Wall friction angle - Seismic case
Wall friction angle - Seismic case
(OP)
I'm currently doing a design for an anchored sheetpile (combi) wall. Retained height approx 15m, marine environment so mostly submerged soil conditions, cohesionless granular material.
I've setup spreadsheets using pseudostaic loadings and limit equilibrium analysis. As well I'm performing a WALLAP analysis of the wall to confirm results. When running WALLAP it requires the wall friction angle to be zero during seismic conditions.
When thinking about it, it makes some sense that due to the shaking you can no longer rely on the friction, possibly. Ignoring wall friction makes a large difference to the passive pressures in particular, basically reduces it to rankine. Thus significantly increasing wall embedment.
Any thoughts/advice appreciated.
I've setup spreadsheets using pseudostaic loadings and limit equilibrium analysis. As well I'm performing a WALLAP analysis of the wall to confirm results. When running WALLAP it requires the wall friction angle to be zero during seismic conditions.
When thinking about it, it makes some sense that due to the shaking you can no longer rely on the friction, possibly. Ignoring wall friction makes a large difference to the passive pressures in particular, basically reduces it to rankine. Thus significantly increasing wall embedment.
Any thoughts/advice appreciated.





RE: Wall friction angle - Seismic case
RE: Wall friction angle - Seismic case
At the opposite extreme would be liquefied sand with phi=0 and c={something small}, or acting like a fluid that weighs 120 lb/ft3.
Haven't got a good answer for you - just compounded the question.
RE: Wall friction angle - Seismic case
WALLAP lets you input both a horizontal and vertical acceleration, the seismic load case uses 'wedge stability' to compute active and passive pressures, it doesn't use M-O, you are required to manually input Ka and Kp parameters as well. After looking further into WALLAP it appears that you can run the analysis with wall friction if you set the vertical acceleration to zero (this is a limitation of the software rather then a strict requirement). I'm happy to Kv to zero, though it still gives a stern warning when analysing that the wall friction may not be able to be relied upon.
Regarding M-O, yes I do believe the original was for a non-saturated case, as you say there are in fact two issues one being the submerged condition and the other being the wall friction during shaking (dry or saturated, I can't find a good answer). Originally I accepted that the wall friction should be neglected, but it seems all the examples I've seen are allowing its use, either through the M-O equation or modifications of it, or the log-spiral method (I believe is a modified M-O equation as well), the log-spiral charts available in AASHTO, FHWA etc all include a wall friction angle.
For my hand calc/spreadsheet I'm allowing for submerged by following guidelines from USACE - Seismic Design of Waterfront Structures, which modifies the M-O equation and effectively increases Kh using: Khe=(Gamma.dry/Gamma.buoyant)x Kh. Basically doubling the Kh. (this is for 'free water' case, slightly different for a 'restrained water' case).