Geotechnical Engineering 1

[Daryoush]

Soil creep due to wetting drying cycles on a slope would exert a force on drilled piers, piles and retaining walls. Does anyone know of a methodology for determination of this force?

 

[Focht3]

Ah, the infamous soil creep! Please excuse my cynicism - some "local" engineers (central Texas) like to claim that damage to residential foundations in my area is due to this phenomenon, even though there is no evidence of any significant horizontal movement. Bad engineering, if you ask me. (Most aren't geotechnical engineers, so they are really outside their area of expertise.) But that's not you -

In general, you will need to estimate the depth of soil movement, then estimate the ultimate passive pressure that the moving soils could exert on a pile. This should give you a "worst case" analysis. Keep in mind that the force will be quite large; you are, in effect, holding up a portion of a slope that is trying to move. And you should understand that since soil creep can be associated with the progressive failure of slopes, the depth of the unstable zone that you assume will be a critical part of getting the "right" answer.

Please describe your site structure and approximate location, as well as the subsurface conditions. And tell us why you think the problem is soil creep -

(I assume that this is a well recognized local problem.)

 

[Daryoush]

The site is in North San Farncisco Bay Area in Northern California. It is a downslope(2H:1V)lot with a car port. The bedrock is Franciscan Sandstone and Shales and the overburden is clayey sand and gravel (colluvium)about 7 feet deep. Depth of creep would be probably not mor than 3 to 4 feet (assuming there is a potential creep occuring). I can think of the creep taking place as a viscoelastic movement. Therefore, passive pressure does not make sense to me, because the force required to move the soil around the pier would be far less than the passive pressure developed when pier is imagined to cut through the soil.

 

[VoyageofDiscovery]

Hi Daryoush,

Try this website and check out the pdf file "Response of flexible piles under laterally linear movement of the sliding layer in landslides", this should answer your question.

http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_tocs_e?cgj_cgj1-03_40

Focht3, some more info on flexible piles since our last discussion.


Regards

VOD

 

[Focht3]

VoyageofDiscovery:
Good - I'll take a look.

Daryoush:
Your slope declination does make creep a possibility. And you're in earthquake country - so extra caution is advisable. However, I have some trouble with creep in clayey sands and gravels. It seems much more likely that your slope's FOS approaches 1.0 when the perched water table above the Franciscan rises. Perhaps lateral drains might help the overall stability.

Viscoelastic flow can occur in an earthquake (particularly if the GWT is shallow), but I doubt it is the movement mechanism under quasi-static conditions. Remember that if you miss out on predicting the magnitude of the lateral forces, the slope will have the last laugh.

 

[Daryoush]

Hi VoyageofDiscovery, Hi Focht3,

Thank you for the reference and insights. I read the reference. It considers a failure mechanism. I know that there is a tendency to look at soil creep as a slope sliding failure mechanism, and it may very well be that the progressive creep eventually leads to a slope failure, in which case then the force becomes that due to the sliding mechanism. However, when creep is still just a creep and not yet a mechanism, the forces applied to the pile or pier will be due to the viscoelastic flow. The creep takes place when the moisture content of a clayey (expansive) soil increases during the wet season causing it to expand perpendicular to the surface slope. Therefore a sandy soil would have a smaller creep and consequently exerts a smaller creep force on the pile. A clayey soil, on the other hand, significantly softens when moisture content is increased. Therefore, the creep force cannot be considered as a function of global shear strength of the overburden material. If we can envisage the creep phenomenon as the overall movement of about 0.1 inch per year of the top 3 or 4 feet of the overburden soils, then the force I imagine should not be much greater than the active earth pressure acting as an inverted trangle (1/3 H from top; H is creep zone depth)on the pile.

 

[VoyageofDiscovery]

Hi Daryoush,

It is difficult to design a structure for this type of load. Where would you draw the line wrt the maximum years of service and the unique residual strains in the structure due to various viscoelastic cohesive values.

Can you remove this layer? If you feel this is a real problem, it may better to tell the owner that the layer of soil needs to be removed due to its movement. It may be a copout but its better than designing in uncharted territory as far as I know.

The other thing what if this viscoelastic slope is subject to significant e/q loading, will it liquify endangering people or property downstream?

Regards

VOD

 

[Daryoush]

Hi VoyageofDiscovery,

I fully agree with you in terms of the design approach. My point was the actual evaluation of the creep force. I agree that when we are designing for a long term service, then we need to consider ultimate stability under long term and transient load. And therefore if creep leads to sliding failure, then consider the force on the piers due to that condition. My question was to determine what is the force due to the creep factor alone. How do we evaluate a creep force (force directly resulting from soil creep before it leads to a slide) on the pier.

Regards,
Daryoush

 

[Daryoush]

Another type of creep is the very slow movement of the colluvium layer over the bedrock under the force of gravity. This movement could be also influenced by increased moisture in the colluvium layer, but it is totally different in nature from the creep due to expansion and shrinkage of clay. However, this type of creep could also lead to strain softenning on the plane(s) of the movement and ultimate sliding failure. For this type of creep, we would need to evaluate the force on the piers using a different model. Again, at this point, I am not concerned about the force on the piers due to an already sliding soil mass. Once we evaluate these forces, we could examine the effect of pier reaction on the local stability of the slope.