Enhancing slope stability using piles
Enhancing slope stability using piles
(OP)
We have piles supporting a bridge abutment. The batter slope for the abutment is 1V to 1H and FOS = 1.4. merely based on Slope Stability analysis excluding piles. In real life situations, will those bridge abutment piles help to increase the slope stability. Rather to implement other forms of design to increase the FOS to meet the Dept of Roads Requirements, we are toying to increase the numbers of Abutment piles to bump it up to FOS = 1.5.
Q1. Is there are reference materials on using piles to increase the Slope FOS?
Q2. If using a computer model, how do I insert the so call abutment piles at the top of the slope?
Appreciate any information and thank you.
Q1. Is there are reference materials on using piles to increase the Slope FOS?
Q2. If using a computer model, how do I insert the so call abutment piles at the top of the slope?
Appreciate any information and thank you.
RE: Enhancing slope stability using piles
All of that said, it is more likely that the end effects due to having a relatively short section (parallel to the slope crest) for the slide to form in is much more significant than the piles. However, without doing a 3D slope stability analysis, which I have never seen done well, there is no way to qantify the effect.
RE: Enhancing slope stability using piles
There are lots of information required to provide a discussion onthe topic - such as height of fill, slopes, type of failure plane determined from stability analysis.
Debate of F.O.S 1.4 versus F.O.S 1.5 should not occur. I see no reason to determine additional piles to achieve the magical 1.5.
RE: Enhancing slope stability using piles
RE: Enhancing slope stability using piles
He references a paper by Poulos in Canadian Geotechnical Journal, 1995.
RE: Enhancing slope stability using piles
If piles do contribute to increasing the FOS by small amount, it will be a economical solution rather than using other means. Increasing the FOS from 1.4 to 1.45 which is closer to 1.5 maybe acceptable to them.
How do you carry out enhancing the FOS using computer analaysis?
thanks
RE: Enhancing slope stability using piles
RE: Enhancing slope stability using piles
Typically, the geotechnical consultant will give recommendations for improvement of overall slope stability if it is less than that required by the DOT.
Some DOTs allow short-term FOS of less than 1.5 if it can be shown that long-term (drained) FOS is greater than 1.5.
Talk with your geotech about how severe the problem is. For a recent project, we were finding that for undrained condition and side slopes of 2H:1V, the FOS was computed to be less than required by the DOT, but was ignored because it was considered a mostly cosmetic issue - shallow sloughs on the slope surface. Deeper failure surfaces under the same conditions had the minimum FOS required.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Enhancing slope stability using piles
RE: Enhancing slope stability using piles
RE: Enhancing slope stability using piles
A few years back, the Corps built a seismic rehab for a dam in MS (not far enough from the New Madrid Fault) and used piles (which were really beams installed like piles) to span a fairly thin weak layer of liquefiable silt. They did a boatload of 3D numerical analysis to verify the concept (especially transfer of force and moment from beam to soil), and they had to use an awful lot of resteel in the beams, which were not off the shelf. Anyway, it can be done, but only for fairly thin layers, and expect to fight the analysis all the way. The analysis is more complicated than rock bolts or piles for vertical loads.
For the probabilistic discussions, remember that not all factors of safety are created equal. Most of us (especially geotechs who are used to great uncertainty in material properties) tend to add minor conservatism at every step of the way - ignoring the cohesion, using the highest piezometer of the set, etc. - so a calculated 1.2 may be absolutely bulletproof. Is your 1.4 really 1.4, or is there unnecessary conservatism built in? If you only have one material parameter to deal with, like phi' for a dry sand, it could be reliably stable with something less than 1.2 -- UNLESS -- there is some overlooked detail, in which case higher FS may not do any good. On the other hand, if you're dealing with a cut in soft clay or something like that, a calculated FS of 1.5 may not be very reliable at all, as in the example cited by BigH. With undrained shear strength, there can indeed be uncertainty in strength that exceeds the margin provided by FS=1.5, especially if the clay is at all sensitive. About 20 years ago, I saw an analysis by one of the early leaders in geotechnical risk analysis that showed an undrained FS of 1.3 for an upstream-type tailings dam gave a more reliably stable slope than 1.5 with a drained analysis, by incorporating the uncertainties associated with each of the input parameters. This was done for the purpose of showing the state regulators that a good 1.3 was better than a typical 1.5. This was NOT smoke and mirrors, but a realistic depiction of the uncertainties of soil parameters and different forms of analysis.
An appropriate probability of failure? Depends on how much a failure would cost and how much it costs to raise the FS. If failure could kill people, real small. If it means just a few $K for regrading, not so small.
RE: Enhancing slope stability using piles
the one you mentioned is a very true and relevant point: due to customary conservativism in evaluating soil parameters (or neglecting some of them) the usual deterministic analysis would yield an output value nearest to say, the 25th, 30th percentile rather then to the mean value.
By the way, are you in on the "riskanal" mail list (risk analysis forum?) your monicker appears to ring a bell.
RE: Enhancing slope stability using piles
Indeed, Mr./Ms./Dr. McCoy, I am part of riskanal. So if you read that one, you probably know exactly who I am, and that among my main (professional) interests are embankment dams, earthquake engineering, and risk analysis. I can no longer hide behind the anonymity of the internet. (A recent cartoon in the New Yorker or somewhere showed two dogs sitting at a computer. One says something like "On the internet, nobody has to know you're a dog.")
RE: Enhancing slope stability using piles
I'm a Mr., and my monicker is derived from the legendary jazz pianist McCoy Tyner. I always use the same, except in riskanal where a full name is required (I rarely show up there, though, and I'm glad the geotech field has some representatives).