Retaining wall - base slab stresses

[amghma]

Hi ALL

I am designing a cantilever wall which supposed to retain 6.3 m height of soil from both sides, but one side is to be back filled directly while the other is taking time maybe a year.
i designed for the worst case( in terms of stability) which is active pressure only.. but when i check the active with passive pressures the base slab reinforcement was shocking !?
please share me your opinions about the worst case for design and the reason why the base slab moments doubled ?
I have uploaded the analysis

 

[BadgerPE]

You don't need to use active/passive when it is a balanced fill condition. Your lower diagram is basically saying that the force on the left side (passive) is roughly 5.6 times higher than the force on the right (active). This is not possible due to the direction your "retaining wall" would want to move. In the lower orientation with fill on each side, it should be designed using at-rest/at-rest which would cancel each other out. Your cantilever reinforcing for the active only condition would more than cover any minor inconsistencies in backfill pressure on either side, including the surcharge loading.

This is pretty basic retaining wall design info. Please make sure to consult with your superior to verify that your design is correct.

 

[amghma]

I agree with you, we should design it using at rest condition in case the back filling is done simultaneously on both sides, but here it is not few meters to be back filled..

 

[BadgerPE]

I understand that. However, you can't have a full active pressure on one side with a full passive pressure on the other side. This is because to use active pressure, it is assumed that the wall will rotate out away from the soil mass a small amount, thus reducing the amount of earth pressure. For passive pressure the wall is assumed to rotate into to the soil mass. Therefore, when you have full backfill against both sides, you should use at-rest on each side + surcharge, or just ignore it.

I fully agree with your design when there is full backfill on one side only.

 

[BadgerPE]

Additionally, when you look at your design moment for the active only case (434 kN*m? - not a metric user) it attempting to rotate the wall stem counterclockwise. In the active+passive approach at the bottom of the page, the design moment of 2033 is attempting to rotate the wall stem clockwise. That doesn't make any sense to me how that can occur.

 

[amghma]

Usually when we design at rest conditions we recommend a max difference in back filling level between the back and the front of the wall to maintain at rest condition
but your assumption (about rotation of soil mass and its effect) is realistic , full passive case wont be reached.

 

[oldestguy]

Why even look at the second case, fully enclo0sed with fill? Suppose the wall fails when it is first filled? Second case is then meaningless. Better design for first case and no fill in front.

 

[amghma]

This case considers a road across a valley , the road is connected with stairs with a slope about 55% .
so the retaining wall is located at the end of that road.

According to the fact that every soil has its own angle of friction and failure wedge as well as the need of retention, even if we construct another retaining wall for the stairs it will contribute with pressure to the road's retaining wall. so the solution is to make the back fill simultaneously.

But what is the criterion upon which we distribute our retaining walls ? is it the width of the failure wedge ?

 

[BadgerPE]

A sketch would help significantly here. I'm not really visualizing what you are asking.