Dry-Stack masonry Walls 5

[ClarkBart]

Hi:

I am working on a project that involved a significant amount of blasting. The owner wants to use this stone for some retaining walls around the new construction. several of these wall will be over 4' and as much as 8'. The wall will be constructed as a dry laid/stacked stone. The building inspector wants stamped design and drawings for the wall citing the International Building Code requirements for engineered wall design over 4'. The International code 1806.1 states roughly that the wall needs to resist overturn and sliding and not have excessive earth pressure.

The design procedure for a typical reinforced concrete wall or even segmental wall is well known. How have you approached this problem for a dry-stacked wall? Thanks,

 

[concretemasonry]

As I see it, the problem you will have to resolve is the unknown ability of a dry stacked variable stone wall of unknown shapes and sizes would be the ability to have shear resistance. I can visual a wall where the individual courses or units just slide off the lower units, especially with moisture and fines getting into relatively horizontal joints providing a weak shear plane. Coming up with some proven history of strength for whatever type of

The segmental concrete masonry units (SRWs) have features such a holes for pins between course or molded shear lugs. Because the various units and are manufactured in the same molds, the shapes are consistent, so it is relatively easy to do testing and instrumentation for different heights and types of backfill materials/conditions. With that information, design standards could be developed for both unreinforced or reinforced soil backfill and has used for a couple of decades. - I know the effect of geo-grid between the courses and lubricating fines effects has also be tested.

Dick

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

 

[SteelPE]

I usually leave retaining walls designed in materials other than concrete and masonry up to the site engineers (it looks like you are one of those).

I have always pondered how one does the design of such walls. I'm sure someone out there has some information. I imagine the only information you would need to know is the coefficient of friction of rock to rock. Then you would need to calculate the overturning/sliding force in the wall and then size the wall large enough to resist those overturning forces. I don't know what size rocks you are getting but I imagine you could limit the size of the rocks used in the wall to say.... 1000-2000# (or what ever would work) which would yield a rock of certain dimensional properties. From that point you could be conservative.

Of course if you wanted to cheat the system and the site allowed you could always have the contractor construct 2-4' walls and be within the limits of the code. That's probably what I would do.

 

[ClarkBart]

Thanks Dick:

I have had many of the same thoughts as you. I've thinking that this problem is kind of like the bumble bee. Scientist and engineers think that the bee shouldn't be able to fly, but, there is plenty of evidence out there that shows it does. Dry-stack walls are out there and have existed for ages, but, we haven't spent the resources to be able to predict what makes them stand-up. It is an art form that experienced and talented artist/technicians can build with reasonable assurances of success unless you talk to an engineer.

 

[ztengguy]

I would design the same as the keystone type walls. use geogrids behind and engineer as such.

 

[concretemasonry]

ztenguy -

Do you have verified numbers/test results to use in evaluating the effectiveness of the grid holding strength when sandwiched in an irregular blasted rock wall material?

Would you sign off, since it is generally accepted that engineered walls should be used beyond the usual limits of 4- - 5' in height?

You will need adequate length behind the backfill and the backfill must be compatible with the type of grid specified for the SRW design methods to work. It may be good to check with a few geo-grid suppliers since they are aware or did participate in some of the testing to insure continuity of the wall system.

Dick

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

 

[woodman88]

ClarkBart - Scientist and engineers know bumble bees fly. It is explaining the means of that they have had a problem with. As for dry-stack retaining walls, engineers know they work. It is defining a code accepted means that is the problem. Or more to the point, if it fails do you have a code acceptable calculation to show that you have done your due diligent in its design.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RacingAZ]

What you have described, I believe is called rockery wall. Where is the project located? The Southern Nevada Building Officials has some design guidelines, not sure if it will be accepted in your jurisdiction if you're not in NV. Try to google it.

Once you get above a certain height, it's going to require MSE + geo-grid behind the wall.

 

[RFreund]

Is the stone of uniform dimensions or are these various shaped boulders?

There are a few design methods I would consider depending on geometry:
1.) Design as a segmental wall (See NCMA - Design manual for segmental retaining walls)
2.) Design as rockery
3.) Design as a GRS type wall. This is more of an extension from a GRS-IBS system. But basically the concept is that you space reinforcing geogrid so close that there is almost no pressure on the face of the wall. The soil pressure resembles that of 'bin pressures'.

My first choice would be a segmental wall using a well know and tested system such as keystone, versa-lok, anchor, etc.

EIT

http://www.HowToEngineer.com

 

[CANPRO]

Heres a good reference for rockerys that I've used before...

http://www.cflhd.gov/programs/techD...ries/documents/01_Rockery_Entire_Document.pdf

 

[dhengr]

Woodman:
You hit the nail on the head, or the bee in the bonnet, or whatever. The point is that there is plenty of engineering judgement and experience, and well reasoned details which can be applied to this type of problem. But the owner who wants this and the inspector who should know that there is no well established design procedure for this type of problem, should not hold the engineer to the same level of legal liability as might be the case with well established bldg. systems or materials. The person who wants this should shoulder most of the potential liability. I’m tired of owners and BO’s asking for the impossible to prove and then expecting an engineer to sign-off in the normal fashion, and then to also act as the insurer of last resort, or the deep pockets when something does go wrong. Just because it hasn’t been codified doesn’t mean it can’t safely be done, within reason. Someone has to use some engineering experience and judgement in setting height limits, material sizes and shapes, means of controlling soil pressures behind the wall, foundation and drainage details, etc. etc.; and an experienced GeoTech guy should be involved, along with a mason or craftsman experienced in this stone laying art. This isn’t rocket science, but neither is it a simple conc. cantilevered retaining wall with a pretty set of calcs. The engineer’s function and duty should be to bring it all together, and know where to draw the line, and provide a report on same, with plenty of caveats.

 

[Michel60]

Great reference via CANEIT and I do see a PE signature (no stamp) up front!....so if it all goes into the handbasket we can just blame the Federal Government then, right?

 

[ClarkBart]

Thanks for all of your responses. They have given me a lot to think about. I will review the reference that CANEIT provided. It looks like there is a lot of useful information in the publication.

It doesn't seem as if there is a lot of engineers out there that are providing engineered drawings for walls over 4' unless it becomes a critical structural component. The popularity of the segmental block walls must provide for engineering designs from the manufacturers' with each wall.

Thanks Again