Retaining wall design: need help

[longisland]

Hi all,
I'm trying to design a wall retaining a slope of 1:1 slope. Coulomb equation doesn't work in this case; any suggestion?
I'm not familiar with geogrid system. Is this a type of gravity wall? Where can I get info on the design concept?
What is a good ball park figure for angle of friction for both cohesionless & cohesive soil?
For L shape drain design, what are the design considerations? I believed sliding is not a problem, other than overturning, what else should I worry? I can think of bearing failure, settlement & uplifting. How to over come these problems?

thanx in advance

 

[ishvaaag]

You may be wanting to use reinforced earth schem which in concept will be close to geogrid.

On reinforced earth, used some times to hold enbankments of your slope, you can find information on

La Ingeniería de Suelos en las Vías terrestres, Volumen 1
Rico - Del Castillo
p. 265

I don't necessarily agree on that Coulomb's can't be used for that. If you want you can use one freely downloadable Mathcad sheet for that using the primary concepts of Coulomb's theory for active push by mere substitution of one more complex definition it has of the surface by straight line slope.

Of course you will have to have a backfill with angle of inner friction going very soundly upwards of your 45 deg to be stable. Otherwise you need more analyzing the stability of your slope than worrying about the retaining wall, for which you have also some sheet isn the collaboratory that help.

Respect typical data for soils I suggest look AASHTO 1992 standard table 5.5.5b

 

[geomo]

It will be difficult to keep the 1:1 slope stable, as ishvaaag noted, and the slope will also put a surcharge on the wall, making it more difficult to calculate earth pressure on the wall unless the phi angle is greater than 45 degrees.

There are many companies that supply materials for building mechanically stabilized earth (MSE) retaining walls and reinforced soil slopes (RSS). Having been involved in both design, supply and construction of such wall/slope systems, I can tell you that it is a low-bidder business, and there is substantial risk if you are designing for someone else to supply materials and build the wall. The material supply companies do the design as a cost of selling their products. It will be easier for you to contact them. They are the experts and will essentially give you what you need for the opportunity to sell their products.

 

[Zulak]

Try contacting Versa-Lok retaining walls for some design guidelines. Also geogrid manufacturers usually have some software for such applications. Try Tensar geogrids.

 

[Rjakiel]

I have successfully installed retaining structures supporting a 1:1 slope using tiebacks/soldier beam construction methods. Is this an option you are considering.

 

[ed34]

You did not mention whether your retaining wall will be in a cut or fill situation. If you are in a cut situation and the soils are acceptable for the technology, I would look at using soil nailing. I have designed soil nail shoring systems to support 1:1 slopes in the past.

 

[layfieldman]

You can build a tiered system such as you want but would require a cut and fill. Depending on the height required you could build a SRW (Segmental Retaining Wall), probably reinforced with either geogrid or geotextile, then build a 1:1 reinforced slope, again with either geogrid or geotextile. Generaly the reinforcement lengths for the geotextile/geogrid will be around .7 of the height of the wall. This is just a guide and a full design by a qualified engineer who is familiar with geosynthetic reinforced walls/slopes is needed. As far as it goes, for SRW's NCMA has a very good design guide, as well as design software available on their website.

 

[solutioninc]

On a 45-degree slope, construction of a retaining wall will make the backfill slope of the retaining wall flatter, as flat as you want depending on the height of the wall. Then you can use any of the classical equations to calculate the pressure on the wall. Only aspect to note is that the flatter slope of the backfill will have to be long enough to prevent the original 45-degree slope from causing surcharge effects. Multiple retaining walls may also be tried converting the original 45-degree slope to a series of steps, the vertical rise of the step being a designed retaining wall. Does this apply to your situation?

 

[cphi]

longisland,
My advice to you is to hire a consulting geotechnical engineer to advise and assist you in the design. A geotechnical engineering test boring exploration and analysis is recommended to evaluate the "angle of friction for both cohesionless & cohesive soil" rather than using a "ballpark" values. The test borings and analysis are recommended for the existing on-site foundation soil below the retaining wall, unexcavated retained soils, the backfill soils planned, and the soils planned to construct the 45° slope above the wall. The 45° slope soils will need reinforcement as mentioned in earlier replies. Very few if any soils will be stable with a 45° slope angle without reinforcement. Granular soils for the backfill and 45° slope soils are recommended due to the strength characteristics and construction considerations.

 

[DBNodurf]

Please don't do this!!! As one of the respondents asked, is this a natural 1:1 slope? If so, get the geology studied to make sure that cutting along the toe won't bring the whole thing down on your head! Then, get a geotechnical engineer to tell you how to stabilize the slope above prior to cutting the toe away. Then, use soil nailing (as one of the obviously talented respondents noted) to stabilize the nearly vertical part.

Analyze the slope with a friction "circle" methodology, and don't forget global stability.

Now, if the slope is a proposed constructed slope, make the slope fit the area available even if it is steeper than 1:1. I've designed several 1:1 slopes for Ga DOT, FL DOT, and for other clients in NC. I've also designed slopes way steeper than 1:1 for mining clients in WV.

Don't even think about using a segmental block faced wall; even if you use a layer of geogrid every layer of block, over the long term you could still get face destablization. Forget NCMA; their method is ok for walls where supported slopes are 2:1 or flatter, but that is only because the thrust from the slope tends to be kicked down to, or below, the toe anyway.

I would not let the geogrid manufacturers design such a wall; their people tend to be more concerned with moving product than with good design (if they are even capable of it at all - yes, I know this because I once escaped this netherworld)

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