I'd suspect that the friction angle of the interface between a soil and the geotextile would depend on the friction angle of the soil as well as the surface texture of the textile.  I would not imagine there is a simple answer to this question.  Additionally, anchor trenches can offer some additional slope restraint - so the height of the slope would also be a factor from the overall stability of a geotextile on a soil slope.

f-d

¡papá gordo ain’t no madre flaca!

FYI, I just found an article online that looks promising:
 http://www.geomembrane.com/friction/ShearStrengthPVCInterface.pdf

I have done a lot of work with landfills and the associated geosynthetics.  A typical value for the interface friction you will get in just about any publication is worthless.  This is very dependant on the manufacturer, manufacturing process, and the soil materials you intend on using. Do some testing.

As far as the anchor trench goes, check for pull out.  Also, in a layered system, work the calculations through the various layers.  This will help you find the weak interface, and also tell you if you have over stressed a layer.  

I have seen synthetics placed vertically in quarry wall linings, and have seen some that require a 4:1 slope to be marginally stable.  In most landfills I have worked around, you will find the maximum slope to be 3:1. Factors to look at include: 1) Will you fill against and buffer the possibility of a slide?  2) Is this a cover system? 3) What is the drainage / sill the soil get saturated? 4) The list goes on.

Putting a fabric on a drainage net can give you an extremely low friction angle along the direction of the bars of the net.  Once saw a landfill where the designers had relied on the tensile strength of the needle-punch filter fabric for slope stability.  Apparently, the fabric got nicked during fill placement, it tore, and a large portion of the slope failed.  They had to tear out a lot of the work in place and redesign it before construction could be finished.  It was a 3:1 slope as I recall.  That was my only experience with landfill liners.

TDAA - Changing the subject slightly, is it considered good practice to rely on the tensile strength of the fabric to make a slope stable?  I could see using it as "gravy" to show a higher FS for a slope that is at least marginally stable without it.

Sorry if I implied that was to increase the slope stability.  Not my intention.

Say you have something such as a cover system that generally looks like: trash/dialy cover/synthetic liner/drainage synthetic/soil cover.

When looking at the veneer stability of the system, you have to check each interface, and each will have an associated frictional force.  In the example above, it can be likely that the interface between the two synthetics will have a lower frictional force than between the soil and the drainage synthetic.  Where this occures, you will get a resultant tensile force. Just draw the FBD for the Drainage synthetic and you will see. Not that this should be used for adding resistance, but it should be checked in a complete design.