Rock Gravity Retaining Wall 2

[dpa]

I was recently asked to design a rock retaining wall whose height varied from about 4 feet to 16 feet along a 150 foot course. I turned it down because I had never done one before. I have a bunch of questions about this type of project. Does anyone know a good reference or design example of a rock gravity wall? How do you insure that the thing will be stable in an earthquake zone 2B? Does any software address this specific type of wall? How do you spec the rock for such a project. An engineer at one of the geogrid manufacturers thought it was a really bad idea. Was he right? I know this is a lot of stuff for one question. Even partial answers would be helpful.

Thanks
Dave Adkins

 

[Focht3]

Good for you! I once heard it said, "It is more important to know one's weaknesses than one's strengths." You certainly have my respect and admiration. I wish more engineers would follow your example.

Others with specific recommendations on this subject will respond - give it a little time. You might also do a key word search of this site using 'rock gravity wall' or something similar. I know that there have been quite a few discussions on this topic over the last few years.

Again, good luck. And my hat's off to you!



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

The main question is if it is allowed, I mean, if the owner doesn't weaken in his intent.

If it is not allowed, you show him the code regulation and this is the end. If not, all comes to prove that the code requirements are met. If this is expensive, it was the owner who wanted that.

But if required you may use every trick in the trade, because just stone and mortar might not work along the specifications of the code. Stones were already held together by clamps in the Age of Bronze, and magnificent baroque domes stand perfectly well tied this way. But now a more rational thing would do to make some reinforced concrete or so backup (or container) and then nail with earqthquake resistant anchors everything unlikely to otherwise stand the shakeout.

As long the moneys are there ... this reminds me a contractor charged with scrapping a CIA's SR-71 or so Blakcbird after just flying (it is reported) one mission:

"It is their thing"

 

[dpa]

Hi all,
Thanks for the quick responses. I have tried various key word searches. Found a lot of interesting stuff most of which was not related to rock walls (or any other walls) at all. However, a lot of it was usefull and is now added to my favorites list.

A friend of mine who is a physicist said the other day "It's better to be lucky than smart." (She wasn't referring to this project) I just don't feel that lucky. After all whether a big rock falls on someone's head has at least as much to do with the skill of the heavy equipment operator as it does the design.

Even though I am not doing the design and may never do one I am curious. Hope someone with specific experience (good or bad) reads this thread someday.

Thanks again,
Dave Adkins

 

[Mudcake]

Hi dpa,

Rokery is very common in the Pacific NW, Seismic zone 3. It is normally used to retain a cut, not fill. Wall height of up to 12 feet or so is not uncommon. If rockery is used to retain fill, the state of practice is my area is to limit the wall hright to no more than about 4 to 5 feet. Having said that, howver, rockery has been used to retain up to 10 to 12 feet of fill with geogrid reinforcement (i.e. rockery was just a facing). The 16 feet that you mentioned is taller than any rockery that I have seen. A local engineer put together a design guideline for rockery and selection of rock, etc.. If you can give me your address/fax, I can forward a copy to you.

 

[dpa]

Hi Mudcake,
I would really appreciate it. Maybe next time I won't have to pass up the job.

My address is
12011 20th Ave NE
Seattle, WA 98125

or fax (206) 440-8663

Thanks a bunch
Dave Adkins

 

[wallguy]

Dave
I can give you a name of someone who has build a rock system. You are referring to a stack rock with mortar laid between the stones? If so contact Ryan Fuetch @ 615-451-0095 with Retaining Walls of Tenn. Tell him Ken Headley told you to call to find a design for them.

Ken

 

[dpa]

Thanks Wallguy. I'll add him to my Outlook Express.

Dave

 

[jdmm]

I was posed with the same request about 25 years ago and the concensus in our office was that you couldn't safely build a stacked rock wall. Unfortunately there was lots of evidence around that said that you could. Many of the railway grades through the mountains were built over small ravines with stacked rock walls that were up to about 60 degrees to the horizontal. The rock was carefully placed so that they all interlocked. These walls still support rail loading and have for over 100 years.

I visited Japan and saw walls several hundred years old in a seismically active area that were preforming well.

I have just completed a series of stacked rock walls for a development in a very steep environment where blasting for roads and highrises was required. The blasted rock wall was split into two categories, very large angular facing rock with diameters of 1 to 3 metres and backfill consisting of about 600 mm minus.

For a gravity wall the thickness of the facing rock at each level must be sufficient to provide an adequate factor of safety against sliding at that level. The rock has to be placed in a tight 'brick work' structure such that there are no contacts between rocks that slope out of the wall. In this regard it is necessary to have a very good operator that will try rocks and if he can't make them work by turning them over and around then will choose a better rock. The walls are erected using an excavator with a 'thumb' that can lift the large rocks.

For small walls up to about 3 to 5 metres we have allowed slopes of the facing up to 60 degrees. For higher walls (up to 10 metres) we normally reduce the slope angle to about 45 degrees.

These last walls were constructed using HDPE geogrid assuming a factor of safety against damage of about 1.25. Metal grids such as Terramesh by Maccaferri are also resistant to installation damage. Don't use PVC coated polyester grids because they are easily damaged in a heavy rock backfill.

Hope this helps.

ps: When I took on my first rock wall job no one was building this way. We did small scale model tests and approached it analytically from about every direction to satisfy us that it could be done safely. Since that time we have done hundreds of walls up to 30 feet with condos on top of the walls. Again the walls are generally sloped back from the horizontal at about 45 degrees.

 

[dpa]

jdmm,
Thanks. That is helpful. I guess a lot of it boils down to how much you trust the skill of the builder.
Dave

 

[grits]

Your last statement was right-on. The quality of the wall is only as good as the quality of the mason. Without knowing a topnotch builder, I too would not attempt this project. I wonder if you could excavated along the back of the wall, construct your wall with the appropriate sized stone, shape, etc. and then loosely backfill with a granular material once a geofabric was laid along th back of the wall. Or perhaps, build a staircase with the material to be retained, and as the height of the wall rises, place the stone on the next teir/ledge, continue vertically, and then repeat. So essentially you have laid a reverse staircase (upside down) as you build. The last step would have to end perhaps a foot below ground surface, cover all the stone with fill to the desired thickness of the top of the wall, and then continue the remaining foot lift (hiding the staircase effect). Just a suggestion. Good luck.

 

[wallguy]

I know the desire here is to build a stone wall. If the main concern was to support the slope or cut then there is another great alternative. It is a modular "crib type" segmental retaining wall system by Westblock Systems. It uses concrete anchoring pieces like lincoln logs and creates a mass much the same way as the stone. No geogrid and less excavation than trying to use grid. Being an engineered system you can sleep better after the construction.

And if you are thinking that since I am "wallguy" that it could be one of my products....You're right it is.

Ken

 

[dpa]

Thanks Grits,
Those are both good suggestions. Your idea about not trying it without knowing the builder is right on.

Hi Wallguy,
Do you have a website where I could get a look at this sytem.

Thanks,
Dave

 

[wallguy]

Dave

We do. This is a link to our old site. We are working on a more interactive site that will work with our CD. We hope the new site is up in October.

http://www.westblocksystems.com/gravitystone.htm

Thanks for asking

Ken

 

[DBNodurf]

You can't insure anything unless you are in the insurance business; what you want to do is ensure... the geogrid guy would think its a bad idea especially if it had no geogrid in it. If it does have geogrid, then the rock wall isn't the gravity wall; the entire reinforced soil mass plus the rock wall is a gravity wall.


D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC
dnothdurft@carolina.rr.com

 

[structbear]

There was an organization that disappeared in the late 90's that call ARC (American Rockeries Contractors) that put out a design manual for Rockers that is republished in a section of the "Retaining Wall Design Guide" published by the US Department of Agriculture - Forest Service EM-7170-14 or FHWA-FLP-94-006 1-202-205-0820. Once this organization went defunct a large rockery builder took it over based out of the Seatle are and a few years ago was still passing out this manual 1-425-481-3456.

The construction of rockeries is site sensitive and drainage is important. Constructing this type of wall system is an art and it is important that the rocks be placed securely.

I design a few but have come to a decision that this is not the type of wall I care to design and usually recommend someone else.

Best of luck

 

[pjnp]

The type of wall you are probably talking about is quite common in Sydney where they are built from large blocks of Hawkesbury Sandstone with no mortar . The blocks are typically about 1.5m by 0.6m by 1m and usually come from bulk excavations made bt ripping or impact breakers. The walls are simply designed as gravity structures from first principles. The look pretty good and may be up to 7m high.

pjnp

 

[RickAstoria]

Dave Adkin wrote: "dpa (Civil/Environme) 16 Sep 03 12:23
I was recently asked to design a rock retaining wall whose height varied from about 4 feet to 16 feet along a 150 foot course. I turned it down because I had never done one before. I have a bunch of questions about this type of project. Does anyone know a good reference or design example of a rock gravity wall? How do you insure that the thing will be stable in an earthquake zone 2B? Does any software address this specific type of wall? How do you spec the rock for such a project. An engineer at one of the geogrid manufacturers thought it was a really bad idea. Was he right? I know this is a lot of stuff for one question. Even partial answers would be helpful."

I have analyze and may have some insight on rock retaining walls which is a form of "stone masonry wall". Now, there are different types of stone masonry formation which can act as a retaining wall. Any wall designed to retain anyform of earth or matter can be clearly defined as a retaining wall. There is two prominate types of stones used for this kind of wall for structural. Granite and Basalt. (There is other types of stones used. Particularly, here in Astoria - a stone retaining wall built in the 1905-1910 era (prior to building construction) was built to hold back densely packed bouldery basalt rock fill and earth soil and a school building completed construction in 1911 and currently used by a community college.

This is a rubble fill retainment wall. Which I am evaluating the use of large cut stone masonry blocks with large custom diameter rebar and rock bolting as an idea for further seismic retainment. So far the wall has survived several large earthquakes in the Northwest region without any failure. This would be ideal for keeping retainment from breaching and breaking down and causing a substantial slide of retained material.

Now, back to a general point, a coursed masonry of cut blocks of stones and layed like brick/concrete blocks will give you the most flexible option and allows for implementation of similar retainment methods as used in concrete block retainment walls and brick masonry.

If you want properties of basalt and granite. They will have greater compression strength than that of a solid concrete block of the same dimension as concrete is weakened by the cement itself being not nearly as hard as the rock aggregates themselves. Historically, crushed basalt or granite gravel is used as the aggregates but like anything - concrete will crack at the cement itself before the stones themselves crack under compression or tension. Basalt is known to have a little bit of tensile strength in tests. Of course, you would want to compare the basalt blocks as being similar to solid concrete blocks (with no hollow chambers). To insert rebar vertically, you will need to drill the cut stone without cracking the stones. (Hydro-jet drilling perhaps).Then insert the rods and fill the area around the rods with cement.

Now, this will help to lock the stone blocks vertically from sliding around. So, through careful planning - a stone masonry wall will not pose much seismic threat. You would also want to implement weep holes like you would with ANY retaining wall for water to pass through. The thickness will probably be thicker if the fill may pose a substantial amount of force against the wall.

General rule - the height of wall to thickness of wall should not exceed 10-to-1 ration for above ground. Preferably for total vertical length including sub-surface portion of the wall. So if your total vertical length of wall is say 25 feet - for a 15 ft above ground wall. The wall thickness should not be less then 2.5 ft thick. So 3-5 ft. would be recommended. All reinforced with rebar and weep holes if conditions require such. The money willing to be payed will be the key. Stone masons are relatively hard to come by these days.

Historically, stone masonry walls have a relatively good and EXTREMELY long durability. The choice of stone and quality of cement/motar for the weather/environmental conditions will make a difference.

I hope this helps on a general sense of things. The structural make up can be as simple as with concrete block masonry. Just a tid bit more expensive due to material costs and getting good stone masons to cut the stones nicely.

I will leave it at that for now. End result can be a beautiful wall that can potentially last 1000+ years.