retainging wall sliding factor problem

huangyhg

retainging wall sliding factor problem
i have 10' high concrete block 2.5'x2.5'x5' (w,h,l).  dl= 3750#/ft
p= .5(.32)110(10)^2 = 1760#
when i check the sliding factor, then i never get the safety value over 1.5.
u = .5;
f = .5 x 3750 = 1875
sliding factor = 1875 / 1760 = 1.1 which is less than 1.5
does anyone know why? or any other method to solve sliding problem?
tia
don't have time to look into your calcs, but if you can't satisfy sliding without making your footing ridiculously big, i would suggest a shear key on the bottom of the footing.
sliding by considering friction is a gross approximation, personally i only rarely use it...
are you looking at your passive pressure pushing back?  don't forget that takes away from your net horizontal load.
these are huge concrete block size 2.5'x2.5'x5'
they are stacked on top another.  crushed rock is located at the bottom.  there is no concrete footing.
if you don't have a footing, i don't think you're ever going to get 1.5.  traditionally, i add to the heel of the footing to get more weight of soil to increase the friction. if the wall hasn't been built, maybe you can add some layers of geotextile to tie the wall beyond the soil wedge.  and .5 is a pretty large friction factor.  if you can't get 1.5 with that, you're in trouble.
to develop passive pressure, you need to have the wall move some, so be careful.
in my neighborhood ( oregon ) these blocks go by several different names. "ultra block" is one of them.
the simple solution to your problem may be to simply rotate the lowest course of these blocks so that the 5 foot dimension is perpendicular to the exposed face of the wall. then check for sliding based on this configuration. if that doesn't work you may need to rotate two courses to get a f.s. of 1.5 or you may need to embed the first course so that you can get some passive pressure at the toe of the wall.
there is some good design guidance usually available from the block supplier.
if you can't get this let me know.
good luck
you can also improve the performance of such walls by designing them to be mechanically stabilized earth (mse) walls employing geogrid fabrics.
ignore cohesion to be conservative since "c" depends on soil moisture content remaining constant.
good luck
i would agree with most of the comments here.  i would rely on passive pressure either with backfill in front of wall or a shear key developing it.  
probably 1' of backfill in front would be ample to develop sliding resistance.
sliding is something that i always have trouble with satisfying and have often thought to myself that maybe a friction co-efficient of 0.5 is a poor representation.
generally passive resistance is ignored because there may be an excavtion infront of the wall (or block in your example) sometime in the future.
i would also like to hear of whether others are using a higher friction co-efficient than 0.5.  
friction is a poor representation?  where have i heard that before?
it's not like i said that two days ago and bothered to type in a full lenght cohesion, passive pressure, etc, formula...
oh well, maybe if my name was younggeotech... lol!
i just hope that kenrbc can solve the problem.  and i agree with you asixth, for the record, that passive pressure (ie: in the kp·0.5·g·(k+d+s)^2 term) would solve this problem very well.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...