Factor of Safety for Natural Slope above National Railway
Factor of Safety for Natural Slope above National Railway
(OP)
Hi All,
I would like to have your thoughts about what would be a minimum factor of safety for a natural slope above a national railway.
We are thinking about 1.3 instead of 1.5.
The railway belong to Canadian National Railway, but I could not find any reference from CN Rail mentioning the FOS.
it's going to be a development at the top of the slope with some storm ponds which is lined (no houses)
Thanks for your contribution,
I would like to have your thoughts about what would be a minimum factor of safety for a natural slope above a national railway.
We are thinking about 1.3 instead of 1.5.
The railway belong to Canadian National Railway, but I could not find any reference from CN Rail mentioning the FOS.
it's going to be a development at the top of the slope with some storm ponds which is lined (no houses)
Thanks for your contribution,
RE: Factor of Safety for Natural Slope above National Railway
RE: Factor of Safety for Natural Slope above National Railway
If you are representing the developer of the property; then it depends on how well you know the soil/rock conditions and what the railway will accept.
Another approach is to calculate the pre-development FOS and design the development not to lower the FOS.
The design FOS should always be chosen based on what you know about the site, the unknowns, the impact of failure, and the owners risk tolerance.
Mike Lambert
RE: Factor of Safety for Natural Slope above National Railway
The development is outside or CN right of the way, however if anything goes wrong they will be affected. Then, should I still seek their approval you think?
As you mentioned, the level of uncertainty will define FOS, I will consider that.
Thanks again,
RE: Factor of Safety for Natural Slope above National Railway
The AREMA manual is somewhat confusing. I've spent a fair bit of time trying to see where they provide specific design approaches for slopes. It's not too obvious. Basically, you'll see reference to FS=1.5 for embankments. But, they are talking about bearing pressure! Not global stability!
The nature of the design soils for embankments makes it easy for them to claim design slopes of 2:1 or 1-1/2:1. Mostly, in these instances the reference is to toe slopes where the critical circle is within the embankment.
For private development next to a public service, I know what I'd recommend: Keep the safety factor unchanged. So, do what you want, but make sure it doesn't change any aspects of the global or toe failure modes. How could that be challenged?
Good luck.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Factor of Safety for Natural Slope above National Railway
So, the development will involve digging a pond and line it to avoid water infiltration and then fill it with stormwater.
If the liner is constructed properly with sufficient QA/QC, then I think that we are basically removing soil and fill it with water at the top of the slope. So, the unit weight of water is almost half the soil and since it's at the top table land, the safety factor will increase.
This is proven by slope stability analyses as well.
Do you think this is a sound reasoning?
Thank you very much
RE: Factor of Safety for Natural Slope above National Railway
Safety factors calculated for global embankment stability often mean nothing- unless you have a precise knowledge of what's in that slope, you have no way of knowing whether you've assigned the correct shear parameters to your analyses, long-term phreatic surface, pore pressures, cohesion in clays, the variables are many...
You need to be confident with the stability of your development, preferably confirmed by the local geotech guys. Even if you did produce a 1,5 factor of safety on paper, that won't stand in court when the slope fails and it turns out you used the wrong parameters in your stability analyses. Just a small tweak on the water-table in those analyses is the difference between 1.3 and 1.5!!!
Chances are you have no problem at all- as you say you're unloading the crest of a slope which should only increase its factor of safety. However, you still need to consider the construction phase and the potential for heavy rains while your excavations are open.
All the best,
Mike
RE: Factor of Safety for Natural Slope above National Railway
Under these conditions, I would check the long term FOS assuming the liner leaks, i.e. I would run an analysis with a high water surface in the slope. If this FOS is sufficient high, then I would probably say ok. As for what would be sufficiently high, goes back to how good I feel about the slope stability model and how well I feel I understand the soils in the slope.
Mike Lambert