Foundation piles and slope analysis

[Mccoy]

I recently found that, from the result of a slope stability analysis on a situation of building on large-diameter foundation piles, the pile group appeared to contribute significantly to the strenght of the soil volume inside of it. that is, after inputting the pile group in the analysis, some failure surfaces disappeared or the FS's increased.
I'm a little cautious over these results, since the distance between piles axes is an average o 5 meters in both directions and it des not work as a proper retention pilesheet. Piles lenght is 12 meters, diameter 1 meter, high shear strenght.
Have you come across similar situations?

 

[eric1037]

I have heard of piles being used to reinforce slopes in situations like you describe. However, I don't have any direct experience of references at hand.

 

[dgillette]

15 or 20 years ago, the Corps of Engineers used piles to stabilize the upstream side of Sardis Dam in Mississippi, which happened to be located too close to the New Madrid fault. Actually, they were heavy concrete beams installed as piles. A number of papers were published on the analyses, but the only one I have the ref for is Said Salah-Mars, Lelio H. Mejia, Robert Fleming, Wayne Forrest, Samuel Stacy, "Rehabilitation of Sardis Dam," in Geotechnical Practice in Dam Rehabilitation, ASCE Special Geotechnical Publication No. 35, 1993, pp. 881-895. That paper does not contain much detail, or references other than for the mechanics of the numerical modeling. That makes me think the numerical modeler in the group wrote the paper. I think there were other papers that would have given more information on the analytical results.

Because they were trying to pin the slope through a thickness of liquefiable material, not just a distinct rupture surface, they had to design the beams for very large bending moments. I have not seen this technique actually applied elsewhere, although it's been proposed for a number of dams. The lack of use may result from the bending issue being worse for thicker layers (thicker than they had at Sardis). I've seen people try to analyze it as a direct shear problem, but that doesn't work when the beams have to connect the slide mass to the foundation through 5 or 10 or 20 feet of liquefied material. So, look at moment capacity as well as shear capacity of the piles.

Try googling "Sardis Dam"+earthquake.

Regards,
DRG

 

[Mccoy]

thanks dgillette, there is lots of material about sardis dam remediation,
they actually used almost 2600 r.c. square piles, with spacing from 8 to 12 feet.
This shows that is not unreasonable to expect some shear resistance from a pile group under a building. The software wasn't wrong, after all, at least in principle.

Only, on second thought, let's watch out what happens around the building, where there are no piles !!

 

[BigHarvey]

I did some barrettes in UK to stabilize a slope for a road project. Barrettes were 6.5 m long, 1 m wide and 15 to 20 m deep. Reinforcement was a nightmare with 50 mm bars touching one another where they overlapped, with no room left for concrete ( I wonder whether we can still call that reinforced concrete ). Just to point out that forces are generally tremendous in these situations. Piles are even worse because their inertia is not adapted to this situation. Nevertheless, it can work. You have a very good program called TALREN ( sold by TERRASOL )which is adapted to calculate slope stability with inclusions ( nails , piles, anchors, etc )

 

[BigH]

I remember that there were a number of papers years ago about using "pin piles" for supporting slopes. These were run in at all sorts of various angles to create a reinforced web. ASCE Geo paper - or conference - say mid-80s??? Also, there has been some work about using stone columns in the same way. I remember a few papers years ago - and have them but in deep Toronto storage. Hope this helps with some other directions to look.