Neglecting Passive Pressure in Frost Depth 1

[azogr]

When calculating the passive resistance of the soil below the frost depth is it proper to consider the passive pressure based on the actual depth or should the passive pressure be calculated on the actual depth minus the frost depth?

For example if the frost depth is 2' and my passive pressure is 200 psf/ft. Does my passive pressure below the frost line begin at 400 psf and the triangular pressure area above is ignored or does it begin at 0 and I begin a triangular pressure curve from there?

 

[CarlB]

I've seen this "ignoring contribution of soils in the frost depth" but I've never heard a valid explanation as to why. I practice in an area where design frost depth is 10 feet. When frozen, soils have increased strength, generally. Only when thawing is there a loss of bearing strength due to excess water/saturated layers. And there may be a small decrease in density if soils were fully saturated, then subject to freezing. I think you could just use a reduced strength in the frost zone, but about the same unit weights.

 

[fattdad]

I always discount the upper 2 ft of burial on passive pressures, just because folks seem to like digging. To me it's added safety factor (just coincidentally our frost depth is also about 2 ft).

f-d

¡papá gordo ain’t no madre flaca!

 

[aeoliantexan]

If you are simply correcting for the loss of strength during thawing, treat the soil in the frost zone as a surcharge. The passive pressure at the frost depth is equal to the overburden pressure times the passive pressure coefficient. For your example, yes, start the pressure diagram at 400 psf. This is still conservative, because the pressure in the top 2 feet is rarely zero. You should be safe also if someone excavates a narrow trench 2 feet deep along your wall.

Of course, if there is a chance that the top two feet will actually be excavated, the pressure diagram will be reduced by 400 psf.

 

[azogr]

Thank you for the responses. texan, this does seem like a reasonable approach to me, you worded it better than I did. I agree I would have to make an assessment of the likelihood of excavation. In a case I am working on now there is going to be a parking lot paved right up against the building so I could feel relatively good about it not being excavated. Other situations might not be as protected from digging.