Do I need to count the active pressure on key of retaining wall?

[tcaoz]

When I working on a retaining wall, a key seems necessary for resisting sliding. I am wondering do I need to count the soil pressure on key? I use a software called retainingpro, it doesn't include that force.

 

[ishvaaag]

Forfeiting any software issues, one needs to care of stability, strength and equilibrium of forces. You can see in cantilever walls that the active forces are counted down to the bottom to be equilibrated by passive forces (to one of both sides), and, maybe, some tie or ties. Hence (for overall equilibrium) better to consider all active forces at one side, footing and key depth included. In cantilever action, however, and for the stem of the wall, counting this or not has no effect (for the same push with depth) on the stem's sizing, but not counting it for overall equilibrium would be I think except some justification arises out of standard practice.

 

[asixth]

Yes, for your global stability of the wall I would include all overturning forces such as active earth pressure down to the base of the key and then equilibriate the overturning forces and moments with the passive soil resistance (...is equilibriate a word???).

I was arguing with an engineer in my office onetime that it should not be including because I believe soil below the level of the retaining wall footing will be in an in-situ state and that the active earth pressures will not be developed. I did not win that argument and now go with the more conservative assumption that active earth pressure does act on the key.

 

[tcaoz]

Thank you all and I will take the advise.

 

[ishvaaag]

In my perevious post... it must read

... (to one or both sides)...

 

[BigH]

http://books.google.co.id/books?id=...sult&ct=result&resnum=23#v=onepage&q=&f=false

 

[hokie66]

Interesting in the reference BigH cited that 7/8 of the resistance to sliding is by friction, with only 1/8 by passive pressure. Also, in regard to the OP, active pressure on the key is neglected.

 

[darthsoilsguy2]

if you made the key 10 feet deep, you'd feel pretty silly putting the pivot point below the footing i imagine. the idea is to design retaining walls knowing that passive soils above top-of-footing may disappear because of ignorance or change of use. If an owner undermines a cantilever retaining wall footing, their punishment will fit their crime.

the key is below the pivot point of the wall structure and any active pressure acting on it should counter the overturning moment... however this benefit is ignored unless you're designing sheet piles.

 

[ishvaaag]

It would be interesting to know the technical reasons for the omission of active push in Hicks' Handbook. But that is one of the things that happen with earth push, the representation of equilibrium is only approximate and sometimes trying to get the things together is like the work of a child trying to mend some of his overflooded dams.

 

[tcaoz]

BigH thanks for the reference. I actually have the first edition of the book. It is good to know you can use inner friction factor of the soil at toe side of key to calculate friction which is a great benefit. Just wondering if there is a minimum height requirement for the key in order to take advantage of this?