Slope stabilisation using sheet piles 3

[Myoho]

I have been asked to review a proposed design to stabilise an embankment reservoir using sheet piles.

The embankment is about 7m high with an upstream and downstream slope of about 26 degress. The embankment material is a soft to firm sandy clay. The crest of the embankment is about 7m wide with a 5m wide road built upon it. Tension cracks have appeared at the crest of the dam (near the side of the road) and bulges have appeared at mid slope level. The proposed solution is to sheet pile through the crest into the stiff clay founding layer to intercept the slip planes.

Ive not used sheetpiles to stabile slopes in this way before and have a couple of concerns. Im worried that a tension crack could form between the sheetpile and the embankment material which could fill with water and create hydrostatic pressure onto a slope which is already unstable.

Is it the right solution? Any advice / recomendations would be very welcome.

 

[GeoPaveTraffic]

How are you going to install the sheet piles? Driving piles in an unstable slope is a bad idea all the way around.

If you could install the piles, think about how they would resist the slide forces. The piles would need to take load from the moving portion of the slope to the unmoving portion. It is very unlikely that sheet piles are stiff enough to do this with out bending.

Lastly, as the piles take load the interlock stress will increase. This will prevent water from draining through the piles and cause the water head in the slope above the piles to increase. At some point this is likely to cause the entire embankment to fail.

All in all, sheet piles to stabalize a slope = not a good idea.

 

[blueoak]

This sounds like a very scary design. Do you know why the slope is failing? Is it seepage or just poor design/construction? I am most familiar with a drainage blanket and a stabilization berm on the downstream toe for slope instabilities. Sandy clay isn't always the best material for long term stability. Have you tested the slope to see what is going on? Is there water at the bulge or coming out of the face of the structure? What does the original design say about seepage and slope stability. Depending on the slope bulging you may want to have the reservoir level decreased.

 

[ntschwanz]

The project you briefly described does sound scary. The downstream slope of about 2H:1V, seems overly steep especially when combining seepage effects. Use of steel sheet pile to stabilize a slide of the size you describe may be possible because it is probably small so that stabilizing loads may not be large. But you need to design for an acceptable factor of safety rather than make the situation marginally stable. This often negates the use of sheet pile as a stabilizing alternative.

 

[dgillette]

Add my name to the naysayers for sheet piles. Sheet piles could be of benefit if they were only required to provide resistance in shear, but they would probably be required to provide moment resistance in the materials you talk about. They just don't have the moment capacity. (You might have a quick look at the literature on Sardis Dam, where the Corps used very heavily reinforced concrete beams, installed as piles, to connect a possible slide mass to the underlying foundation through a layer of liquefiable silt or silty sand. The analysis was very complex, and the cost was quite high. Those were not off-the-shelf H piles or a common size of prestressed concrete pile - they were massive and required very heavy reinforcement to develop the required moment capacity.)

2H:1V is not necessarily too steep of a slope, nor is SC necessarily bad material for a 2:1 slope. It all depends on whether drainage is provided within the embankment and whether the fill was properly compacted. If there is water up to the surface of the slope, as might well occur if the dam is not zoned or drained, 2:1 is too steep. Without knowing the age and design of the dam, piezometric/phreatic levels in the dam and foundation, foundation characteristics, etc., it's hard to say what the best solution would be, but after looking at the data, I would most likely start by looking at drainage or an earth berm, possibly keyed into firm foundation material below any weak zones in the fndn.

Rereading your post, I see that the piles were to go THROUGH THE CREST, but it's not obvious that they would intercept the sliding surfaces if they did; you might just see pullaway scarps on the downstream side of the sheet. They might have some benefit in reducing the seepage, but if they are exposed to both air and water, you'd have to worry about them rusting away and making things worse. Also, I would be very hesitant about drilling or driving anything into a dam embankment except well downstream of the water barrier.

 

[WePushPiles]

GeoPaveTraffic, I'm glad you brought up the point of not being able to install pile on unstable embankments, and you're exactly right if you're using a conventional vibro hammer. The company I work for, however, can stabilize slopes using tubular sheet piles and have done it countless times in Florida and all around the East Coast and Mississippi Valley. We have specialized equipment from Japan that requires no temporary staging if access concerns are present.

Our machines utilize hydraulic jacking methods which installs piles vibration-free and can easily install pile in firm sandy clay conditions. Myoho, if you are still in the design phase for this project, check out our pile driving methods and see if they would be appropriate.

Alex Sotelo
Giken America Corporation

http://www.gikenamerica.com

 

[VAD]

It appears that the stabilization approach used was to prevent loss of the roadway rather than the dam. dgillette, you are correct in saying that the soil will peel back behind the sheet pile i.e on the failing side. This is something that is often not considered or forgotten when even using piles. The idea of placing the piles at the crest is also to reduce the forces on the piling.

Based on my experience this is a short term solution and would be monitored. As soon as the soil begins to peel back, I am sure that there would be a tieback system used or that they will try to fix the downslope by material replacement. Sometimes you can buy a year or two with the procedure intended.

Re driving versus silent installation. If my thoughts about the stabilization are correct then it probably does not matter which way you go. However, if the silent option is available and cost effective then I would certainly take this option.

Seems to me like funds are the problem and overall risk may not be considered great. Just my views.

 

[jdmm]

If this isn't too late:

A method imployed by Dr. RM Hardy and attributed to him is to align the sheet piles parallel to the direction of movement. This is counter intuitive but the purpose is to effectively increase the shear strength of the soil by providing shear resistance on the sides of the sheet piles and transmitted to the stronger more resistent soil beneath the zone of movement. I carried out field review of his installations and can confirm that they work well.

Sheet piles when installed perpendicular to the direction of movement have very little support compared to the load impressed. If it is possible to supplement this with anchors near the top of the sheet pile they can be effective. Usually this is not possible and the Dr. Hardy approach can be employed.

 

[asiandude]

Hi Myoh

Interesting problem you got there. I think my opinion here might be too late to be heard but never the less just for discussion sake. Based on the slope failure that you have describe I would say that the failure occurs due to weak shear resistance at the toe of the slope. I dont know the extend of the soft soil layer of your embankment but im guessing its near or maybe further below the toe of the embankment.

Using sheet piles does sound like a sollution to solve the problem ofcourse PROVIDED that the selected sheet pile can withstand the bending moment due to the weight of the soil pushing along the failure plane.

Theorethically it sounds symple but construction wise can it be done? Can your sheet pile be constructed on an embankment which might still mobilize during construction work? Can the sheet piles be embedded deep enough to provide that bending resistance you require?

Sounds tricky but maybe if we use a simpler approach it might prove to be workable and cost effective. I was thinking of maybe just to strengthen the toe of the embankment as that is maybe the main cause of the problem. Balancing berms might due the trick. However proper modelling and checking has to be carried out for stability. The length of the balancing berm and the height have to be analysed to determine the best balancing berm configuration. I believe this sollution is much more cheaper and safer to construct.

Im guessing that this sollution might not be preferrable if you have very limited space for the balancing berm. If this sollution does dot suit your site condition I would advise you to divert your focus to the toe of the slope before you start doing any construction on the crest of the embankment. And make sure you carry out proper monitoring works during and after construction to ensure that the stability of the slope is secured.

Hope that this help. Have fun!!

 

[dgillette]

I would NOT NOT NOT put in sheet piles perpendicular to the dam axis (i.e., parallel to the direction of movement), unless they are limited to the downstream 1/3 of the downstream slope (or upstream 1/3 of the upstream slope). %-( Hardy's way - using the sheet piling as shear walls - might be fine for other slopes, but putting a line of sheet piles through a dam would create a new Achilles' heel for seepage and piping. Along the piles, there would be a disturbed zone that could concentrate seepage and increase the chance of erosion by the seepage flow.

 

[BigH]

Agree with dgillette. Don't remember this from 2006, but sounds like it might be a perfect situation for using a downstream stabilizing berm - with drainage zone sandwiched between the berm and the existing slope.

 

[dgillette]

Well, in that case, I agree with BigH. I'd start out looking at something like he describes, but also look at whether a "key" could be excavated through a thin weak layer and backfilled with stronger material (without destabilizing the whole slope - need to dewater while working?). This might be needed if the space beyond the toe is too limited. Consider also relief wells or other means to lower the piezometric/phreatic line.

 

[asiandude]

Hi everyone!

Glad you like the idea of using balancing berm / stabilizing berm rather than sheet piles, BigH. Its a good idea to sandwhich in those drainage zone by the way. Im guessing sand might do the trick. Just to add thou, make sure that the toe drain of the embankment is properly extended around the balancing berm's toe and well connected to adjacent drains. You surely want the water to keep on flowing

Creating a key at the toe also sounds like a good idea. However I am curious of how it can be done if your slope is already showing tension cracks..... hmmm ..tricky....How about using heavier material for the balancing berms? granular fill maybe?

 

[BigH]

Asiandude - one way that the toe key can be constructed in a marginally stable slope - is to carry out excavations in very limited widths - so that the slope doesn't "see" the excavation. You can do this in leapfrog fashion - then come back and fill in. I've done this successfully in front of a large tailings dam in northern Ontario years (make that eons) ago. Still, if you have the room, stabilizing berm and the drains are the best way to go - you can even excavate in finger drains (excavator bucket width) into the embankment to draw the water further back inside the embankment.