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Dewatering for slope stability
2

Dewatering for slope stability

Dewatering for slope stability

(OP)
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!

RE: Dewatering for slope stability

In concept I like the idea, never seen it done, but I like the idea.

RE: Dewatering for slope stability

I have had experience in a similar, but slightly different way.  We had a slope that was sliding on a well defined thin clay layer.  The sliding was induced by elevated pore pressures within this layer.  We directionally drilled in slotted drain pipes which relieved the excess pore pressure, and slowed the movement.  Slightly different problem, but we did have good results.

RE: Dewatering for slope stability

(OP)
What type of slotted pipes did you use?  Do you have a specific manufacturer?  My concern is that the pipe will be damaged or clogged when it is pulled back through the bore hole.

RE: Dewatering for slope stability

We used 1.5 inch diameter sched 80 HDPE pipes with #10 slots.  I can't remember who we got it from, but anyone who sells standpipe casing should be able to help out.

We drilled into the clay shale material horizontally, then pushed the HDPE pipes up.  I believe we only went approximately 40-50 feet per hole, and the shale was self supporting enough to keep the holes open while the drill stem was removed and the pipes were pushed into place.

Hope this helps.

RE: Dewatering for slope stability

The description is for horizontal drains, which is a slotted pvc pipe ... We have drilled these drains in California 300 plus feet with no problems for dewatering slopes and landslides.

If you search the web there are several references, and I believe ASCE has some old articles on the design and spacing.

Good luck.

RE: Dewatering for slope stability

We've used horizontal drains at a number of sites for stabilizing landslides with mixed results.  Big problem has been maintenance to reduce clogging with roots and precipitates (especially gypsum in desert areas where we've gone and built a dam on bedrock with a lot of soluble minerals just waiting to be mobilized).  The biggest root problems were with black iron pipe installed in the '60s and '70s, with holes burned for perforation.

I like your idea for several reasons: 1. can easily get a larger diameter (or more importantly, larger circumference and screened area) than drilling from the toe.  2. can use conventional well-drillers' equipment for installing and cleaning the drains, and you can just let the debris flow out the bottom instead of having to bail it.

I don't like your idea for one reason: 1. you won't be able to get as much drawdown of the water table away from the face of the slope.

Maybe you should just skip the fabric and use slotted pipe with the slots sized to retain the sand.  The gradient will be small so you shouldn't move many fines out of the clay, and so what if you do?  They will just drain out the bottom, and any clogging of the drains could be cleaned out with water jets.  It's not like a dewatering well that gets pumped hard and develops high gradients.  With slotted pipe and no fabric, you could even develop the formation outside the wells with water jets for greater efficiency.

RE: Dewatering for slope stability

(OP)
I was thinking of using 4-inch diameter corrugated plastic pipe, but I am very concerned about damaging it when pulling it back through the bore hole.

I guess smaller diameter pvc or hdpe would be acceptable, but not as effective.  However, it would probably be easier to install.

RE: Dewatering for slope stability

I had a 45deg slope with layers of clay on top of bentinite layer on top of coal seams.  We installed two 6 inch colverts into the side of the slope below that bentinite layer about 100 meters into the slope. Well the slope has stoped moving even with the one meter of rain we had in Alberta last month. The water poured out the colvert for about three month streight before slowing down.   

RE: Dewatering for slope stability

Since you have a layer of sand beneath the site that is not saturated, have you considered drilling vertical drains that will intersept all of the perched water and drain it by gravity to the toe of the slope.  This has the advantage of not requiring any pumping.  I would think of large diameter wells 12 inches or larger.  You could construct a line of them at close intervals to provide a vertical cutoff of groundwater to the slope.

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