Steep slopes behind retaining walls

[Vinny7]

Hi all,

If a bank (approx. 8m Ht.) of cohesive soil is excavated leaving a slope of about 60deg. (remains stable) and the owner wants to later add a retaining wall to serve as one wall of a building (garage). Should the wall be extended and the embankment re-graded to 2:1 or can the bank be assumed to be self supporting and a new wall only retain the cohesiveless backfill material?

 

[fattdad]

For "cohesive soil" (eh, fat clay, lean clay, elastic silt, 0 percent sand, 49 percent sand, LL=100?, etc.) I'd likely form the permanent slope at 3H:1V and then design the retaining wall for a sloping fill, at rest condition (Good luck finding a chart solution for that - I just use the chart solution for active earth pressure and multiply by 1.5 to develop the at-rest earth pressure for sloping backfill) I don't like 2H:1V slopes using clay as over time the cohesion can relax and you can have failure. Maybe it's a sandy lean clay with a PI of 18 and 45 percent sand? That would be different. . . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i'm with you fattdad...

 

[BigH]

First off, your 60deg slope is not stable (over the long term) - when porewater pressures have a chance to stabilize, it will assume a friction angle of 24 to 30 degrees likely and, of course, 60 degress is too much. If you want to put a retaining wall in front and backfill with sand, and the retaining wall is not prevented from movement, then, obviously active earth pressures can be designed for - but, you must be sure of the material in which the sliding plane would form (granular backfill - or in granular backfill AND cohesive soil). To be safe - and for a small project, using an "at rest" earth pressure is reasonable (the added costs will not be significant). I would say that the slope will have to be regraded - 3H:1V or 2.5H:1V - matter of choice and aversion to risk. Due to the sloping backfill in at-rest condition, fattdad has a reasonable approach - again as the project is small. There was a thread a while back that you could search for where this was discussed in some detail.

 

[RWF7437]

Cohesion exists ONLY when there is water present in a narrow range of percent moisture. Never count on it.

good luck

 

[oldestguy]

While the above cautions may be the safe way, take a look at the area cut slopes in the same soil types. If these slopes stay at the steep excavated slopes without sliding or excessively eroding, you may look to them as examples of what works in your area.

I have worked on steep slopes under conditions where you cannot flatten them because of terrain or property ownership (or lack thereof).

These generally have to maintained to prevent erosion, but it is an ongoing thing that cannot be neglected. We generally have found it is not practical for high slopes to consider a full height retaining wall. A few have worked OK with a gravity retaining wall only at the base mainly to keep that area from dribbling earth into a work area below. Erosion above is what has to be controlled. Keeping runoff from above seems to be the main need to limit this erosion and local slips.
6
I also have seen several 60 degree slopes "paved" with heavy stone, treated with drainage filtration behind them. One near here where I did the design 27 years ago, is still holding fine, but neeeds a little maintenance now and then. Height of the stone "paving" was about 10 to 20 feet Land above that is a 39 degree slope wooded for a long ways. In that case the owner US Govt. agency knows of the risk and what is involved for maintenance. The treatment is mainly for limiting erosion, not for holding back anything. Soil type is silty clay, weathered loess. If it comes down there is only a parking lot below.

 

[Vinny7]

Hi,
The situation is more or less as oldestguy says. My problem would be that i don't think this approch can be justified via calculation (slope angle is much more the the soils angle of internal friction.

 

[oldestguy]

One more about that 60 degree slops.

Cohesion from simple unconfined compression tests was quite good. Critical height on that basis was pretty hig also, but there was no way the funds would pay for a better job.

We didn't go so far as to figure losing this cohesion with time, but this got the job going for now. It seems to be rather "permanent" in spite of possible failure some time later.

I suppose we could have told the owner "no way" and walked off.

 

[oldestguy]

More on this subject.

Thread 255-186861 has a reference to a US Govt. publication for Rockery Walls, generally under the class of gravity walls. However, some of the reference work is analysis of these walls with sloping front face. Sort of applies here I think.

Interesting to see the ratio of base width of the rocks to height can be much more than the rule of thumb (3)for sloped back face of wall situations.

 

[civilperson]

Some loessal soil slopes with the steep angles described can last more than 100 years if protected against heavy rains or moisture infusion. The Mormon Trail through east Nebraska has some near vertical cuts that are still stable.

 

[BigH]

civilperson is correct - I tried to get a contractor in China to use the loess as his wall and footing formwork - why put in formwork when we had the loess? However, I also so a rathole develop on the slope (and a decently large one) by someone relieving himself just above the crest of the slope.