Retaining Wall Review 1

[tclat]

Hi,

I'm doing a design review for the stabilisation of a 30 foot cut face. The slope of the land beyond the face is fairly steep at 27-30 degrees. The topsoil layer is about 3 feet thick but the rest of the face appears to be cemented granular material of varying consistency. See attached picture. The engineer has proposed a 20 foot high masonry retaining wall and the final 10 feet, has proposed a geomat. The engineer has also specified 1% cement by volume to the backfill and the width of the backfill zone varies between 3-6 feet. The design looks very lean to me and the only way I can say the wall works is if the cement stabilized backfill lowers the active pressure on the wall significantly and if they have determined that the cut face is stable and full active wedge is not developed. What friction angle would one consider for the backfill described below?

crushed stone + 1% cement by volume
sand + 1% cement by volume
coarse grained gravelly/sandy soil + 1% cement by volume

Thanks

 

[oldestguy]

What will the slope of the cut face be? Assuming the cut face is stable, all you are faced with is some covering to prevent erosion. It all depends on the slope selected. I'd look into the "silo effect". I did a slope like this once and it took some estimate of "backfill height". As recall "not much".

 

[tclat]

The cut face is near vertical. Any thoughts on what the friction angle would be for the cement stabilised fill?

 

[oldestguy]

The wall I refereed to was a new cut sloped back about 1 horizontal to about 6 or 8 vertical and stable in itself. The wall itself was large cut blocks of dolomite, no mortar. That was typical of the area (Dubuque, IA) but about 40 to 50 feet high. Somewhat unusual for the area. Drainage in back was required to avoid building up any water from above. That could be at the cut slope behind all that fill. The sloped cut of the soft weathered rock in back made for a much more stable situation than vertical. In your case, I'd be much concerned about any drainage from above getting behind a somewhat cohesive sand backfill and pushing whole thing over.
Thus lab tests for permeability as well as strength (friction angle and cohesion)could tell a lot as to what to do. Adding binder to the backfill may be a major mistake. For sure that wall ought to be porous, full height, as with precast blocks. I'd not use a cast concrete wall. Give thought to tied back construction also. A complete stability study, along with lab tests would be my thought. In connection with this, our recent area wide few days of 11 plus inches of rain might have taken down walls, but so far the news doesn't mention that.

 

[oldestguy]

For what it might be worth, the wall I refer to was built about 30 years ago. A Google view a few years ago shows the wall, top just below the top of the photo. Its at the Administrative site a mile north of McGregor, IA. Mississippi R. is to rear of camera.

 

[bdbd]

tclat,

This is a nice question that I don't have answer too. Let's keep it updated. I would like to know the binder effect on the backfill too.

 

[oldestguy]

OG again. As I view this, it is an unusual kind of wall or earth cut protection, due to the lack of slope, width of backfill and a watershed up above. There maybe more things unknown to us here. Thus, I'd get back to the geotech with your question and inquire about any analyses done. Their experience with this treatment also would be good to know. I'd not try to second guess the geotech, but certainly ask for a complete explanation. You should be satisfied before doing the job.

 

[oldestguy]

Hey, I'm still skeptical. Looking up Internet search for GeoMat comes up with info on its use for foundation wall drainage. I see nothing for using this as a covering for exposed earth near vertical surfaces "way up there". Documentation as to successful examples of use for protecting earth cuts from erosion, etc. definitely would be needed if I had anything to do about this job. Sit back and re-think what to do.

 

[tclat]

OG....the cut face is about 1H:10V. Why are you skeptical about the binder? Forgot to mention there is a cut off drain at the top of the wall.

 

[oldestguy]

I say once more:
Documentation as to successful examples of use for protecting earth cuts from erosion, etc. definitely would be needed

Any protection from erosion helps, but none are perfect.

Oh, very important to check the geotech's insurance. They may need it.