Weight Limits?

[TDAA]

I have looked at a slope that has failed. That is, there has been movement in both wet and dry times. At the time I saw it, there was about 1 foot of vertical displacement at a gravel access road (bench)near the top of the slope.

We are looking at ways to stabilize the slope, but the client has asked about posting weight limits for the road to avoid having heavily loaded semi's crossing. This access road is not open to the public and sees occasional heavily loaded semi trucks and more frequent pickup / tool trucks.

No drilling or analysis has been done yet, but the slope is moving, so the FS is <=1. If you add more weight to the top, this will decrease, but obviously not guarantee there will be movement.

So, would / could you assign a weight limit to this situation? I could see doing it if an analysis had indicated a marginally stable condition (no movement, acceptable FS up to a certain load).

Second question: Would you consider a FS <1.5 for this slope? There is nothing below. The access road is the only piece of infrastructure affected by this current slide.

 

[PSlem]

Posting a weight limit might be interpreted as you saying it is safe for vehicles below that limit. I'd avoid a weight limit. If the owner wants a sign, give him a caution unstable slope.

 

[Riggly]

Posting weight limits on a slope that has failed, continues to fail, and has not been analyzed does not make much sense to me. How will you know what limit to set? Definitely, caution signs,but as for load restriction it's a big guess, but I can't imagine truck load will significant lower the stability of the slope. In the mean time, I would have taken precaution to limit infiltration at the scarp, and control runoff as much as possible away from the slope.

Concerning the factor of safety, you can go with less than 1.5, say 1.3. A 1.5 SF could result in much more expense fix.

Anyway, how high is this slope? What is at the toe of the slope? Is it a fill material or native? Does the site experience a lot of rain? What type of failure it looks like? Circular, sliding etc? Is it well vegetated?

 

[fattdad]

The only way to justify lowering the design safety factor is if you have reliable and representative design parameters.

Irrespective of posted weight limits, continued movement is likely.

f-d

¡papá gordo ain’t no madre flaca!

 

[TDAA]

Thanks guys, you are reaffirming my thoughts. I want to avoid the whole weight limit thing. Anything that would be provided would have to be based on design with appropriate sub-surface data.

The problem I had with it was that if the FS is already <1, as in the slope is moving, how could you justify that something less would be safe as well. I think the client believes that this is more predictable than it is, even though I have tried to explain otherwise. It is not like we have a stable condition, and could calculate the changes that would make it unstable - it is already there.

Anyway, how high is this slope? What is at the toe of the slope? Is it a fill material or native? Does the site experience a lot of rain? What type of failure it looks like? Circular, sliding etc? Is it well vegetated?

- ~70 feet high
- Nothing at the toe. There is a highway further down in the valley, but it is outside the are affected by this.
- Native material. "Mapping performed by the USDA Soil Survey classifies the soils at the site as Fughes-Curecanti stony loams. These soils are generally present at slopes of 10 to 40 percent, according to the soil survey. The surficial soils are composed of old alluvium and/or glacial outwash and/or complex landslide deposits." The material in this area is definitely composed of landslide deposits based on visual assessments of the site.
- Not much rain. probably <20" per year. Snow melt in the spring will have much more effect.
- Generally circular. Tension cracks at the top. Other slides have left about 20 foot "near vertical" faces.
- Vegetation varies.