Cast In Place Concrete Retaining Wall With Piers

[OAPE]

Hello--

I have a project coming up where I will be replacing a cast-in-place retaining wall that is failing.

The wall will be 10 feet at the highest point. I am somewhat new to cast-in-place walls on piers so any advice would be greatly appreciated.

I have done some preliminary modeling and I can't make anything work above 8.5 feet. Even then it will require 24" piers at least 10 feet into supportive material.

Without getting into details, I wanted to see from your experience how tall do you generally build cast-in-place retaining walls before you need tie backs or another option?

Thanks for any advice.

 

[oldestguy]

As you might know, we don't do design here for you. However, with some details, we can suggest alternative, etc. For me, I don't know exactly what you mean by piers. I suspect that is the same as the usual term here (English) as piles. I also suspect you might be trying to use a computer program. Do you have access to someone who is more experienced? Sometimes these questions finally resolve themselves that outside help is needed. In any case to have useful information,we need ALL possible relevant info. For instance, why the replacement? Photos will help.

Edit: Looking at your info, it seems that those large diameter "piers" seem to be way over sized for a wall of this height, but of course I know nothing of site conditions yet.

 

[OAPE]

Piles=Piers(American) as far as I know.

I am using a computer program, for walls in the past I have done the calculations by hand and then with the computer program so I feel pretty good about it. Of course, using the program causes its own problems but that is another issue.

I am working for a small geotechnical company, I have access to a very experienced geotech but not direct access to an experienced structural. I am trying to push myself and learn how to do more complicated wall systems as I would like to move in that direction.

I have designed every style of wall except large cast in place walls with piers. Because my background is in civil/geotech, I'm missing some of the structural required for that. I have, "Foundation Engineering
Peck, Ralph B." and "Design Guide for Cantilevered Retaining Walls CRSI" on the way.

Site conditions:
Allow Soil Bearing = 3000 psf
Active Heel Pressure = 55 psf/ft
Passive Pressure = 390 psf/ft
Soil Density = 110.00 pcf
Footing||Soil Friction = 0.400
Soil height to ignore for passive pressure = 12.00 in

Criteria:
Retained Height = 8.00 ft
Slope Behind Wall = 5 to 1

Here are my current piers, as you said they seem way over-designed. I'm not sure if the program is being very conservative with a factor I'm missing. I guess I'm looking for what an experienced SE would expect the wall to look like with these conditions. Thanks for any help.

Pier Foundation
Not Restrained at top
Spacing = 4.50 ft
Diameter = 24.00 in
Pier Reinforcing = 8 - #7
Embedment = 12.00 ft
fc = 4,000 psi
fy = 60,000 psi
Added Lat. at pier top = 500.0 lbs
End soil bearing allow. = 6,000 psf
Skin friction = 500.0 psf
Allow. passive press. = 300 pcf
Ignore Pass. Pressure
from Pier Top = 1 ft
Mu = 146,528 ft-lbs
<phi>Mn = 169,543 ft-lbs
Vu = 74.05 psi
<phi>Vn = 94.87 psi
Total Vertical Load = 20,726 lbs
Vertical Capacity = 53,393 lbs

 

[OAPE]

I believe the wall failed due to age and being under-designed. Additionally, the adjacent building could be surcharging the wall but let's not get into that. My above question is more important to me than this specific wall. Thanks

 

[SlideRuleEra]

I have a project coming up where I will be replacing a cast-in-place retaining wall that is failing.

...the adjacent building could be surcharging the wall but let's not get into that.

Why would you want to ignore the facts?
Find out, or at least make an informed assumption.

If the building is creating a surcharge force on the existing wall there are at least two problems:

1. If (when) the existing wall is removed... the existing building may "move"... not good.
2. If the replacement wall is constructed in the same place, the replacement wall design needs to include the surcharge load.

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[spieng89]

How is the new wall design failing per the design results? In what manner? 10ft tall retaining wall is not too high, but as others have said surcharge from adjacent structure and any unintended water drainage from that structure can impact the load the retaining wall experiences.

 

[OAPE]

How is the new wall design failing per the design results? In what manner?

"The applied moment in the pier exceeds allowable."

"The applied shear in the pier exceeds allowable."

"The torsion of the grade beam exceeds allowable."

It works when the wall is 8ft high and spacing is 4.5 ft o.c. That is way too close for 24" piers, 6 ft or 10 ft would be much better.

So something isn't adding up. There is already 8 - #7 in the pier, the program is using IBC 2015.

 

[OAPE]

Why would you want to ignore the facts?
Find out, or at least make an informed assumption.

Absolutely, I agree, but regardless of this specific wall, I want to know what design would make this kind of wall work without the extra surcharge. It's really two sperate issues, I shouldn't have brought up the above example but this job made me start thinking about this type of wall.

I know the program I'm using is producing overly conservative results and beefy designs, I want to get a ball park baseline for what it takes to build a 10' ft high cast in place retaining wall on piers in moderate conditions.

 

[SlideRuleEra]

OAPE - Since you are using software in which you have confidence, for a moment, stop trying to design the wall. Instead, change the input and see what results come out. The very first thing I would try is to assume the piers that are 6' on center with 20' embedment.

Edit: Believe I have found the problem... or at least a large part of it.

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