Rotated Segmental Retaining Wall 3

[PittEng88]

Hi All,

Our company recently received a submittal for a segmental retaining wall, that will be approximately 10-12 feet in height. Typically these submittals are pretty straight forward. However, the design engineer decided to rotate the entire wall, including the foundation base, 5 degrees back towards the retained soil on top of the 5 degree rotation he had from offsetting the blocks (see attached). Also, his calculations do not reflect this rotation properly, he is still using the equations as if the wall were vertical.

My main concern here is the global stability of the entire system. I feel that rotating the wall this way may cause the entire slope to give way. Is this thinking correct or am I way of base? Also, besides the fact that his calculations are incorrect can the wall even be constructed like what he is showing?

Also, there is no geogrid.

 

[emmgjld]

A 'Large Block Segmental Wall' such as this is possible to 12':
IN SOME SITUATIONS (overall geometry)
WITH SOME SUBGRADE SOILS but not the majority, in my experience
WITH SOME BACKFILL SOILS but not the majority, in my experience
WITH SOME RETAINED SOILS but not the majority, in my experience
WITH SOME OVERALL DRAINAGE CONDITIONS but not the majority, in my experience
Sorry! (No, not really) for the capitalization!
Get the ENTIRE STORY

 

[PEinc]

I think you may be a little "off base." Battering the retaining wall reduces the lateral pressure to the wall and reduces the vertical surcharge to the subsoils. I would expect a plumb wall to have a lower global safety factor than a battered wall. I assume that nobody has done a global stability analysis. In my experience, segmental wall designers rarely do global stability. They usually assume that global stability is OK or that it is someone else's problem. If the subsoils look less than competent, you can ask for the analysis. However, if the wall designer is designing a wall in the same location and to the same height as already schematically shown by the project engineer on a project's site drawings, it is not unreasonable for him to assume (but he should confirm) that the project engineer already determined the wall to be globally stable in the final condition.

http://www.PeirceEngineering.com

 

[oldestguy]

Your question "Is this OK?" No.

You state this also.
"My main concern here is the global stability of the entire system. I feel that rotating the wall this way may cause the entire slope to give way."

If this is your concern, do that stability analysis for two different wall lay-back slopes. My bet: the designer is safer, due to less weight above an otherwise uniform slope. Also, that fore slope and the up hill info should be extended to show what is there during the global check.

 

[PittEng88]

Oldestguy, do you mind expanding on why this is not "OK"?

PEinc, thanks for the info! I'm not a geotechnical engineer, so I appreciate the help. My thinking here was that due to the shallow cover of the toe and the batter of the wall, there could be the potential for a global stability problem.

Also just so I am aware in the future, what factors play into the global stability of a wall?

 

[oldestguy]

OG here. Reason for "no" is that the result of the global stability study will show there is less chance of failure with the wall leaning back more, per my bet. Remember the force of gravity is vertical. Think about the wall built at a 45 degree angle to the horizontal. Safer?, yes. The global stability study should look at all situations, not just in the vicinity of the wall alone.

 

[PEinc]

Geometry and soil properties affect the global stability, assuming all other loads and surcharges are the same for each wall. Geometry (wall height, slopes, wall batter, etc.) determines the weight of soil that drives the slip circle. The more a wall is battered, the lower the driving force and the higher the safety factor. The degree of battering should not affect the required depth of wall embedment below finished grade.

http://www.PeirceEngineering.com

 

[PittEng88]

Thanks everyone for your input, it was very helpful!

 

[Doctormo]

All - I think you will find that heavily battered walls have lower global factors of safety than vertical walls. As a wall approaches a slope, the "slope instability" gets greater. Global stability has nothing to do with earth pressure and in fact is totally unrelated.

If you can imagine the slip circle analysis, the weight of the wall structure over its base is primarily a resisting force. If we batter the wall, that weight is reduced with no effect on the driving forces from the slope behind the wall thus lower factor of safety.

The common way to "trick" an earth pressure calculation is to heavily batter a wall structure. The crib walls commonly did that with up to 4:1 batters (14 deg) which permitted a single crib to go quite high. The large block systems employ the same method reduce pressure and increase gravity wall height.

All that being said, global stability failures are not all the common for simple walls but many of the battered walls have low factors of safety when checked for global stability.

 

[PEinc]

With all due respect to Doctormo, there's is not much that I agree with in his above explanation. The more the ground surface or wall is battered, the less driving force you have and the more stability. The weight of the wall is not significantly more than the weight of the soil it retains.

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[RFreund]

Maybe we have different arguments here but....

If there is a vertical wall in a slope and the global stability indicates that the slope is stable. Keeping all else equal except that you now battered the wall, the slope stability will not decrease. You are removing surcharge on the soils below. Unless I'm missing something here...



EIT

http://www.HowToEngineer.com

 

[Doctormo]

PEInc - I think the problem is related to thinking retaining wall earth pressure is related to global stability analysis when it is not and that what was being asked originally. Your statement is exactly correct for a retaining wall sliding and overturning analysis but is not correct for a global stability analysis.

The best example of this is breaking a single wall into two tiers. The two tiers have less load on each of them than the taller single wall but the global stability FS of the tiered walls is much less than the single wall. The steeply battered wall example is similar to the tiered example although the difference is less. I would be happy to prepare two examples for you next week if needed to convince you.

RFreund - your example is interesting. If you place the wall at the bottom of the slope, the batter will reduce the weight slightly which is primarily a resisting force thus the factor of safety will go down slightly. If you place the wall at the top of the slope, the wall is primarily a driving force thus the batter would reduce the driving force and increase the FS slightly. If you batter the wall in the middle of the slope, negligible change.

Keep in mind that putting a wall in the middle of a slope is the worst thing one can for a global stability situation. It is much more common for the wall to be at the bottom or top of a slope and most slopes go for a distance then level off.

You can do a global stability analysis on a wall that is level on top and bottom and the factor of safety will go down based on the amount of batter modeled.

 

[hokie66]

Doctormo,
To a simple structural engineer, your theory is illogical. To quote Tschebotarioff, "Any measure which decreases the overturning moment...will increase the stability of a slope. This may be done, for instance, by cutting it back...to decrease the angle of the slope."

 

[MotorCity]

Ask yourself this question....if I remove the soil from behind the wall, is the wall stable? Absolutely not.

Would you stand behind a leaning pile of blocks not mechanically fastened together? The wall relies on the soil behind it for stability. Without the soil, you will have a pile of blocks.

 

[oldestguy]

Seems like a can of worms now. Do two evaluations. One of the wall itself. The second is the condition of the overall slope. called "global stability". My tip: Erase all the other comments from your mind.

 

[PEinc]

Attached are quick slope stability analyses, one for a plumb excavation face (no wall, soil only) and one with a battered face (again, no wall facing) where I set the closest termination limit at the top of slope as if a wall facing prevented sloughing of the sloped excavation face. The sloped case has a higher safety factor.

This is just a quick example with some soil properties I already had in an old file. I don't want to argue soil properties.

www.PeirceEngineering.com

 

[PEinc]

I thought we were talking about global stability, not the internal or structural stability of a leaning block wall that has no soil behind it.

http://www.PeirceEngineering.com

 

[emmgjld]

Watching how this discussion has gone (with some amazement) & then the discussion at the crosspost on the Structural forum (with more amazement), I will repeat the last line from my original answer: Get the ENTIRE STORY.
If you have serious questions,the entire story will include stability calcs,
1. global, 2. for footing & 3. retained mass.
and the design for GeoGrid or Fabric inclusions.
and any required subgrade improvements or geometry.

It should be obvious that increasing the face setback or even a face rotation should normally improve the global stability and the retained mass stability. The question often involves the footing stability. It is for the reason of footing (in)stability why I have had to place a mechanically stabilized fill matte beneath some walls. Now I commonly deal with collapsible soil horizons, which colors my perception.

MotorCity (Structural) asked; Ask yourself this question....if I remove the soil from behind the wall, is the wall stable?
Assuming you are utilizing 5 + 5 degree 'tilt', as you are considering the removal of the retained element (soil) you have not considered the reality of the construction. The original 'tilt' of 5 degrees creates a 12' high wall whether constructed with the 2'x2'x? blocks or a smaller block (Versa-Lok or whatever) would be an isolated structure which is marginal to unstable for tipping. Increasing the 'tilt' is, of course, worse but, is this realistic? The issue of pin connections or shear locks is immaterial. The wall is a system: Facing, Soil Backfill & any required Grid or Fabric inclusions.

 

[MotorCity]

@emmgjld:

I agree that it works as a system, as long as all of the components of the system remain in tact. My concern is that over time, there is a very real possibility that water accumulates in the soil behind the wall. This may result in erosion or voids developing behind the wall. When this happens, you no longer have the soil you are relying on to hold up the wall.
In my opinion, the scenario I describe above is a load case that must be considered.

 

[PEinc]

MotorCity, your last concern seems outlandish to me. If the soil behind the battered block wall washes away, what difference does it make if a couple of the blocks fall over? The bigger problem is that the water was not handled properly to begin with in the original design. Falling battered blocks, due to soil erosion, has nothing to do with global stability, as was questioned in the original post.

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