Slope stabilisation using sheet piles
Slope stabilisation using sheet piles
(OP)
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.
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.





RE: Slope stabilisation using sheet piles
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.
RE: Slope stabilisation using sheet piles
RE: Slope stabilisation using sheet piles
RE: Slope stabilisation using sheet piles
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.
RE: Slope stabilisation using sheet piles
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
RE: Slope stabilisation using sheet piles
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.
RE: Slope stabilisation using sheet piles
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.
RE: Slope stabilisation using sheet piles
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!!
RE: Slope stabilisation using sheet piles
RE: Slope stabilisation using sheet piles
RE: Slope stabilisation using sheet piles
RE: Slope stabilisation using sheet piles
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?
RE: Slope stabilisation using sheet piles