Horizontal drain alternatives 8

[Hbone]

I have a fairly steep cut embankment, stability problem.

The embankment consists of varying sized cobblestones and alternating layers of varved clay--glacial till soils--with an underlying layer of firm gray clay. The slide is of a circular / rotational type and is visually heaving at the toe almost four feet high. The top of the embankment has a 4-5 foot scarp, and accompanied by tension cracks throughout the upper portion of the bank.

At the bottom of the embankment is a truck terminal lot, light poles--which are now leaning at 15 degrees--and a water main about 15 feet ahead of the tip of the slide. Through test boring, and utilizing the PASTABLE Slope Stability Analysis Program, we have determined that the major cause of the instability is due to a large amount of underground (an artesian head) and surface water.

Recommendations include the installation of horizontal drains, and from what I have been reading throughout the threads here, that horizontal drains in this type of soil may not be feasible. Could this endeavor be an expensive risk and unnecessary risk?

Limiting possible solutions is the property boundary at the top of the embankment--eliminating the ability to cut a shallower bank.

Any ideas or thoughts would be greatly appreciated.

 

[Hbone]

I was researching the diference between varved clay and till when I came upon this site. This slide is about 45 miles north of here (Binghamton). I am sure BigH knows about it, having lived next door to it.

I have visually seen clay like they are chopping into about 15 feet deep (we excvated some holes near the top of the bank)it was solid gray clay.

http://pubs.usgs.gov/fs/fs13-98/

The Hammer comments had me laughing out-loud, as I feel like the poor guy in the emoticon sometimes too!

Thanks again for such great insights!

 

[Focht3]

Hbone:

I'm glad that the hammer made you laugh; and I hope everyone understands that I use it as a humorous element - I do not mean to insult or hurt feelings.

Myoho:

My statement was,
Your description sounds like deep-seated movement to me; my best guess is that the slide is at least 20 feet deep that far in [10 feet] from the original slope toe. (I added the [10 feet] for clarity. )

Since the entire slide is 46 feet high and has a declination of 21 degrees, the original slope was about 120 feet wide. The failed section is about 300 feet long, so at least 350 feet will need to be treated. So 10 feet in from the toe is a long way from mid-slope, and I was talking about the thickness of the slide at that point. I assumed the slide was deep-seated; look at the post-landslide cross-section in Figure 3 of Hbone's USGS link and you will see that the USGS people came to that conclusion for their slide 45 miles away.

I did some rough CAD work to look at the length of soil nail required to simply get from a wall at the original slope toe and through the slide zone. That length alone is 75 to 100 feet, depending on the assumptions. If an angle of less than 20 degrees is used, the length goes way up.

If you are counting on pullout resistance from the nail, they will need to be at least 150 to 200 feet long - and you will need one hell of a lot of them! Myoho, I'm curious. What size nail would you propose? How much allowable friction (in ksf or kPa)? Your best guess of how many per foot of slope will be needed?

 

[BigH]

 

[SirAl]

Truly a great thread. Full of insight, experience, and (best of all) engineering! Hats off to Hbone for bringing a challenging problem to this forum. Equal acknowledgment to the respondents. I have benefitted from your discussions.

 

[ANTHAE]

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Many years ago we worked on the Barometer Mountain
landslide in Kodiak, caused by excavation for road
fill at the base of the mountain. It bulged at the
bottom and cracked at the top, and many high-level
people flew out and looked at it, measured pore-
water pressure, measured the crack width and li da.
In the end, after countless expensive studies, it
was decided to stack crushed car bodies at the toe
of the slope, as steel is 3x denser than concrete.
If looks are a problem, you can always bio-engineer.

USER ASSUMES RESPONSIBLITY FOR USE OF THESE OBSERVATIONS