Dewatering for Slope Stabilization

[eric1037]

I have a project that is on a bluff overlooking Lake Michigan. The slope is having some sloughing issues.

We performed soil boring and the soil profile generally consists of alternating layers of sand and clay to a depth of about 40 to 45 feet. Many of the sand layers are saturated. Clean sand was encountered below this depth. The slope is about 100 feet from the top to the toe and relatively steep. (Too steep for equipment to efficiently work on it.)

There are areas on the slope face where water is emanating out of the face of the slope from the sand layers.

I am considering using a combination of dewatering, vegetation, and geotextile/gravel to help slow down the sloughing.

The dewatering aspect is what is getting me.

First, I don't believe all of the sand layers are connected laterally. Therefore, I don't like the idea of using deep dewatering wells because there would need to be too many and would be relatively expensive.

I considered wellpoints, but the dewatering contractor I contacted told me they are only really effective to a depth of about 22 feet.

What I am considering now is doing directional boring from the top of the slope to the toe and installing perforated pipe wrapped in geotextile that daylights at the toe of the slope.

I see several advantages with this. First, no pumps will be required. Second, several drains could be installed relatively close to each other, thereby capturing more of the water.

Does anyone have experience with this technique? Any positive or negative experience? Things to watch for in design, construction, operation or maintenance?

Thanks in advance!

 

[BigH]

Eric1037 - first, I would think that you should address the reason for the sloughing. This reminds me big time of the Scarborough Bluffs in Toronto. Major studies have been done and some of it has been put out in the literature. I will see if I can find something. In the meantime, do a search on Scarborough Bluffs and Landslides and MTRCA (Metropolitan Toronto Regional Concervation Authority) and you should get some hits. One name that pops into mind is MAJ Matich and RD Powell. There had been interesting failures in the past in the area. One reason is that the beds of clay dip back "slightly" into the slope creating a "bowl". When the slope sloughed, it released the stored water (even though the head wasn't that high) and it caused a rather severe slough - back in the 1970s I believe.
I think that doing such background searches will give you an idea whether trying to stabilize the slope as you are suggesting or stabilizing the base of the slope to prevent erosive action undercutting and causing the sloughing is an important aspect of your overall project.

 

[eric1037]

BigH:

Thanks for the reply. Fortunately, the U.S. Army Corps of Engineers is doing a research project relatively close to my project site. The soil profile is practically identical. They only have one year's worth of data, but at this point it looks as though dewatering is helping.

They currently have three systems to accomplish the dewatering. Deep wells, horizontal drains installed in the face of the slope, and a system like I am considering. (I will call it vertical drains.)

Their system of dewatering wells are doing quite well. The vertical drain system is also performing, but not as well as the deep wells. They said the horizontal drains are not really helping at all. Since our project is relatively small, we can not justify the cost of installing deep wells and maintaining them long-term. In addition, since the layers are not connected you would need them very close together to be effective. That's why I am considering a system that is gravity drained.

The Corps thinks that the installation technique they used for the vertical drains is contributing to the reason they are not producing as much water as the deep wells. They think the fabric was smeared with clay and clogged.

I am considering oversizing the bore to help reduce the amount of smearing and make the pull easier.

As always, undercutting of the slope is an issue. Fortunately, at this site, there is a relatively large beach and the water levels don't ever reach the toe of the slope. They may sometime in the future though. Therefore, it may be wise to armour the toe of the slope in the future.

However, I want to address the main cause of the sloughing in the short term. Obviously, budget is an issue, as always.

Thanks again for the input and I will be sure to check for additional information.

 

[VAD]

Depending on wether the vertical holes can stand open, another technique is to use a texoma rig to drill verical wells and fill these with clean gravel. the wells are to be taken down to sand layer. This will allow water to be linked with this layer. If you have seepage below this sand layer then this may not work. This technique has been used in slide areas where such conditions have prevailed.

You have to weigh the costs of the operations re directional driling which should work as well.

Sometimes placement of geotextile and heavy rock on slope face can mitigate problem. However, this may have constraints as well. Armoflex can be used as well. Anyhow you are the best judge as you are aware of the site conditions. The above are only some suggestions

 

[BigH]

eric1037 - first off, a project of drainage of 1 year is not sufficiently long to tell if the wells are "working" or not. Bluffs don't usually slough on a regular schedule. They might go three to five years, then slough; then maybe even 10 years and slough; then maybe slough the following year. It depends on the erosional processes, yearly climate/weather, etc. Still, it may be positive. I'd still check out "Scarborogh Bluffs Matich" on a search engine.