Huh?  
What is the significance of 'bioengineering' in this case?
Slope stability generally relates to geometry, moisture/groundwater, and exisitng weak zones.  

Are we talking mulch and manure?  : ]

I think lcw818 may be talking about surficial stabilization due to plant roots.  I ran into this in the past trying to asign a strength value in a stability analysis due to the root zone, will take a look for my references.  A couple aspects you need to consider are that you need to be sure the roots are permanent (plants will not die or be eroded away), and the depth of the root zone to consider in the analysis.   

Sounds like slope armor.  Rootmat should generally reduce erosion,
but the effect of tree roots is somewhat controversial
with regard to embankment stability.  
Trees blow over during storm events causing root balls to erupt,
embankment then erodes significantly.  

Yes, I thought I might get some of these answers coming from a bunch of geotech's.  I am one also.  It is controversal.  The erosion protection is taken care of.

Generally, it is stream bank stabilization of a 10 to 12 foot (vert) slope to be laid back at my determined slope angle.  The stream bank restoration guys (envir/water engineers) are putting in a 4' thick zone on the slope face and will stick tree cutting into this zone ("live stakes").  Eventually, the roots will be throughout the zone (so they say) providing additional stability.

The research I have done says they account for this by adding cohesion to the soil zone (makes sense although we acknowledge that it is temporary geological time speaking).  But they do not say how much.  I have been using 5 to 50 psf and get good results but I don't have anything to substantiate these numbers.  

Any help would be appreciated.
lcw818 PE

 

Just off hand,
I would think that shallow stabilization might reduce erosion,
but would have little effect on global slope stability.  
Surely someone has run a direct shear test on rootmat.  
But it also sounds like you need to understand the interface friction
between the stabilized layer and the underlying slope material.  
Sliding block for that condition, Bishop/Janbu etc... for global?  

 

I like your approach and it is essentially what I have been doing.  Get an overall slope angle that is acceptable using a surcharge load for the 4 ft layer.  Leave the stability (shallow circles) of the root zone to the water guys.  

A sliding block on the interface is also a good idea.  I will use geotextile interface strengths since it separates the two layers anyway.  I think I now know what will control the slope angle.

thanks

Still interested if anyone has done and analysis on a rootmat.  Or if they have seen any thing published.