Retaining Wall Height Concern??? 2

[bcdinh]

Hello engineers,

It just happened that I stumbled into this forum and I need help with a retaining wall detail. I am trying to build a house and the structure engineer gave me a detail for a retaining wall around the garage and front of the house due to a sloped downward lot. The initial detail is 7' from the top of the footing to the top of the slab and now I realize that I needed 8' instead. The engineer allows me to go up as high as 8' max but I have a concern that this wall is bit too tall. Could some experts out there confirm if this retaining is safe to go up as 8' tall? Please see attached PDF for retaining wall detail.

Thank you everyone in advance for your time!

 

[SlideRuleEra]

The engineer allows me to go up as high as 8' max but I have a concern that this wall is bit too tall.

The detail is not attached, but that's ok. Check with the Engineer again, I'll tell you why. The force on the wall from the soil behind it tends to make the wall both slide and overturn. This force is proportional to the square of the height of the soil behind the wall. Going from 7' to 8' height increases this sliding/overturning force by 31% (8² / 7² - 1) x 100 = 31%

There can be other loads to, maybe equipment or a building sitting on soil just behind the wall (called a surcharge load). The detail drawing probably does not include this information, but it is critical.

Get the Engineer to review their calculations so that nothing is overlooked.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

This is no simple wall thing,especially if there is any slope upward from the top of that wall. Any engineer dealing with this job should do the design work for ANY change of his plan. If any change on the job from his plan is made, the engineer may have no responsibility if a failure occurs, etc.

 

[bcdinh]

That's weird that my attached PDF didn't work. Would you please take a look at the PDF again? The slab remains at 7ft from top of footing to top of slab but the wall/stem is now 8ft tall. I found this spreadsheet online that calculated retaining wall detail and was just curious on how the engineer achieved the calculation. Please see attachment. I was reading a lot online and in this forum mentioned that the surcharge load is about 250-300psf. However, I can only achieve 105psf to maintain a and a 4ft backfill on the low side to maintain a positive safety of margin. Please advise is this check is adequate for a 8ft wall/stem.

I will insert the spreadsheet on the next post as this doesn't allow me to insert to files in one reply :-/

Thank you so much for everyone input!

 

[bcdinh]

Attached is the spreadsheet.

 

[oldestguy]

Is this a building basement? Also what is the slope of the ground going away from this wall? There may be a slo0e failure situation with the fill behind the wall as well s building weight. I'd get a geotech involved, since we really do not have a detailed knowledge of all possible problems that must be addressed here. If the engineer has done that, fine, but we can't critique without full info.

 

[bcdinh]

I'm attaching a sketch for better visualization. All the highlighted pink will be 8ft tall walls and in front of that will be filled with dirt to level it out. Currently, the lot is sloped down about 8% toward the back of the house and this required me to raise the front of the foundation taller to accommodate for the elevation. The back of the house will be short foundation wall (2-3ft) and pony wall it up to catch up the height of the front of the house.

 

[dik]

In an area with no frost penetration...

Dik

 

[JoshPlumSE]

It's a very rough sketch, but I don't think it's as much of a concern as others have suggested. There won't be a slope when the back fill is complete so that's not a consideration like "oldestguy" was worried about. Also, this may not really be a "cantilever wall". I suspect the house slab may provide restraint at the top of the wall. So, there is likely some conservatism in the assumption of a cantilever wall.

That being said, there is no doubt that you should still have your engineer review it. He / she is the one who knows the most about this project. We're just providing some EWAGs (educated wild ass guesses) based on the very limited information you have provided.

 

[bcdinh]

I agreed with JoshPlum that my cartoon isn't the best but do you need additional info? However, reading the threads do help broaden my view on how engineer community think about it. I find it more beneficial to have more input than just one but in reality not all designs are bullet proof. Some engineers may overlook something and it's always better to ask then shot myself in the foot.

I also forgot to mention the the soil guy said is good for 2000psf.

 

[dik]

JP:

Isn't that eWAGs, or with Apple, iWAGs.

Dik

 

[PEinc]

EDIT: Don't pay attention to this response. I looked at the wall section sketch incorrectly. I corrected my response farther down the thread after Hotod10 "woke me up."

It seems to me that you need to decide if the 8' high wall will act as a cantilevered wall or a top-supported wall. This decision should affect the location of the vertical reinforcing bars in the wall. As shown in your first CAD sketch, the steel is closer to the inside face of the wall which indicates a top-supported wall. However, the footing width looks like it is sized for a cantilevered wall.

http://www.PeirceEngineering.com

 

[BridgeSmith]

PEinc, the inside face of the wall, the side with the pressure against it, is the tension face of a cantilevered wall. If it was restrained at the top, you could possibly have tension on the outside face, requiring reinforcing in both faces.

 

[bcdinh]

Hi PEinc,

Thank you for responding to my thread. What I meant if I wanted to go 8' tall wall from the top of the footing to the top of the wall is still structural acceptable? My calculation on the spreadsheet said I need to backfill the low side at least 4' to not having the wall overturn if I wanted to a 8' wall. I think there need to be more rebars place at the bottom of the stem and wider footing is I go to a 8ft tall wall.

 

[BridgeSmith]

Even with the best backfill material (which phi = 30 degrees is not), I've never seen a ratio of footing width to wall height of less than 0.5 work. We almost never count on passive pressure from fill in front of a wall either, since there is rarely certainty of that material remaining in place. Fairly sure you need a wider footing.

 

[oldestguy]

Helping considerably is the many corners or jogs. I'd more look upon this as a house basement, but use those wide footings also at the outside garage wall as helping as a cantilevered wall. Then, more important, is those highest walls should have horizontal re-bars in the top sections, fully continuous, with the required laps and around the corners. Beyond the highest wall, I'd use a typical basement wall with no re bars except at the top in he front of the house only

 

[PEinc]

HotRod10, I was wrong. I must have had a "brain fart" or dyslexia when looking at the CAD sketch. The stem's vertical bars shown are on the correct side of the wall for a cantilevered wall, but not for a top-restrained wall. Also, the CAD sketch shows a SOG at the top of the wall but not a connected floor slab. Therefore not top support. I guess the high side of the wall is inside the building and the low side is outside the building. So, based on the sketch, it is currently a cantilevered wall. I agree with you that the footing width looks questionable for the given wall height. My bad! It's been a long day.

http://www.PeirceEngineering.com

 

[BridgeSmith]

Yeah PEinc, I figured it was something like that - you're used to looking at basement walls that have pressure from the outside. Our walls are next to highways, so there is no "normal" direction. I run into the same issue when someone posts a question about a frame system for a building - I keep wanting to put the biggest load on the top (where the bridge superstructure would be).

 

[SlideRuleEra]

I found this spreadsheet online... Please advise is this check is adequate for a 8ft wall/stem.
...always better to ask than shoot myself in the foot.

Ok, I have reviewed the Engineer's wall section drawing, your sketch, the "free" spreadsheet, and all of the posts on this thread to date. Have reached the following conclusions:

1) The spreadsheet seems to work reasonably well... but...

2) You are putting "garbage in" the spreadsheet and, not surprisingly, getting "garbage out".

Take your own advice, stop "shooting yourself in the foot". Talk to the Engineer who did the design.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[slabongrade]

As mentioned above, the forces increase with the square of the height, and also the moment arm
increases with height. So an 8 foot wall may not have a sufficient safety factor using the 7' wall design.
I didn't look at the drawing, but the footing width is "typically" around 60% of the height for a cantilever retaining wall.