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# What is passive wedge?

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 geo6 (Geotechnical) 9 Apr 12 20:12
 We have two inclinometers on the same line of defense, both of which have been sheared. However, it appears that the shear plane is not in the same elevation considering they are quite close to each other. One eng mentioned that this could be possible if one of the inclinometers is installed in the passive wedge. Can someone explain the concept of passive wedge in soil?Thanks,
 AussieGeoEng (Geotechnical) 9 Apr 12 20:41
 When referring to a passive wedge it is typically the wedge of soil in front of a retaining wall.
 DRMABZ (Geotechnical) 17 Apr 12 7:43
 You will find a reasonable explanation and some drawings that will help illustrate this in most basic geotechnical text books. Look under retaining wall design or earth pressure. Although keep in mind the simple methods, Rankine, Coloumb, etc, involve a idealization of the problem. It is not inconceivable that on a complex site, with 3D effects and variation in geotechnical properties that it will be a bit more complex in geometry.The "passive" bit, refers to the soil i.e. the soil is not moving (i.e. is passive) the wall is being displaced into the soil. That I don't understand from what you describe, unless the inclinometers are downhill of the wall. Uphill of the wall would typical be an active wedge, if only gravity loading is relevant.
 fattdad (Geotechnical) 17 Apr 12 11:40
 I'd be more interested in providing some comment if I had a better explanation of the problem, a sketch showing critical data points and the data from such location.If you want to understand the location of the passive wedge, you can draw it out yourself (knowing friction angle) by taking the lowest elevation of the structure that's applying laterial pressure and casting a line that's set at the angle of 45-phi/2 as measured from the horizontal.Then again to account for wall friction, you may want to consider a log-spiral passive zone.  Then again, you may have some unique boundary condition that will require some alternate method to assess the passive wedge.f-d ¡papá gordo ain't no madre flaca!
 geo6 (Geotechnical) 15 May 12 13:57
 This is actually for an earth-fill tailings dam. My interpretation is that the passive wedge refers to an isolated geological layer outside the toe of the dam.
 Ron (Structural) 16 May 12 8:42
 Think of the variability of your soil profile. As DRMABZ notes, your issue is in the "real world" not idealized. There can be multiple shear planes and there same shear plane can be sloped which might account for your elevation difference.

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