Stable Feature MSE Wall

[RFreund]

We are looking at a couple different options for installing an SRW/MSE wall. This wall was originally designed as an SRW type wall. It is near the bottom of steep slope and in front of a river. Thus there are relatively long grid lengths. It varies in height up to maybe 14' tall and the slope is about a 1V:1.5H or 1V:2H. They did some soil 'probing' and found:

“Approximately 1-foot of low bearing capacity soils overly what appears to be a combination of very dense slope wash soils over a layer of limestone over shale. The slope wash contains many large limestone slabs and from visual observation of the limestone in the slope it appears to be approximately 6-12 inches in thickness and is underlain by shale. Based on the DCP values it is doubtful that helical coils would be able to penetrate to sufficient depths for adequate resistance”

To reduce the excavation into this shale/rock, we are looking into designing the wall using the Stable Face MSE wall method as outlined in the FHWA-NHI-10-024 and FHWA-CFL/TD-06-001. Or possibly anchoring some steel pipe (running parallel to the wall) into the shale/rock with rock bolts (?). Then lapping the grid around the pipe and back to the wall.
I have a few questions and was hoping for some feed back:

1. While the Geotech will most likely give us their insight, I am wondering what the stability of limestone over shale is like? And/or can shale be "rock bolted" to?

2. Any other ideas for this situation?

3. I see Hilti has a product line for "rock bolts" however it is not found in the US. I'm having a hard time find any other similar products, any suggestions? Basically there would be a good number of these so the capacity does not need to be extremely high. Also I'm trying to keep the complexity to a minimum although I'm sure testing will be required. Some sort of threaded rod / epoxy anchor?


EIT

http://www.HowToEngineer.com

 

[cvg]

try williams for rock bolts. most rock can be bolted, length, diameter and number of bolts is one variable that geotech can give you. you might also want grouted soil nails, not rock bolts. can you just use soil nails and shotcrete and forget about the MSE?

generally shale is not very good and slope wash limestone also sounds questionable. you need a geologist to evaluate this

 

[GeoPaveTraffic]

Agree with cvg, also I typically use Dywidag bars.

Limestone over shale, actually any shale, can be a real problem for slope stability. Depends on the degree of weathering and the dip on the beding planes.

All in all, I would not be looking at any type of MSE wall. Soil nails might work, but I would be looking at a cast in place wall or maybe a soldier pile type wall.

So much depends on what the soils/rock really look like, degree of weathering, etc.

Mike Lambert

 

[RFreund]

Thanks for the replies.

Yeah we are going to get more input from the geotech.

CIP and soldier piles seem like a lot of excavation into 'rock', no? Are you suggesting these systems to improve global stability?

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

CIP would likely require some rock excavation, hard to tell with as little as I "know" about the site.

Soldier piles wouldn't shouldn't require much.

As for these options "improving" global stability, not sure. Unless there is a current stability problem, in case you have a completely different set of problems, the goal would be to not make global stability worse. Slope like the one described can be very sensitive to changes in geometry.

Mike Lambert

 

[RFreund]

Mike -
I guess what I was getting at is if the slope is the problem than a CIP system and maybe the soldier pile system would just go 'along for the ride' the same as an MSE system would, no? I thought you were suggesting these as a way to handle a global stability issue. I believe the solider piles would need to be drilled/driven(?) into rock below as well.
Essentially any of those systems would satisfy internal stability so isn't it really a matter of global stability or am I missing your point?
If all systems are the 'same' as far as global is concerned than really I'm just trying to use the most 'economical' system to satisfy internal stability. I did/am thinking about soil nails as well, however I think this option may come higher but may result (as you have indicated) in a better system.
Could you may express some reasons you dislike the MSE idea and prefer the other systems? Unfortunately I still do not have much soil info to go on other than what I described above.

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

I obviously wasn't clear.

You are correct that any of these wall types can be designed to provde adaquate internal stability. The issue is global stability.

If there is a global stability issue, MSE walls will be very difficult to install in a way that improves global stability.

Either CIP wall bearing on stable rock or a soldier pile wall with the piles anchored into stable rock will be much more capable of improving global stability.

It all comes back to what is really going on, and better soil/rock information will be required.

Good Luck.

Mike Lambert