Stone Retaining Wall Reinforcement

[kxa]

A client has come to me for help to get C.O. from the town for a retaining wall that he has put up without the Geo-Grid Fabric that the architect had specified on his sealed plan.

The wall is 4' high and the stones are 12 to 15 inches wide. Top or the wall is 15" wide and the bottom 18". the bottom stone which is 8" high is completely buried and is place on 8" of compacted 3/4" blue stone gravel.

The client does not want to remove the backfill to install a geo-grid. I was thinking if I could strengthen the slope by driving 5/8" dia. rebars behind the wall to go below the bottom of the wall and perhaps at a normal angle to the shear failure surface, I can prevent it from future sliding or failure. Any thought or suggestions?

Thanks.

 

[dik]

If the wall is like a segmental precast wall, the design may be prescribed by the stone supplier. You might want to have the Architect review the design with the supplier. It may be less costly for the lower part of the wall to be reconstructed.

Dik

 

[oldestguy]

What's all the fuss about?

This appears to ba a gravity retaining wall. The rule of thumb for these, without reinforcement, is that the width should be about 1/3 of the height. Which it is.

If you wish to get fancy, ask the "designer" to show calculations that the wall will fail? Unlikely result.

Or better yet, do the gravity wall calcs your self and show it will stay put. Lateral pressure depends on the material there, but unlikely exceeds an equivalent fluid unit weight of 40 pounds per cubic foot. If sand, it may be as low as 33 to 35.

Keep your resultant within the middle third at lower ground level.

Sliding is unlikely, but then you need some soil shear strength data.

I am assuming the backfill will not be saturated.

 

[TDAA]

What are the requirements in your area for designed walls? In the several areas I have lived, walls 4 feet or less, with only one wall and no other oddities such as steep slopes above/below etc., do not even require a design. They are considered landscape walls, which do not pose a significant risk to the public.

 

[kxa]

The stones are relatively thin (2" +/-) and could slide on each other. There is also a 1:3 slope to the backfill. Any type of failure (structural or esthetics) will not be happening right away but could happen in the near future. If I just go ahead and certify the wall, I will most certainly be the first to hear from them when the wall starts to move.

The material behind the wall is sand. I just remember that there was a method of soil reinforcement that was accomplished by driving rebars in the soil at spaces calculated based on type of soil, height of backfill and.... I just can't recall the text or the reference material. Any help is appreciated.

 

[oldestguy]

If you really want to do a design, the Navy NAVFAC DM-7, (out of print) shows the equivalent fluid changes from a density of 30 to 32 #/cf going from zero slope backfill to 18 degrees. Vertical component then changes from zero to 10 #/cf. This is for clean sand backfill.

For all practical purposes, the slope is not important.

Slipping of stones also is unlikey if some of the sand gets in there to add to the friction. Wall failures generally are bulging and tipping ou, even for no grout bonding.

Reinforcing with bars seems like a nonpractical solution and not easily designed for this simple a job. You would need a lot, maybe one every foot of wall length and some mode for securing to the wall to have a noticeable effect.

If you do a simple stability check with a triangular horizontal pressure distribution and take the wall at a density of about 145 #/cf, I'd be surprised if you see any problem. Actual stone density is higher than this, about 165 or higher.

Any beginning soils engineering text ought to have all that you need for this.

Add in that slight vertical component and it is even better.

Does the wall have any "lean back" slope? Even a little makes a big difference in stability.

 

[kxa]

I will be going to inspect it next week but from the sketch I have, it is supposed to have a 1:12 batter. The backfill is RCA and sand with an underdrain. If I do an estimate on the loads I get about 800 plf for the wall weight with a lateral soil resultant of 360 plf based on 45 pcf efp and 145 pcf for the stone. if I use a friction coefficient of say 0.5 for the stone, I get 800x0.5=400 plf of resistance load. If I want to have a FS>=1.5, then this is not enough.

 

[oldestguy]

Friction between stones and also to earth is probably more, such as with a friction angle of 33, the tan is 0.64

Even with say 30 degrees friction angle in stones, which is unlikely, Tan is .58

then comes the soil equivalent fluid pressure. 45 p/cf is very very conservative for clean granular material. Try 30 or at the most 35.

Then use vectors showing location and direction of the forces and it is unlikely that there is a problem.

That "lean back" also is likely to help noticeably.

My quick calcs show "no problem" now.

That drainage also is an advantage to keep these numbers as valid.

 

[miecz]

oldestguy-

"the Navy NAVFAC DM-7, shows the equivalent fluid changes from a density of 30 to 32 #/cf going from zero slope backfill to 18 degrees."

Where can I find this in DM-7?

 

[oldestguy]

I hope some other guys can jump in here and help.

I went to the Navy web site and find that DM-7 apparently was not publised again in their latest publications.

The military are combing all theri manuals into those usable by all services.

You can search in this area if you like.

I did find that part of DM-7 is published as a pdf file.

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_220_10n.pdf

That title is "Soil Mechanics".

I looked through this one and much of it has not changed.

You can do a computation using a soil mechanics text and use the Rankine method to find it, assuming a density of the sand, say at 105 p/cf and a friction angle say at 33 degrees.

The Corps of Engineers may have similar stuff than Navy

Here is the index page where the current Navy Manuals are listed.

http://www.wbdg.org/ccb/NAVFAC/DMMHNAV/engineering_criteria.pdf

 

[oldestguy]

Hi again:

I found it.

Vulcan Hammer folks have a bunch of stuff that can be downloaded.

The 1986 version of DM-7 is there.

http://www.vulcanhammer.net/download/foundations.php#dm72

 

[Boonie]

Who designs stone "rubble" walls with geogrid anyways? The walls are inherently suited for gravity walls. A wet stacked, rigid retaining wall facing is not suited to the flexible characteristics of a reinforced retaining wall system. As Oldest said, do the gravity calcs and see if it works.

 

[kxa]

How about the loss of soil through the openings/gaps between the stones. Wouldn't a geomembrane be required?

 

[dodggy]

why is the slope of the backfill not important? cuz we provide timber/steel post retaining walls 2 residential houses, and slope of the backfill normally affects the depth of the concrete footing pad