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SOE using slope stabilization methods?

SOE using slope stabilization methods?

SOE using slope stabilization methods?

(OP)
I am an experienced geo-structural engineer who is considering the use of soil stabilization methods for SOE. In my area, we have recieved quite a few SOE designs that have very similar conditions:

15ft Excavation
Palisades Diabase rock at 15ft
Glacial till (silty sand/sandy silt/Fill/Not very favorable surficial soils)

Currently, the most common methods of SOE has been rock-socketed cantilevered or tied-back micro piles with timber lagging between piles. Given the hard Diabase that must be socketed, this SOE is fairly expensive ($250k-$350k for approx. 100ftx100ft hole). The heaviest cost in this SOE is the approximately 5ft - 6ft rock sockets that need to be drilled into the Hard diabase rock. It eats through several $15k bits and only 2-3 can be installed a day, and that does not include grouting. Alternatively, smaller/faster to drill micro piles require tie-backs that produce similar cost dis-advantages.

For most of these SOE's, however, we can excavate past the ROW and into the sidewalk areas which generally extend about 10ft beyond the required excavation line. Having previously worked on larger-scaled projects and performed slope stability designs, I am considering an alternative SOE method that would stabilize a 3/4:1 or even 1/2:1 (if conditions allowed for it) excavation slope in these areas. The design would incorporate a grid pattern of traditional soil nails or helical ties that would be tied to the slope using plates connected to geogrid and steel mesh.

From a design prospective, I am fairly confident that I can make the back end of the tie (behind the failure plane) work however, I am not so confident on the face side of the slope given the little amount I have to work with to grab the soils.

Before going down the rabbit hole on this and spending a few days running calculations, I wanted to get a sense from this community if a.) is this a common SOE technique, b.) is this a viable option c.) any alternative recommendations on tying the face of the soil nails/helical ties to the face of the slope and d.) Any alternative SOE recommendations?

P.S. Thank you in advance for your consideration. I have been "lurking" on this site for years and am posting for the first time. Please note that I am a Licensed Engineer with a Masters, and significant experience in Geotechnical Engineering. I am posting this question as a "sanity check" for an application of slope stabilization that I do not commonly see and, on paper, should work.

RE: SOE using slope stabilization methods?

Quote:

I am considering an alternative SOE method that would stabilize a 3/4:1 or even 1/2:1 (if conditions allowed for it) excavation slope in these areas. The design would incorporate a grid pattern of traditional soil nails or helical ties that would be tied to the slope using plates connected to geogrid and steel mesh.

Please sketch out what you’re saying here. I think I understand what you’re going for but want to make sure.

RE: SOE using slope stabilization methods?

(OP)
I did some more research last night and found the NHI4007 and other references that gave me more confidence in this system. Below are some snips from an "abg creative geosynthetic engineering" technical note that will give you a better visual of what I am describing:



Basically, a toe or face failure due to the hard rock that the slope will sit on top of, and the excavation will be down to this rock.

Does this help?

RE: SOE using slope stabilization methods?

I don't have experience with the nailed geosynthetic but differently seems like it would work. I'm not sure if the cost-benefit is really there for the drill bit wear and tear since it's very close to a soil nail wall. The main items that stick out for me are:
1. Nailed geosynthetic would likely have a cheaper material and labor cost to construct than a soil nail wall since there is no shotcrete work, unless nail spacing has to be tighter.
2. Additional excavation will be required for the nail geosynthic than the SNW. This would add excavation cost, and you lose potential valuable space due to the difference in sloping nailed geosyntheitc vs near vertical SNW.
3. Mesh dimension and placement looks to be key. You would need at least 2 nails per piece of mesh.

RE: SOE using slope stabilization methods?

It’s probably too late, but going back to solve the root of the problem (avoiding the rock socket) I can think of a few other options. I assume SOE is temporary. Soil freezing and also an internally braced system (ring beams - but you would still need some rock socket, just less I would think).

I don’t have direct experience with it, but temporary soil nail walls are used for zero lot line big digs in Seattle. They are vertical and they still have shotcrete. But they can be cheaper than traditional tied back soldier pile walls.

RE: SOE using slope stabilization methods?

Your choices are 1) drilled-in micropile soldier beams with lagging and tieback anchors if needed and 2) a soil nail wall above the very hard palisades rock. Your sketches show a soil nail wall. The soil nail wall would be a cheaper solution than micropile soldier beams as long as the overburden soils, including fill, can stand open and unsupported long enough to install soil nails and shotcrete. Also, if there is ground water above the bedrock, soil nailing would be more problematic than installing lagging between soldier beams would be. The soil nail wall could have either a vertical or sloped face. A sloped face could be helpful if you have concerns about the stability of each approximate 5 foot lift that needs to be excavated, nailed, and shotcreted.

www.PeirceEngineering.com

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