Soil Creep 1

[ChinookGeo]

I am currently working on a project involving soil creep and I was wondering if there are any equations/ calculations available. So far I have been unsuccessful in finding an equation telling me how to calucalte the lateral forces from soil creep and if anyone knows of an equation or if there even is an equation it would be much appreciated.

In addition, please feel free to post any information or suggestions when dealing with soil creep.

Thanks,

Brian

 

[fattdad]

Creep (as I understand it) is the gradual loss of cohesion over time. For the case that somebody bases lateral earth pressures (or slope stability) on the cohesion intercept (or undrained shear strength), there is a strong likelihood that this shear strength will not be there for teh long haul. Rather design using the effective friction angle. For the case of stiff fissured clay, design using the residual friction angle.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[ChinookGeo]

I'm looking at the downhill forces on piles and grade beams due to soil creep. Is there a range of typical soil creep forces?

 

[palmahouse]

I have tackled this problem before. A mentor of mine recommended doing a wedge analysis over a perscribed "failure surface" using a peak phi angle and ignoring cohesion, assume there is no earth downslope of the piers, and find the unbalance force that corresponds to a factor of safety of 1.2. The rational is that creep occurs when the driving stresses get within about 80 percent of the resisting stresses, so the calculated unbalanced force should prevent creep if it is applied to the earth (in this case, structurally). Then, I applied a creep load on each pile equal to the calculated unbalanced force times the pile spacing, and applied it to the pile at a depth below the ground surface equal to 2/3 the depth to top of support (2/3 the depth to top of rock in my case). Then, I recommneded that the structural engineer design the pile to resist that force and any other lateral loads (seismic and wind) using the UBC pole formula. Nowadays, I use L-pile, do the analysis myself.

In my case, prescribing the failure surface, or a surface over which creep occurs, could be reasonably estimated, because it was coluvium creeping over top of rock, and the rock was thestable top of support. May be difficult for your problem if there is no rock.

Good luck - not an easy problem. My gut feel is that the approach I describe is reasonably conservative and captures all the soil mechanics that influence the problem. Incidentally, the projects I worked on held up fine.

 

[slam00000]

In classical geotechnical books Creep is explained under "Secondary Consolidation" . So look under these keyword

 

[fattdad]

secondary consolidation is change in void ratio with out a change in effective stress. The loss of cohesion over time is a different behavior.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

fattdad, you know of any good references on the topic?

 

[fattdad]

msucog, are you referring to the topic of secondary compression? Virginia Tech has a "Settlement Manual" by J. M. Duncan, that has a good section on secondary compression, which is the tailing effects of a 1-D consolidation test. My statement pertaining to change in void ratio with out a change in effective stress is fundamental to the 1-d consolidation test. The end of primary consolidation (T100) occurs when all pore pressure is dissiated. The inflection point at T100 marks the beginning of secondary compression (with the slope of C-sub-alpha).

If compression occurs in the absence of excess pore pressure, then all of the original loading has been conveyed to the soil grains (i.e., no further excess pore pressure, T100). However, any observation of C-sub-alpha reveals there is continued change in void ratio.

From my way of thinking, however, this has nothing to do with the original post. But, sometimes I'm stubborn - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

no no...not secondary consolidation...creep. i agree that both are different animals. but since creep is very complicated and not "fully" understood, i have not been able to find a lot on the topic. i have my thoughts on it but i'd like to see what some of you that are much wiser than me think about it.