When to consider active/passive state..?
When to consider active/passive state..?
(OP)
Hello all,
When I design retaining walls supporting some kind of structure, I usually use the at-rest earth pressure to limit possible deflections.
However, a colleague of mine came up with a question about the design of one of his walls. At what point can we consider active/passive pressures on a retaining wall? In other words, we can consider it when the wall has "moved" for a given deflection but what is this value? 1 inch for each 10 feet of height?
When I design retaining walls supporting some kind of structure, I usually use the at-rest earth pressure to limit possible deflections.
However, a colleague of mine came up with a question about the design of one of his walls. At what point can we consider active/passive pressures on a retaining wall? In other words, we can consider it when the wall has "moved" for a given deflection but what is this value? 1 inch for each 10 feet of height?
RE: When to consider active/passive state..?
RE: When to consider active/passive state..?
The suggestion for the latter case was well established. The argument for the former is that though it only takes slight deflection on top of the wall to allow the development of active pressure, however, the soil will reach an new equilibrium back to the at rest state. Then, the wall deflects further more,...active pressure...so on so forth, until the soil reaches its final equilibrium state. At that stage, the total deflection could be significant. or the wall failed before reaching the final. Thus, it is suggested to use at rest for the cantilever wall as well.
For passive pressure to occur, it requires the wall move into the soil, such as pressure in front of a free moving retaining wall, or a deadman in tie back system.
RE: When to consider active/passive state..?
RE: When to consider active/passive state..?
Passive is a resisting force only activated by large movements in the order of .01H. You wouldn't want that much movement except for retaining walls where movements are not critical.
RE: When to consider active/passive state..?
- Active: general walls (for phi=30º -> k=0.33)
- Rest: bridge abutments and important walls that cannot have great displacements (for phi=30º -> k=0.50)
- Passive: I usually model the soil with springs and ballast coefficient and the passive means the maximum possible reaction of the soil in piling walls (for phi=30º -> k=1.33)
http://NotOnlyBridges.blogspot.com
RE: When to consider active/passive state..?
JedClampett: Do you have a (book/article) reference for the 0.001H criteria? That doesn't seem so bad, 4 mm on a 4 m wall..?
RE: When to consider active/passive state..?
Bear in mind, there also has to be movement at the toe to mobilize passive earth pressure.
Bear in mind also, if you are designing a sheeting system, Terzaghi and Peck found that the earth pressure envelop is either rectangular or trapazodal, depending on whether you are restraining fine- or coarse-grained soils.
Some folks design cantilevered retatining walls using at-rest earth pressures on both the "active" and "passive" sides as they don't want to count on any movement. This would then require stability from base shear or a gravity mass above the wall footing.
I'd hope that a geotechnical engineer would be able to explain this to a civil/structural designer, but then again. . . .
f-d
¡papá gordo ain't no madre flaca!
RE: When to consider active/passive state..?
In Winterkorn and Fang, "Foundation Engineering Handbook" Table 12.1
Sand:
Active Pressure: Parallel to Wall .001H
Active Pressure: Rotation About Base .001H
Passive Pressure: Parallel to Wall .05H
Passive Pressure: Rotation About Base >.1H
Clay:
Active Pressure: Parallel to Wall .004H
Active Pressure: Rotation About Base .001H
Passive (No values given)
My favorite Geotechnical reference, but I'm sure it's been out of print for a while. My copy is dated 1975.
RE: When to consider active/passive state..?
"Bear in mind, there also has to be movement at the toe to mobilize passive earth pressure."
That actually can't happen for the wall I'm designing at the moment.
RE: When to consider active/passive state..?
Contains a table of minimum movements.