Reinforced concrete retaining walls

[fargofarmer]

Friends,

Does anybody know how to design cantilever poured concrete retaining wall with radius? I've designed straight walls in the past but not the curved walls. It's tiered retaining wall. Do the tiered segmental wall design rules apply for the concrete walls or is there any different procedure? What I mean is how to consider the top tier wall load on the bottom tier wall?


Regards

 

[TXStructural]

Design would be the same for a curved wall, unless it is tightly curved like an arch, with resistance against thrust at the ends. And that would be a bit excessive for a retaining wall.

In a tiered wall, each wall must retain the surcharge load of the tiers above. The type of wall is unimportant, it is the weight pressing on the soil.

 

[FixedEarth]

See sketch.

http://www.FoundEng.com

 

[FixedEarth]

not sure why it is not attachning?

http://www.FoundEng.com

 

[fargofarmer]

TXStructural and FixedEarth,


Thank you for your suggestions. I have been in the field for the last 10 days and I didn't get chance to discuss this topic further.

FixedEarth: Thank you for the surcharge distribution sketch.

TXStructural: Do you think that tight curves in the retaining walls are not recommended?

Thx

 

[RFreund]

Fixedearth -

For a tiered wall condition don't you think its a little "unconservative" to assume that steep of an influence/failure plane? I know this is always highly debated. Also I know you have more experience than myself so I am not trying to say that you are wrong. I would just like more info on the subject so just for conversation, as this is one of my favorite topics...

It seems like whenever you are trying to establish a vertical stress in the soil then the assumption of a 2V:1H distribution is used. If you are looking for the Horizontal stress then a 1V:2H (very conservative) or a 1V:1H may be used. As if you use an elastic method (boussinesq) then you will see a lateral pressure no mater what the offset. I usually hear people say that the Rankine failure plane could be used but I believe this failure plane is different with a surcharge loading offset. I different approach (maybe overly conservative) would be to assume a 2H:1V or 1H:1V (I really am not sure which to use) from the back of the lower footing upward. If it does not intersect the upper wall then you are ok to design them independently. If it does intersect the upper wall then you have two options: 1.) Apply boussinesq using the bearing pressure of the upper wall over the width of the footing making sure the height of the lower wall is limited to the bottom of the upper wall. 2.) Apply the height of the soil above the lower as an offset surcharge with a width = the width of the upper wall heel + a 1V:1H from the back of the upper heel.

Like a said maybe not the most accurate way, but it should work, no?

Also if you are using passive pressure for the upper wall design than you should also account for that in the design of the lower wall if the active and passive failure planes intersect. Either Cap4000 or FixedEarth posted a good reference for this situation on a previous thread I can try to find if you would like it.

FixedEarth - Not sure if you had Bowels 3rd Edition but if so is the 4th edition any different/better?

EIT

 

[FixedEarth]

RF;

For a tiered wall, say 3 short walls (<6 ft stem Ht.), the surcharge of one wall to the other is only one concern. We also do static and seismic slope stability of upper wall onto the middle wall and then jump the middle wall and do a static/dynamic slope stability of the upper wall into the lowest wall,and then do a global static/dynamic of wall 1, 2, and 3 combined.

Again, if the walls are short and the slope stabilty calculations work out, generally, there is no concern. For walls, say talller than 6 ft, you can do Boussinesq lateral stress analysis.

Then there is individual wall stability, like overturning and sliding factors of safety. For tiered walls, it is best to suggest a shear key below the footing, possibly placed directly below the stem. This will avoid large heel footing and still will give you adequate sliding resistance. Also try to discount toe soil cover, unles you are in frost condition. Watch for daylight distance (descending slope height/3, min. of 2 m)requirement.

As an option, you can select gravel backfill above the heel to reduce your active thrust. If the lateral setbacks are tight, then you have no option but to use drilled pier support.

I have Bowles, 1st, 4th and 5th editions. His first edition is more hand calculation style and is my favorite one, except in lateral load analysis. I alos like the 4th ed., because most units are in English and the computer code emphasis is balanced.

Good point about passive/active wedge intersection. If you get a chance draw by hand a 3 tier system and do the geotechnical analysis. Then design the stem, footing, key, and see how all elements influence each other- it is revealing excercise!

http://www.FoundEng.com

 

[fargofarmer]

Friends,

Thank you for your suggestions for the tiered wall system. The goetechnical drilling for this tiered wall system was just finished . Interestingly, we encountered rock at shallower depths. In order to reduce the cost of rock excavation, the client wanted to construct a H-pile wall with precast concrete panels for the lower tier wall, which will be approximately 23 feet tall. The top two tier walls' footing will be on the bed rock. Will there any surcharge effect on H-pile wall in this case?

Thank you.