Pinning a slide with soldier piles. 2

[Civengine]

I work for a state parks system. The system has little money since we need to be self supporting. And the bosses hate anything that looks "engineered."

I have a 600' slope failure along a shoreline I need to deal with. About a year ago, the shoreline was armored. Pre-armor photos clearly show existing slope failures that look an awful lot like translational failures. The current failures also show signs of sliding along a plane instead of rotating into the lake. I haven't done a subsurface examination yet nor surveyed the lake bottom to see what is down there.

In the past for smaller slope failures, we have cut off the top of the slide and installed cantilevered retaining walls. They work (if you use deadmen- but that's another story ;-)), but they're ugly.

I have considered pinning the slope with soldier piles- either steel or wood.

My questions- Can I approach the design as a cantilevered wall using soil arching between piles? I am concerned because of long-term soil deformation.

Where should the base of my wall be considered to be? At the slide interface?

 

[GeoPaveTraffic]

You can pin a slide with piles, however, depending on the slope hieght; it takes a lot of big piles. The process that I have used in the past is to analize the slope by adding a reinforcing line at the failure plane and increasing the force until a FOS of ~1.15 is obtained.

Then you determine the spacing and size of the pile by using a program such as LPile with the pile going from the ground surface to the failure plane. The goal is to determine the spacing needed to develope the required force from the slope stability program. Then you determine the amount of embedment you need below the failure plane to disipate the required force.

It is a lot of work and you need good information on where the slide plane is located and the soil conditions. Otherwise you just have to install piles until the slide stops, then install a few more to get the FOS above 1.0.

 

[epongra2]

The first thing to do is an extensive subsurface investigation. Lateral extents of slide, depth of slide plane, C and phi values for slide plane material, groundwater levels, etc. Once you have at least some preliminary data, then you can start looking at some remedial repair options.

Stabilizing an active landslide with soldier piles, especially at or near the toe, can be problematic. Do a google search on the Portugeese Bend landslide in Palos Verdes, CA.

By the way, rotational landslides also often slide along a well-defined surface.

The location of the landslide, i.e. lakeshore, suggests that groundwater levels are key to the activation of the landslide. Particular attention should be paid to the groundwater levels. Lowering of groundwater levels within and below the slide mass is often more effective, or at the very least should be used in conjunction, with soldier piles. Oh, and the installation and use of passive or active measures to lower the groundwater levels would certainly not give a real "engineered" look to the area.

 

[Civengine]

I thought of rock drains, but I'm space constrained behind the slide- I have a road on top of the slope.

If I do need piles, I'd pin it near the top, since that is what I want to keep. Maybe half-way down.

My first reaction was to pile rock on the toe of the slope and lay back the surface angle of the slope. I'll do that where I can. But... I have a road on top of the slope and can't lay back all of it.

I'm trying to get hold of a trackhoe to open cut the slide in a small area and a drill rig to get me samples from other locations.

 

[Panars]

Instead of driven piles, you might consider launched soil nails. Instead of resisting the slide by using the bending stiffness of the pile, it is resisted by putting the nails into tension. The soil nail launcher is very mobile and access is usually not a problem. Several municipalities in Ohio have used them to stabilize slopes.

Check

http://www.soilnaillauncher.com/

for more information.

I have no affiliation with the company, and I personally think launched soil nails are used in places where they are not appropriate, but it sounds like your application might be appropriate.

 

[epongra2]

Regarding dewatering, provided you have access to the toe of the slide area, you could drill gently inclined hydraugers into the slide mass above. This passive method, as opposed to dewatering wells, obviously relies on gravity flow and does require occasional maintenance to make sure they don't plug.

Like most things related to landslide stabilization, dewatering is not perfect. It has failed in the past, despite extensive analyses beforehand. Often, slide materials have a very low permeability and secondary porosity (i.e. fracturing within the slide mass) is not well developed to transmit water in sufficient quantities to lower the effective stress. Also, groundwater is often found in "compartments" within the slide mass due to secondary failures within the larger host slide.

 

[rchanke]

Reticulated Micropile Wall - A common method of slope stabilization. Not an inexpensive method but very versatile in terms of site access and design changes/modifications as the work progresses.