Horizontal pressure on rataing wall from backfilled material

[mar2805]

Hi!
Please see attached image.
How to calculate the horizontal pressure that the backfilled material will have on the retaining wall?
The material will be deposited and will have this trinagular shape behined the wall.
Usualy we are presented with problem that say that the material is compacted behined the wall up to the walls height.
This is a bit different.
Can you help

 

[BridgeSmith]

Below is the soil pressure coefficient calculation out of the AASHTO spec. You should be able to enter Beta as a negative angle corresponding to your slope into the equations, and get a decent approximation. The soil pressure is bound to be really low, though.

Structurally, you could design it for a level backfill condition, and it would be plenty conservative.

For the sliding stability, it would seem that would be conservative, also, but I'm not completely sure of that.

 

[mar2805]

Bridgesmith thanky you very much!
Any chance you could post an image that your text is relating to?

 

[BridgeSmith]

I won't be able to post the Figure from my spec. until Thursday, since I'll be out of the office tomorrow.

Beta is the angle of the backfill from horizontal, in this case, it will be around -30, if your sketch is correct.

Gamma, the friction angle between the wall and backfill can be conservatively approximated as half of phi, the angle of internal friction of the soil.

The angle of the back face of the wall looks to be vertical. If so, theta = 90 degrees.

 

[PEinc]

Look for a reference on lateral earth pressure caused by compaction of backfill behind a wall. Then you need to modify it to account for the downward sloping backfill. You may not find a reference for sloping, compacted backfill pressure.

http://www.PeirceEngineering.com

 

[mar2805]

BridgeSMith...any chance for that picture?

@PEinc
You mean the shifting of the force to the half of the height of the backfill and not on 1/3 from the bottom?

 

[BridgeSmith]

Sorry I didn't get this up earlier, but here ya go. I hope it helps.

 

[mar2805]

I dont think this picture describes my scenario.
What you have on the picture is an full soil profile behind the wall. I dont have this.
Please see my picture

 

[BridgeSmith]

I dont think this picture describes my scenario.
What you have on the picture is an full soil profile behind the wall. I dont have this.
Please see my picture

You're not likely to find a diagram for your scenario. It's not common to build a retaining wall to hold back a diminishing slope. What is the purpose of backfilling this?

 

[WinelandV]

If this were my design, I'd likely just assume that the backfill is level behind the wall(even though it's not) for lateral loading purposes. When checking overturning, though, I would only utilize the actual weight of material on the heel.

Assuming one could calculate the reduced lateral loading due to a declining backfill
1) how much material savings would you actually get?
2) what's going to prevent the end user from filling the retained area flush for a distance of 2*H?

Please note that is a "v" (as in Violin) not a "y".

 

[BridgeSmith]

If this were my design, I'd likely just assume that the backfill is level behind the wall(even though it's not) for lateral loading purposes. When checking overturning, though, I would only utilize the actual weight of material on the heel.

That's what I would do, too.

 

[mar2805]

Ill guess ill just lower the safty factor from 2 to 1,50 for overturning.
Other than that I dont see any other solution

 

[PEinc]

mar2805, no. Look for lateral earth pressure due to compaction of backfill. Look at NAVFAC's Foundations and Earth Structures, Design Manual 7.2, Page 7.2-76 & 77 and Figure 13.

http://www.PeirceEngineering.com

 

[mar2805]

There wont be any compaction behind this wall!
Only deposited material

 

[PEinc]

WinelandV makes some good points. I would use a loose to medium unit weight for the soil, a relatively low soil friction angle, and a Coulomb equation for the active earth pressure coefficient of downward sloping soil behind the wall.

http://www.PeirceEngineering.com