Shoring analysis looking at slope stability only

[CANeng11]

If you have a temporary retaining wall with a near vertical cut behind and the void filled with loose fill, can you analyze the wall to only take the load from a slip plane failure with the resultant acting near the bottom of the wall? Or once the slope stability failure occurs, will there now be an active pressure behind the wall that needs to be designed for?

Hopefully this is clear. I have attached a quick sketch

 

[MTNClimber]

Why would you place loose fill behind the wall and how would you expect it to stand up at a near vertical?

Usually the "slope failure" would really be a global stability failure, something a cantilever wall isn't really designed to resist. If a bottom up constructed wall would experience global stability failure then it would either be buried deeper or switched a different wall system.

I don't have the soil parameters or wall/slope geometry to check the failure plane but I think you have drawn isn't typically the controlling case. In this case, I would assume the "slope failure" will be your near vertical cut or near vertical loose fill sloughing and hitting the wall. Which means you should just backfill the wall like its always done, in compacted lifts without a gap.

 

[CANeng11]

This was proposed to me. I think their reasoning for using loose fill is to avoid having to analyze it with an active pressure on the back of the wall, but I would think that once the slip failure occurred, that would then be a case of active pressure on the wall.

 

[MTNClimber]

Let’s back up a bit. Why is the wall there in the first place? Is it just a “fence” wall or is it an actually retaining wall?

 

[oldestguy]

My bet would be the loose fill will permit the cut bank to move slightly to develop active pressure. However design or "at -rest" to be sure. Likely the loose fill will hang up and any settlement will be slight.

 

[p_kefa]

Make sure the proposal is constructible, if you are being asked to design it.
If wall is cantilevered, conservatively design it for active pressure plus pressure due to possible surcharge (which if this is temporary, should be the normal 2'-0" surcharge), whatever the condition of the fill will be. At rest pressure occurs when the top of the wall is not free to move. I hope you have a soil report. If not, be conservative in value of active pressure which I have seen as high as 45 psf or about 1/3x125 pcf. Geotech engineers tend to be conservative.

 

[CANeng11]

MTNClimber, it is a wall, but it seems I'm being asked to look at it as a fence. Basically, they are saying it shouldn't be looked at with any pressure behind, but instead it would only be there to hold the soil if a slip occurred. I don't believe I can ignore pressures behind the wall because if a slip does occur, then there would now be pressure on the wall. Does this make sense? They claim the pressure would only be at the base of the wall if a slip occurs.

 

[Mad Mike]

I don't believe you should ignore the pressures behind the wall- I doubt that "they" can predict with any certainty where a failure plane would be located with respect to the wall profile.

I've been persuaded to make a number of decisions I never agreed with, and almost always have come to regret them- nobody is going to thank you for handing over a lean design...

All the best,
Mike

 

[MTNClimber]

What they are asking for is for you to design something that is impossible. Loosely placed fill cannot stand up at a near vertical. Even if you were to build it, as soon as some additional loading would occur (water, seismic, construction vibrations) it will likely fail. Soil needs to be sloped back. Depending on the subsurface conditions, loading, erosion potential, it’s typically a 1:1 to 2:1 for temporary and 2.5:1 to 5:1 for permanent slopes.

You will have to design this as a standard retaining wall unless the soil is sloped back correctly. Only if the slope is self supported that the wall can be considered a fence.

 

[PEinc]

Too little information; too many answers.

I am assuming that there is an existing, near vertical slope in front of which someone wants to build a cantilevered, temporary, sheeting wall. I also assume that someone wants to fill the space between the wall and the near vertical slope with loose soil. Maybe I am wrong. If not, first, in the US, it would be illegal per OSHA for people to work in front of a near vertical, unsupported slope if higher than 5 feet. So, how would someone legally build the wall without first having to backfill in front of the near vertical soil? If, before building the wall, backfill is properly placed to support the unsafe slope, then building the wall would be typical, top-down construction with active earth pressure.

EDIT: I just saw the last post about the wall being a "fence" to catch the slope when it fails. Now you could have some dynamic load being applied to possibly the full height of the wall. This would be had to model for earth pressure. You probably need to calculate the force of the failing soil (F = MA). What is A, the deceleration of the failing soil mass? When you get the force, I would apply it uniformly to the full, cantilevered height of the wall. Sounds like fun. Why not just stack some big gravity blocks (Redi-Rock?) and backfill behind the blocks?

http://www.PeirceEngineering.com

 

[aeoliantexan]

I believe walls should be designed not for what the soil pressure will be under ideal conditions, but for what it might be under conceivable conditions. We have all looked at a vertical cut in cohesive soil and asked ourselves why a wall is needed. Because the soil probably won't stand like that forever. Erosion, frost action, creep, vertical cracks....nature hates that cut.

If the cut is standing vertical it must have considerable cohesion and probably has low permeability. After the loose backfill settles and creates a low spot behind the wall, how much rainfall will it take to completely saturate the loose backfill and subject the wall to a fluid pressure greater than clean water? My mentor told me never to design a wall that wouldn't support water. I think there is considerable wisdom in that.