On guarding gravity walls against overturning

[geotechniqa]

Retaining walls are guarded against overturnning by taking the ratio of the driving moments/resisting moments.
all around the toe.
But I noticed that driving moment of the force resultant of the effective vertical bearing pressure at the bottom of the wall (which is equal to the sum of the downward vertical forces) being a driving one is ignored. Why ..?
Scott text book when solving an example of a gravity wall mentions that the point of action of this force may be considered at the toe.( its moment is zero)
But later when checking the FOS for bearing capacity, Scott calculated its arm to be at the central third of the base.

 

[BigH]

Can you give the Scott reference? He has several books . . .

 

[geotechniqa]

text book of 1985 (second ed). I do not have it in front of me now because I red it in the library. Even Bowles (1982) in his examples on overturninig check of gravity walls... doesnot consider this force.. There is something silent about this matter which I cannot hear

 

[csd72]

These are 2 separate checks, but to reassure yourself, take the loads around the central reaction point and you will find that once you include the rotational effect of the bearing pressure then
the FOS is at least the same.

 

[geotechniqa]

csd 72 .. your answer is not clear .
Take a gravity returning wall and check the FOS for overturning around the toe.. does your FOS with and without including the bearing pressure resultant will be the same.. Of course No
unless the resultant pass from the toe. The same thing about the centre ,, unless bearing pressure resultant pass from the centre you will get different FOS : period.

I am aware of the second method of checking which is dependent on the point of application of the resultant. This needs another threat. Please let us focus on the above issue.

 

[kslee1000]

The bearing pressure is a reactive force, it does not push the wall base but respond to the rotation. You can keep a thick book standing vertical up on a desk, then push it, it rotates, but does the desk pushes the book?

 

[geotechniqa]

kslee1000 thanks,

If you have a body and you want to check if it is under equlibrium state or not, you take its free diagram with all the forces influencing this body including reactions and you solve the equations (M: moment, F: Forcein 2D):
SIGMA M=0
SIGMA Fy=0
SIGMA Fx=0

 

[kslee1000]

geotechniqa:

What you have described is "the state of structural equilibrium" - internal reactions = externally applied forces at any point of the system. The earth retaining structure is in a state of equilibrium, when the effect of the externally applied forces (selfweight, soil weight/pressure, surcharge) is balanced by the reaction (bearing pressure/resultant forces).

The stability of the retaining wall is a very special concern of tip-over (imbalance) caused by the external forces, which are further divide into driving force and resisting force. Thus, the stability against rotation (tipping) is defined as Mr/Md with a safety factor to safe guard uncertainties.

Other than the example of a book, you can think when someone pushes your on the back (external force), you would lean, or fall forward, or stand still - if you are a 500 pound American football player.
In any outcome, your weight is the stabilizing force, that has been challenged by an external force, but the earth, on which you stand. Hope all thess helps.

 

[PEinc]

If you replaced the soil beneath the footing with a pin at the toe and a roller at the heel and then checked overturning, the roller would not push up on the bottom of the footing. The roller stays in place. The soil beneath the footing acts the same. The soil bearing pressure does not contribute to overturning of the wall.

 

[BigH]

kslee500 - or former player!

 

[kslee1000]

BigH:

Where is my rest 50%?