rock gravity retention wall question 2

[evilscott]

I am a mechanical engineer who obviously doesn't know jack about geotechnical engineering and have a home project that I'm looking for some seasoned advice. I built a house a few years back backically on the edge of a small plateau/hilly area. Growing up in an area where the biggest hill was a speed bump, I've gotten to learn about hills, slope stability (or the lack thereof, etc ...). I want to
build a tiered mortarless stone wall(s) and need some advice. Bascially, I have two questions

1) Spacing between consecutive walls
2) Better to cut into/disturb existing soil to lean the
stone wall into or to build and backfill as you go
(leaving the existing soil updisturbed)

Background
There is an drop-off/hilly section (I'll call the upper section) ~ 25 ft away from the house which is ~ 50ft long with a ~1/1.5 (vert/horz) slope for ~ 12 ft (7 ft vert drop for 12ft horz)then changes (I'll call the lower section) to 1/3 (vert/horz) over the next ~ 20 feet and then blends down to relative horizontal ground over ~ 30 ft. There is a very small grade from the house (I'll call the top) to the upper section and has grass coverage. The lower section also has grass coverage. The soil type is predominantly sandy.
At construction of the house 3.5 year ago, there was a heathly grade straight from the house throughout this entire area I'm describing above. I had soil brought in and graded to produce my current situation. Before the grass went in, I had some washout problems in a particular location of the upper section initially. I added silt fencing along the top edge of the upper section and this corrected that situation (as a "temporary" solution).
I have kept and eye on this area and other than some slight settling/movement during the first year, it appears to be stable. I want to ensure that when I put the tiered stone retaining walls up, it remains so. Lastly, "my boss" wants the tiered wall less than the 1/2 (vert/horz) distance that I've read is recommended. She wants ~ 1/1.5.

Sorry, for the wordy description.

 

[Focht3]

Better ask "the boss" if she wants to change the carpet to a medium brown color so that the mud from the slope failure will blend in, or would she prefer white for a high contrast?!


The first thing you need to do is hire a local geotechnical consultant. It sounds like the 'hill' is 15 to 20 feet high with variable slope. Your "sandy" soils may or may not be stable on a 34[°] slope, regardless of whether your mortarless wall is in place. (The wall won't add to the stability of the slope.)

With a big enough pocketbook, you could create a vertical wall - if you wanted one. And I can think of at least three different systems that might work for you; but I'd need to see the site - and obtain soil borings - before providing any recommendations.

And don't expect the consultant to provide you with a $500 answer, either -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[evilscott]

Apparently, my description was somewhat misleading based on the response. The tierred rock wall(s) are only intended for the upper section which is a 7 ft drop over a 12 foot horizontal run, not anywhere else. The real question centered around violating the recommended 1 ft vertical to 2 ft horizontal terracing for tierred walls. no one wall would be > 3.5 ft and I'm planning on "leaning" the wall into the hillside on a 1 ft horizontal per 3 ft vertical.
The second question was whether when constructing a rock retaining wall is it better to disturb the existing slope so the wall is "resting" against pre-compacted soil behind it, or it is better to build the wall at the base of the slope and bring in additional backfill, essentially adding to the original hill ?

 

[g7mann]

g7mann [geotechnical]

evilscott:

The 2H:1V setback criterion is one developed predominantly by public agencies to make sure they are "safe." Whilst it is nice to set one stage of wall back twicwe the height of the wall in my more than thirty years of ewxperience it has niot been necessary. In my practiuce we frequently set back the upper stage [or stages] of a rock wall a distance that is determined by an imaginary 1H:1V line between the base of teh upper wall and the base of the lower wall. This is sufficient to eliminate virtually all of the lateral load component of the upper stage on the lower stage.

If you wish to set the walls closer together you can set the base rock of the upper stage at a loweer depth thereby achieving the same 1H:1V setback of the base of the wall. Providing the lower wall is engineerd to accommdate the lateral loads imposed this poses no significant problem.

Alternatively, you can simply engineer the lower stage of the wall to generate more than sufficiant rock mass to restrain the loads applied by the upper walls.

In rock wall construction "size" matters. The larger the rocks, particularly in the basal one or two rows, the more significant the wall mass and the greater resistance to sliding and overturning. However, the most critical element is the physical construction. If teh contractor does not build what you engineer it probably will be at high rick and won't work [over the long term]. If the wall is built as engineered it will even ride out a significant earthquake.

Good luck.

 

[Focht3]

The soil type is predominantly sandy.

I think that precludes the use of a 1:1 slope unless you put in a lot of rock to alter the overall slope stability using the rock mass. I'd be looking at something on the order of 3:1 to 2:1 (H:V), personally. Particularly in earthquake country.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[evilscott]

Guys,

Thanks for the input regarding the tierred wall design guidelines that I asked about. One question though still has not been addresses. If an existing slope is stable as is but I want to introduce a tierred wall system for this area, is it better to leave the existing slope intact and build the wall from the base of the hill and add filler for each prgressive tier ? Or is it better to dig into the existing slope to produce the square-off's for the tierred wall. Intuition suggests that I may be alterring the conditions which produced the stable slope in the first place, though my intuition is based from my experience in mechanical engineering, not geotechnical so it may not be valid. Which is why I ask.

 

[MSEMan]

Whatever you do, don't add an embankement (i.e. filled wall) to the top of the slope. Given your description, the slope seems marginally stable and adding any weight to the top of the slope may just tip the balance.

Free advice for what it is - cut two 3.5'v x 3.5'h benches out of the top of the slope and build the walls into the slope, this will reduce the disturbing force trying to cause the slope to fail. Take the excavated fill and use it to construct two 3 ft walls at the toe of the (second shallower) slope, this weight will add to the restoring forces.

Can't promise it will work, but as a matter of basic slope stability principles, it will add to the overall stability of the slope.

The design of the walls themselves is the next step.

 

[BigH]

MSEMan - for your good comments on surcharging the top of the slope! If I can reminisce a bit - I used to work out of Vancouver BC - oh back about 8tyoh something. One night - during a rainy week, a Seattle TV station had a local news story of how one guy "lost" his house in a landslide. They interviewed this guy - perhaps a barber, maybe not (who knows?) - and he said that he noticed the ground behind his house settling a bit much. He tried to keep it "filled" up to the top but couldn't keep up with the moving mass. He lamented in that had he been able to keep the fill topping up the sliding mass, he would have been okay. I have often wondered about how many other lay people facing the same situation in Seattle/Vancouver areas figured they would try the same thing, based on this "expert's" advice, if it was happening to them? I wrote the TV station with some technical comments - like how could you show an interview like that? - but never heard back. Thanks for the moment.