Factor of Safety for Natural Slope above National Railway

[Silty]

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,

 

[GeoEnvGuy]

Canadian railway have a right of way which is federal jurisdiction and not developed unless it is a crossing. I don't know if they have any design criteria for uphill developments. Outside that right of way you are in provincial jurisdiction using municipal stormwater ponds design criteria, provincial dams criteria or both.

 

[GeoPaveTraffic]

If you are representing the railway and someone other than the railway is developing the property; then I would require a minimum FOS of 1.5.

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

 

[Silty]

Thank you GeoEnvGuy and GeoPaveTraffic for your valuable feedback.
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,

 

[fattdad]

In the United States it's AREMA that publishes design standards. I googled and CN references some of these standards. Just didn't see it for slopes?

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!

 

[Silty]

Thanks fattdad for the detailed response. I could not find anything for slopes from CN rail either.
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

 

[Mad Mike]

Silty,

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

 

[GeoPaveTraffic]

Seems like the real question is how confident you are in the pond liner. The railroad and slope will be there for a very long time. How long will the liner last? How well will it be maintained over the next 50 or 100 years?

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