.

So please indicate whether the labeled active push by the Monobe-Okabe assumption is expected to be higher than the labeled passive or viceversa.

Thank you beforehand.

 

[geomo]

I am not sure, but I think the higher active pressure is a pseudo-static active pressure taking into account the inertia of the retained soil mass acting on the retaining wall. It is a crude way to model a dynamic response using static methods. For the passive coefficient, again I am not sure, but maybe it is a way to limit the shear strength of the passive wedge in case of a loss of shear strength due to pore water pressure build up during siesmic action. The higher pore water pressure (water has zero shear strength) reduces effective stress, which means less pressure available for passive resistance.

 

[ishvaaag]

I think it is likely that the Monobe-Okabe active pressure is to take into account the inertia of the retained soil mass acting on the retaining wall..., and the passive pressure given by the corresponding equation being a limit to the passive pressure in the worse earthquake conditions, that fits me well...yet my question is about if it is likely that the Monobe-Okabe active and passive push are such that the active is bigger than the passive...


In

Static and seismic passive earth pressure coefficients on rigid retaining structures:
Discussion 1
Jyant Kumar

a document you can dowonlad in pdf

I see values for earthquake passive push in the ordinary static ranges, so I still think the passive Monobe-Okabe should be bigger than the active.

...so all points to that I may be taking some angle with changed sign (I think no criteria there is for some).

 

[bredas]

Check your formula. There is a paper by Whitman and Seed, I believe 1975 ASCE Publication on Earth Retention systems where they note that the M-O formula for the passive coefficient has a sign error in it that has "propegated through the literature".

This may be the cause of your result.

 

[ishvaaag]

Thanks, Bredas...

Today I have been starting some part of a design that may need the use of the formulation and have rediscovered your post ... and that the reference you give may be already more or less available for all.

I am looking forward to see what comes within, I have just downloaded it.

http://nisee.berkeley.edu/library/seed_whitman/seed-and-whitman.pdf

Thanks again, and best wishes to everyone here.

 

[dgillette]

It's not implausible that the active pressure would be larger under very large accelerations, particularly as calculated by M-O. It's just Coulomb active/passive with a horizontal acceleration term put in, in effect rotating and increasing gravity. On the active side, the critical sliding surface flattens out as the acceleration increases. At a certain level of accel, it flattens out completely and M-O blows up. On the passive side, the pressure is reduced by the inertia of the passive block pulling it away from the structure. When you get near those accelerations, it's time to bag M-O and go to dynamic FEM or FDM analysis with Plaxis or FLAC.