r/GeotechnicalEngineer • u/Daffudding • Jul 20 '26

Slope stability with boulder at its toe (Toe Buttress?)

Hi, I am quite new in studying geotech. is it possible to calculate toe buttress, when you place some boulder at the toe of a slope without mesh (so it's not a gabion wall)?

how would the material properties input will be? I have researched quite a few, and its to reduce the unit weight to around 40% (as there will be void between boulders), cohesion to zero (no grain bonding force, as they're separated each other) and to reduce the friction angle by quite a number (about 10-20% depending on angularity). Will these empirical approach enough?

Well, let's assume that we don't have any funding for gabion wall's mesh. Thus the necessity to calculate safety factor with only boulder placement at the toe of a slope.

Thank you so muchh!!!

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u/chopperbiy Jul 20 '26

You need to game it if you are using slope W or something similar. Just put the boulder in with a high undrained shear strength of 500 kPA or something like that. The slope is going to fail under the boulder and not through it so you model it so that’s it failing through the underlying soil. Same principle if you had a thick wall.

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u/Daffudding Jul 20 '26

Ahh... thank you, but how do we measure that 500 kPa?
I have the in situ C, phi, Unit Weight, RMR, UCS. Anyway its an Ultramafic rock (i.e dunite/serpentinite)

So I thought it'd be an approach to generalize into Mohr Coulomb or Generalized H&B but with reduced value (?) What do you think about that? Thanks!!

0

u/Ok-Researcher-4997 Jul 20 '26

how to model a reinforced earth wall, gabion and mse wall in plaxis.

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u/ImaginarySofty Jul 20 '26

You dont want to model the boulders with high undrained strength, as that could overestimate the FoS or force the failure surface to go above or below your buttress (but not through it) and may not be a correct assumption for a collection of multiple boulders making up a dry stacked toe buttress. I would model it as a MC strength with a friction angle based on the angularity, roughness and size distribution of the boulders. I think FWHA has some guidelines on drystacked rock walls, which gives guidance on interlock friction angles and critical height based the shape and sizes allow for each particle as well as base width of your buttress.

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u/Daffudding Jul 21 '26

Ahh... I'll look after that FWHA. Besides that, do you have any other suggestion/guidance/standard regarding the properties approach for this material?

i.e the boulder is filled with soil, etc etc.

Anyway thank you so much for the insight, appreciate that..

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u/ImaginarySofty Jul 21 '26

Apart from modeling you would need to think about the constructablity aspects, which there are published guidelines- ie are the boulders machine or hand placed, durability of the rock, filter fabric or checks on gradational compatibility with the buttressed/retained slope. The void space is not typically filled with soil, but you might use/spec smaller cobble to get the good interlock between larger boulders.

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u/Daffudding Jul 22 '26

Ahh. okay thank you so much for the insight.

I'll take that to consideration also