NCEES sample questions - 2011; #111

[Jacob_PE]

I've reread the CERM and All-in-one chapters on retaining walls and am still having issues.  I'm having trouble with the statement: 'The lateral strain required to fully mobilize the soil passive pressure is much greater than that required to fully mobilize the soil active pressure.'

According to the all-in-one, it takes about 10 times the wall movement to develop full passive resistance as it takes to develop the full active forces. Can someone explain this further for me?

Lateral strain, related to steel testing, is the percentage decrease in diameter, what is lateral strain for retaining walls?

How are lateral resistances from passive soil and passive pressures related?

Im also confused on the concept that there is a wall movement required to develop minimum (active) or maximum (passive) pressures. 

Can someone explain why the theory discusses the development of pressures due to movement, aren't retaining walls always at rest and the soil pressure behind the wall generally always the same?

 

[civilized_naah]

I've reread the CERM and All-in-one chapters on retaining walls and am still having issues.  I'm having trouble with the statement: 'The lateral strain required to fully mobilize the soil passive pressure is much greater than that required to fully mobilize the soil active pressure.'According to the all-in-one, it takes about 10 times the wall movement to develop full passive resistance as it takes to develop the full active forces. Can someone explain this further for me?

Lateral strain, related to steel testing, is the percentage decrease in diameter, what is lateral strain for retaining walls?

How are lateral resistances from passive soil and passive pressures related?

Im also confused on the concept that there is a wall movement required to develop minimum (active) or maximum (passive) pressures. 

Can someone explain why the theory discusses the development of pressures due to movement, aren't retaining walls always at rest and the soil pressure behind the wall generally always the same?

If you have a very rigid wall (such as a basement wall that is restrained at top AND bottom, or a bridge abutment that is very massive), then the wall movement is minimal and the condition is called 'at rest'. The earth pressure coefficient is then 1.0 (By the way there is a member on engineerboards whose signature is 'K = 1.0', so he is a very laid back person, I am assuming). When the wall is flexible (such as a cantilever wall, it tends to move away from the backfill, thereby inducing deformation or strain both behind the wall (active) and in front of the wall (passive). The active pressure is less than the at rest pressure (stress relief) and the passive pressure is more than the at rest pressure (stress increase). Hence, Ka < 1 and Kp > 1. The amount of soil deformation required in the active wedge to really reach the minimum earth pressure (fully active condition) is quite a bit more than that required to develop the passive condition.

 

[civilized_naah]

The amount of soil deformation required in the active wedge to really reach the minimum earth pressure (fully active condition) is quite a bit more than that required to develop the passive condition.

Sorry, I stated that exactly backwards. I meant to say - The amount of soil deformation required in the active wedge to really reach the minimum earth pressure (fully active condition) is quite a bit LESS than that required to develop the passive condition. In other words, active condition develops pretty easily but it takes quite a bit of wall movement to mobilize the large passive pressures in front of the wall.

 

[Jacob_PE]

The amount of soil deformation required in the active wedge to really reach the minimum earth pressure (fully active condition) is quite a bit more than that required to develop the passive condition.

Sorry, I stated that exactly backwards. I meant to say - The amount of soil deformation required in the active wedge to really reach the minimum earth pressure (fully active condition) is quite a bit LESS than that required to develop the passive condition. In other words, active condition develops pretty easily but it takes quite a bit of wall movement to mobilize the large passive pressures in front of the wall.

Thanks for clearing that up; I nearly blew a gasket when i got to the end of your first post.

In this sentence: 'it takes about 10 times the wall movement to develop full passive resistance as it takes to develop the full active forces.' how is the 'development' of those forces manifested, soil movement, wall movement towards the passive force, or strictly as a measure of the retaining wall's deformation?

When you say it takes quite a bit of wall movement to mobilize the large passive pressures in front of the wall.' what do you mean by 'mobilize'; is it that the wall will inch forward?

One concept that I think helps me understand the active/passive relationship is the earth pressure coefficients ka and kp, for horizontal backfill and a vertical wall face, kp is the inverse of ka, so for ka = .307, kp = 3.26. -- passive will be way larger. 

 

[civilized_naah]

When you say it takes quite a bit of wall movement to mobilize the large passive pressures in front of the wall.' what do you mean by 'mobilize'; is it that the wall will inch forward?One concept that I think helps me understand the active/passive relationship is the earth pressure coefficients ka and kp, for horizontal backfill and a vertical wall face, kp is the inverse of ka, so for ka = .307, kp = 3.26. -- passive will be way larger. 

First, a soil has the potential to develop these forces, by virtue of the wall geometry and soil parameters such as phi, delta etc. These earth pressure coefficients are given by the Coulomb and Rankine formulas. But that doesn't mean that these pressures (active or passive) will fully develop (or mobilize). That will depend on the extent to which the wall moves. For cantilever retaining walls, it is 'assumed' that adequate wall movement occurs.

Secondly, the Kp is exactly the inverse of the Ka only for the simplified case (delta = 0, vertical wall stem and horizontal backfill). For the more generalized case Kp is only approximately inverse of Ka

 

[Jacob_PE]

When you say it takes quite a bit of wall movement to mobilize the large passive pressures in front of the wall.' what do you mean by 'mobilize'; is it that the wall will inch forward?One concept that I think helps me understand the active/passive relationship is the earth pressure coefficients ka and kp, for horizontal backfill and a vertical wall face, kp is the inverse of ka, so for ka = .307, kp = 3.26. -- passive will be way larger. 

First, a soil has the potential to develop these forces, by virtue of the wall geometry and soil parameters such as phi, delta etc. These earth pressure coefficients are given by the Coulomb and Rankine formulas. But that doesn't mean that these pressures (active or passive) will fully develop (or mobilize). That will depend on the extent to which the wall moves. For cantilever retaining walls, it is 'assumed' that adequate wall movement occurs.

Secondly, the Kp is exactly the inverse of the Ka only for the simplified case (delta = 0, vertical wall stem and horizontal backfill). For the more generalized case Kp is only approximately inverse of Ka

Right on, appreciate the help.