CMU Segmental Retaining Wall Design Earth Pressure

[jagti]

Can we design a CMU block retaining wall for at rest condition or Block Retaining wall by their very nature are supposed to be designed for active earth pressure conditions.

Besides increased Geogrid Length and or Larger block wt what would be the other design implications due to higher earth pressure being assumed to be applied.

If active earth pressure prevails still it would be okay being a conservative design.

 

[dcarr82775]

You can certainly design it for at-rest loads. Whether or not you can get them is another thing.

 

[jdgengineer]

I would recommend reading through the Allan Block Engineering Manual (They are a manufacturer of one type of block walls).

You can obviously, always conservatively design for the at-rest condition, however if your wall is able to deflect, it's typically not economical to design for the high at-rest pressures.

Here's a quote from the AB Engineering Manual

"The at-rest condition occurs whenever a retaining wall is built. Some designers may prefer to take the conservative approach and design for the higher at-rest pressure rather than the active pressure. However, this is not necessary since the amount of wall movement required to cause the pressure to decrease from the at-rest level to the active level is very small. Studies of soil pressure on retaining walls have shown that the top of a retaining wall needs to move only 0.001 times the height of the wall in order for the pressure to drop to the active value."

 

[jagti]

What if the walls deflect more than 1mm per 1000mm wouldn't that cause performance issue.
what are the maximum permissible limits/ tolerances for wall lateral movement.

I have a situation where there as foundation wall & in front approx 4.0ft there is a need to create a block retaining wall for a ramp 7.0ft max ht.

 

[jdgengineer]

I don't think it would create a performance issue if the deflection is greater than .001. The quote I presented was that is the threshold between active to at rest pressures. Allan Block recommends designing for active pressure to lower the conservatism.

I'm not familiar with maximum permissible lateral wall movement tolerances. I would imagine if anything you would want to keep it within a certain allowance so as to eliminate the perception that the wall is failing even though I think strength wise it would be fine. Either way typical block wall is placed at an angle to the vertical so it shouldn't be a major issue. I'm not a block wall expert (we don't design it usually) but the engineering manual I quoted you should have the answers to your questions.

 

[RFreund]

Another good resources for segmental walls is NCMA's Design Manual for Segmental retaining walls.

They are flexible wall systems there are suppose to move, thus they are not taken down to frost. I believe NCMA gives construction tolerances (on the order of 1.5" in 10', 3" max. for hoizontal and vertical elevations and 2+- deg for batter) but I'm not sure they give overall maximum deflection/settlement they just say that it is expected in the first 18 months or so.

For specific questions you could contact one of the big name manufactures, they are very helpful (Allan Block, Keystone, VersaLok,etc).

To answer your question regarding what implications does a more conservative design have due to a conservative earth pressure assumption...none that I can think of. Atleast not from an engineering perspective, now from an owners point of view, possibly a different story.

EIT

 

[concretemasonry]

RFreund -

The firms you mentioned are not necessarily manufacturers, although they may have limited local production capacity in some geographic areas, but extensive expose internationally and experience in the use and standards approvals. There are 4 major firms (Allan Block, Allan Wall Systems, Keystone and Versalok) that are the developers and licensors of their products and use of the systems. They developed their similar systems starting in the 1980's and all have many licensed producers internationally (most of their business) and domestically and have many engineers on staff that work with associations, DOTs, government entities and essentially wrote the NCMA design recommendations because of the wide experience, projects and millions or billions of square feet used.

There are also numerous local suppliers with "knock-off" copies with slight differences because the majors would not license them, but little support and limited expose to real engineered walls 20' to 30' high.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RFreund]

Dick - good point. Sometimes the most confusing part of designing SRW's is the relationships between licensors/developer, manufacturer, supplier, contractor. Who owns who, who supplies what.

EIT