Retainging Wall Sliding Factor Problem 1

[kenrbc]

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

 

[youngstructural]

cap4000:

It is very rare to have a soil with an active resultant angle of 30! Most of the time this is a value ranging from 1/2 to 2/3 of the internal angle of friction, which is commonly taken as 30 when working with clays. That means your active resultant should be taken as a maximum of 20 in absence of specific testing of the soils; Afterall, we are talking about the founding soil, not a controlled fill!

That means you get a responsible maximum of 0.3639, not 0.5 or 0.6, when you ignore cohesion. hen dealing with dodgy ground (or sometimes dodgy contractors!) I would advocate using 1/2 of phi, thus tan(15), and only 0.2679 in absence of cohesion.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[RWF7437]

"How come there are many Ultra Block walls that taller than my wall still standing? I assumed they must have tie back huh? "

The reason there are many Ultra Block walls taller than 10 feet is that they are designed and built to use geotextile reinforced backfill. This makes the wall blocks and the backfill into a large, cohesive gravity wall system. Only by tying the blocks and soil together can such high walls be achieved, economically. These are usually described as Mechanically Stabilized Earth (MSE)wall systems.

Please see this link:

http://www.ultrablock.com/ultrablock_2_002.htm

 

[youngstructural]

Oh, and I am not an overkill type... I was on the side of permitting design to 1.05 in many applications. I just believe that you need to use the most accurate analysis you can when dealing with a situation outside of the norm.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[cap4000]

YS

You are correct with your reduced friction values. Its done for safety reasons per the Das book on pg 319.