Strip load influence on a retaining wall

[mar2805]

thread255-485674

Guys I know this is an old thread, but Im not clear with one thing that has been said.
"Surcharge above the heel loads the stem but helps resist overturning and sliding. Surcharge behind the heel loads the stem to a lesser degree and lowers sliding and overturning resistance"
I can undestand this, but if you have 10kn/m1 strip load above the heel of the footing, thats generating the horizontal load, and you say that since the load is above the heel it for help sliding.
Do you say this becasue it also generates vertical stresses on the heel?

 

[mar2805]

Forgot the picture

 

[EireChch]

Yes, sliding resistance is sigma'v x tan(interface phi)+c' you are increasing sigma'v

 

[PEinc]

The vertical strip surcharge load over the heel helps RESIST sliding and overturning (more normal force), but increases the bearing pressure at the heel which also may decrease the bearing pressure at the toe. Placing the strip load just off (beyond) the end of the heel will give higher sliding and overturning forces than if over the toe or if there is no strip surcharge load.

http://www.PeirceEngineering.com

 

[mar2805]

Can you make a small sketch please of the two cases you are talking aboth.
I just wanna be certain that Im 100% shure aboth this

 

[PEinc]

Here's a quick, sloppy sketch. A surcharge above the heel pushes laterally on the stem and downward on the heel. A surcharge beyond the heel pushes laterally on the wall stem and footing but not downward on the heel.

www.PeirceEngineering.com

 

[mar2805]

I havent seen any software that takes this into account...I might be wrong
Vertical surcharge presure on the heel...how would you determine it?...2:1 rule?

 

[PEinc]

Vertical surcharge pressure x width of heel behind stem = vertical surcharge pressure on the heel. In Design Example 3 in Basics of Retaining Wall Design, 9th edition (came with RetainPro program, now Enercalc; the example includes the vertical surcharge over the heel as a resisting moment when checking overturning. This could be unconservative if the surcharging vehicle is not sitting above the heel. I am not sure how various wall programs address the surcharge over the heel when checking overturning, sliding, and bearing. If you do the design calcs by hand, you can easily chose the worst case load combinations.

http://www.PeirceEngineering.com

 

[mar2805]

Im not claar with what you are saying.
Just to check.
YOu are saying that q=q"?
We are talking aboth strip load with defined width
See picture attached

 

[PEinc]

If you are using p[sub]s[/sub] = (q x ka) as your horizontal surcharge load, there is no width to consider. If you are using a Boussinesq surcharge analysis method, there can be both surcharge width (parallel with the face of the wall) and a surcharge length (perpendicular to the face of the wall).
If you have a 250 psf vertical surcharge load over the heel, there will be 250 psf vertical load on the heel that will help prevent overturning. The 250 psf also will add to the normal force that resists sliding.

http://www.PeirceEngineering.com

 

[mar2805]

Agree with you.
But in the case when the strip load is not directly above the heel?
Like in my previous post.
Maybe just half of the strip is above the heel....

 

[PEinc]

There can be different "critical" strip locations when checking the stem, overturning, sliding, and bearing pressure. Much also depends on whether you are using Boussinesq analysis or (q x Ka) to analyze the surcharge.

http://www.PeirceEngineering.com

 

[BridgeSmith]

But in the case when the strip load is not directly above the heel?
Like in my previous post.
Maybe just half of the strip is above the heel....

The strip load is typically assumed distribute wider as it goes through the soil. The spread angle will be dependent on the soil properties. It can be as steep as 2:1 (V:H) or as flat as 1:1.75, depending on the code or spec used, and the fill material properties. You'd need to sketch the load distribution to get the pressure magnitude and distribution width on the heel to estimate the effect on stability for a particular configuration.

Rod Smith, P.E., The artist formerly known as HotRod10