Rankine Analysis Questions

[dweiman]

Hi All,

I am looking to design wingwalls for a culvert. The height of retained earth is approx. 17.5 ft. (worst case). The soil is a clayey/silty loam with cobbles. Soil assumptions are an E.F.P of 45 psf (Geotech Report), Soil Weight = 120 pcf, and angle of internal friction = 20 deg. (Std Handbook) My question is - What is the difference between a Rankine analysis vs. an Equivalent Fluid Pressure analysis for retaining wall design? I'm running RetainPro 6.0 software and the Rankine anaysis method gives me a much more conservative design. Is there any correlation between the E.F.P of a soil and its angle of internal friction? Is there any reason to prefer one analysis over the other, other than potential savings on concrete? Any advice would be appreciated.

 

[DRC1]

Rankine is a equivelent fluid pressure analysis. Rankine refers to the method to compute the lateral earth pressure coeficent, which is the ratio of lateral load to vetical load. First, there are two types of lateral pressure for wall design: active, where the wall is allowed to rotate slightly, or at rest, where the wall is not allowed to rotate. For a culvert wingwall, I would recomend at rest pressure. This is usually .45 to .55. If you are using active there are several was to compute, of which rankine is the most consevative. The soil is a clayey/silty loam which may have some cohesion, which may have been acounted for in the equivelent fluid pressure value. The best thing to do is call the geotech and dicuss with him. If you are using his values,, you have a right and an obligation to understand what the values represent.

 

[PEinc]

dweiman,

45psf/vf is too little to be an at rest pressure and too little to be a Rankine active pressure using 120 pcf and 20 degrees. Possibly the geotech used a Coulomb active earth pressure coefficient with some type of downward sloping ground sruface behind the wall. As DRC1 suggested, ask the geotech where he got the 45 psf pressure.

 

[dweiman]

DRC1, PEinc,

Thanks for your time. I ended up staying with the Rankine analysis using the 120 pcf and 20 deg. angle of internal friction. The client ended up going to the culvert precaster for the wingwalls, ultimately to reduce the size of the footings, which were going to interfere with some proposed drainage. Thanks again.