Passive pressure exceeds the active one 2

[geo321]

Hello,

I am considering a retaining wall having a 3.5m height of retained soil and 1.7 meters at the base from the other side. I want to rely on my passive earth pressure. As per my calculations, the passive earth pressure induced moment exceeds the moment coming from the active.
In this case, how can I represent the real actions on the wall ? Can the active and passive earth pressures be developed as represented?
Thank you for the help

 

[oldestguy]

I didn't check calculations but let's say you are standing on the ground and the bearing capacity of the ground is something like 2 tons per square foot. You weigh 0.1 tons in one square foot.. That's all that you get from that earth below. The passive is three if you need it, but you don't need all of it. So what.

 

[PEinc]

Passive pressure is a reaction, not an applied force. The wall cannot move backwards just because the available passive pressure is greater than the active pressure (driving force). If the wall could move backwards, then the passive pressure would switch to the rear of the wall and the front pressure would become active - until it again reversed, thereby forming a perpetual motion machine.

http://www.PeirceEngineering.com

 

[geo321]

Ok I understand. So in this case the stability of the wall is ensured without any additional verifications. Right?

 

[PEinc]

Under all logical and reasonable conditions, the wall cannot tip over backwards from passive pressure. In order to have a safety factor against overturning, the resisting moment must be greater than the overturning moment. In order to have a greater resisting moment, the total available passive pressure must be greater than the driving pressures because the resisting passive pressure has a smaller moment arm than the driving active pressures. The wall cannot push backwards if there is higher, relatively similar soil behind the wall compared to in front of the wall.

http://www.PeirceEngineering.com

 

[cyphos168]

If the active pressure exceeds that of the passive pressure then your wall will move or rotate. How is this possible?

1. When a very heavy load acts on the active side like a vehicle or building.
2. When the water level behind the wall rises and pressure is not allowed to be reduced due to clogged weepholes.
3. An unplanned cut on the passive side below the base of the wall hence reducing the available passive resistance.

When designing a cantilever retaining wall, it is not always good to rely too much on passive resistance provided by the soil in front of the wall. Try to use a longer heel or provide a key instead at the base.

 

[Ron]

Ah, yes....the conundrum of earth pressure understanding! PEinc has nicely explained it, but I'll offer this as well.....

There are three states of earth pressure; at rest, active and passive.

For either active or passive pressure to be mobilized (realized) wall movement has to occur. It really doesn't matter how much movement occurs, just that it does occur. So, by definition, if the wall moves (bends, slides, etc.), the force causing that movement is the active pressure (it is "acting" on the wall to cause the movement). Think of the passive pressure as being somewhat like a bearing capacity of soil if you are putting a footing on it.....if you don't put a footing on there and load the soil (thereby causing movement, however slight it might be), the bearing capacity is irrelevant. The passive pressure, as PEinc noted, is then a reaction, just like the bearing capacity reacts the footing load below its failure point.

 

[Ingenuity]

Ok I understand. So in this case the stability of the wall is ensured without any additional verifications. Right?

To answer your above question, you need to check overturning and sliding, and depending on the code in your area, using either: 1) code-specified load factors applied to your actions, or 2) using working/service loads and calculate the F of S with say a minimum of 1.5 for overturning, for example.

In your calculations you appear to be using service level actions, and for overturning you do NOT appear to have a F of S of > 1.5.

You need to check sliding, bearing capacity...

 

[geo321]

Hi ingenuity,
You are right. The main purpose of my thread was to discuss the passive earth pressure. But surely sliding and bearing capacity will be checked.
Regarding the safety factor of overturning and according to the above discussion the wall will move due to the active pressure and the passive will be mobilized. And since the passive counts as a resisting moment so the overturning is ok. (Please correct me if I am wrong and thank you for the help).

 

[PEinc]

geo321, for the type of wall shown in your sketch, passive pressure is usually ignored in analyzing overturning and sliding. You never know if or when someone will dig in front of the wall, thereby removing the passive resistance. The wall dimensions should be sized so that the wall is stable due to its own weight and the weight of the backfill soil over the heel of the wall footing. And, as previously mentioned, don't forget to use a safety factor for service load design or use load and resistance factors for LRFD design.

http://www.PeirceEngineering.com