Rankine versus Coulomb Earth Pressures
Rankine versus Coulomb Earth Pressures
(OP)
AASHTO LRFD seems to flip flop in their cantilever retaining wall figures (Section 3 and 11) showing the inclination of the resultand active pressure to be at angle beta (parallel to backfill slope) or at angle delta (wall friction). This confuses me a bit. I've also seen several figures from random google searches that show the angle of inclination to be angle beta+delta. The link here shows an example of the beta+delta which is almost identical to some of the figures in AASHTO h ttp://www. engmath.da l.ca/risk/ pubs/FHWA_ Manual.pdf
Can somebody explain which is correct?
Can somebody explain which is correct?





RE: Rankine versus Coulomb Earth Pressures
1. Rankine theory does not include wall friction, for sloping backfill, the resultant is parallel to the slope.
2. Coulomb theory considers wall friction and angle of slope, both are included in the equations for earth pressure constant K, the resultant force is making an angle, equal to the friction of the wall, from the plane normal to wall.
Remotely remember the case shown in the attachment, but the details are forgotten.
RE: Rankine versus Coulomb Earth Pressures
RE: Rankine versus Coulomb Earth Pressures
kslee1000 - you are correct; the original Rankine model - remarkable for being developed on the basis of internal friction and failure state of soil decades before Terzaghi played in a sandbox - assumes frictionless walls and level fill. (For that case, it gives the same answer as Coulomb.) I believe, however, that it has been extended to the general case, allowing frictional walls and nonlevel fill. I think I saw that somewhere, but I would use Coulomb.
RE: Rankine versus Coulomb Earth Pressures
Thank you for responding to the call for help. You have clarified my memory - an advice from my geotech professor: do not count on wall friction unless it,s certain, and Coulomb method is used. Also, the use of wall friction shall take into account of the type of backfill materials and fluctuation of water table into account, the potential risks outweighs the gains when wrongfully applied.
RE: Rankine versus Coulomb Earth Pressures
Coulomb method does consider wall friction. Depending on the reference, friction between soil and concrete for granular soils is often 0.5 to 0.75 of phi. For flatter slopes like 3:1(h:v) or less, wall friction can also be equated to beta, the slope angle.
RE: Rankine versus Coulomb Earth Pressures
RE: Rankine versus Coulomb Earth Pressures
Coulomb is older by most of a century, but the concept of coefficient of friction on a sliding surface was already established by then. The rest is trigonometry. I think Rankine was so remarkable because, as far as I know anyway, that was the first, or certainly one of the first uses of the concept of internal strength of mass of soil.
I'm easily impressed by ideas that I would never even have thought about thinking up.
RE: Rankine versus Coulomb Earth Pressures
RE: Rankine versus Coulomb Earth Pressures
That was what I learned from school, glad to hear it again.
RE: Rankine versus Coulomb Earth Pressures
RE: Rankine versus Coulomb Earth Pressures
The first is The Seismic Design of Waterfront Retaining Structures. The second is Retaining and Flood Walls. The latter has two especially interesting comparisons of the relative conservatism (or lack thereof) of Rankine, Coulomb and Log-Spiral.
Both of these can be downloaded from
http
http://www.pz27.net
RE: Rankine versus Coulomb Earth Pressures