building retaining wall on existing slope

[KaBone]

I'm not exactly sure where to begin except for how is it done? In addition to the wall design, would I have to specify the construction technique? How do you cut into a hillside to place a retaining wall without the hillside crashing down on you? How do you dig the heel? Can the heel encroach on an adjecent property?

 

[BigH]

KaBone -Think out of the "Box". There are many types of retaining structures about - many varieties and modifications of each. You seem fixated on a traditional cantilever retaining wall. Have you considered using a nailed wall? I remember seeing an article in a magazine years ago where the Stuttgart subway excavation proceeded from the top down. They cast 1m by 1m panels. Excavated down 5 ft. Placed the panel against the soil, drilled a soil anchor (nail) through the precast panel, grouted, then prestressed the anchor so that the panel acted as a footing pushing into the hillside - hence - reinforcing the soil behind. They then excavated another layer, did the same thing, etc. You can then shotcrete the soil between the panels to keep the soil from falling out. Forms a nice wall - you might want to use a facing concrete pour (or not). Other retaining walls to consider might be to use secant pile walls. But, understanding what you need to achieve, what methods can be used to achieve it in constrution - you can come up with a complementary design.

 

[KaBone]

Thank you for the reply. Can the nails/screws encroach on the adjacent property?

 

[BigH]

They could - but, of course, you would need permission from the owner of the adjacent property to do it - and since it would be permanent, a long term "easement".

 

[born2drill]

How deep is your cut? You may not need tiebacks or nails if you can cantilever a soldier beam wall. Secant walls are awfully expensive, and usually only needed when a very stiff and watertight system is required. A cut into a hillside is just going to be a single run, which can't be watertight.

Can you bench off enough room to get a large rig in to drill in beams? Micropile soldier beams might be an option too if access is restricted.

 

[BigH]

Good points born2drill - I was just trying to highlight that there are many solutions - if one can get past the textbook cantilever wall mentality.

 

[KaBone]

I'm still working out the overall cut depth. Conservatively, I'd say 10'. The adjacent properties are on each side. The cut on them would be 10' down to zero. How large of a bench would the drill rig need? If it's 10', that would be all the way to bottom since the slope is nearly 100%.
I looked into secant walls and to some extent soldier walls. The examples shown were massive projects spanning miles and several meters deep. It's hard to imagine how I would get that equipment on site for a 10' cut that's 70' long.
Since the piles are vertical, wouldn't it appear as the wall is failing since there will be horizontal flexure?

 

[born2drill]

You should be able to cantilever a 10' cut. If you can drive beams or sheets with a crane you may be able to work from the bottom of the slope (where I assume you have flat ground).

Of course the wall will have some deflection, but you're talking about a few inches maximum at the top. It will most likely not even be visible to the naked eye. Also, if it's a permanent wall, you'll probably be pouring a concrete facing in front of it once you've excavated down to subgrade.

 

[KaBone]

Forgive my ignorance, but what does "cantilever a 10' cut" mean? I don't think the neighbors will be happy with the noise level from driven piles. I'm thinking boring. Another thing is, from the bottom the only way out will be a 4' wide ramp. Also, since the back property line is curved, I'd like the wall to follow the same convex curvature. Does that complicate things? The side walls will be straight. Just let me know if you need a sketch.

 

[born2drill]

A sketch would certainly be helpful.

However, if you're not sure what type of wall would be best suited for your site, not sure how to design it, and not sure how to build it, your best bet might be to get in touch with a desigh-build specialty contractor...

 

[MSEMan]

KaBone,

This shouldn't be a big deal unless you are dealing with very poor native soils. Put it out to bid on a design/build basis and save your fees to develop a performance spec and evaluate the alternates that come in. Make sure the contractors have the appropriate levels of E&O, and evaluate the proposals carefully.

 

[KaBone]

Here's the sketch. It's a bit grainy, but you should be able to get the idea. The soil is clay, fairly stiff. I haven't done any kind of soils analysis yet. I want to design it.

 

[ptmoss]

I agree with born2drill and MSEMan. What kind of experience do you have? Are you an engineer? I have always been under the impression that walls over 5' tall need to be designed by an engineer, but I don't know offhand the particular code that comes from.

I understand the desire to design something and not hand it off, but if you're not experienced with wall design methods and working with clay soils then I'd reconsider and follow the design/build advice. It may very well be that you would look at the design and say "Dang, I could've done that", well next time you can!

 

[KaBone]

Yes, I am a PE, although just a lowly civil. I'm trying to beef up my design portfolio. I was going to design this one and have a structural or geotechnical sign off on it. But it's like I can't even buy my own jobs. Am I oversimplifying things by asking which type of retaining structures best fit my application?

 

[BigH]

No, KaBone, I have no problem with your asking about types of walls, etc. One thing I would suggest is that you peruse Engineering News Record, Heavy Construction News, Ground Engineering and other magazines - they typically identify current projects and give descriptions of foundations, structures, etc. You can learn a lot about how certain types of walls fit specific site conditions. The one I remember best was an article about the Stuttgart Subway - way way back in time.

 

[oldestguy]

One added point:

In changing the landscape here, the overall "global stability" need checking. Most geotech engineers can check that. No point in having your whole area, neighboring house, wall and soil sliding down the hill.