Live Load Surcharge Lateral Pressure Coefficient

[Quade999]

Hi Everyone,

I'm not sure which coefficient of lateral earth pressure to use when I have a situation like the one shown below.

When you have a live load near a wall, we have to include a horizontal live load surcharge on the wall. If I had the case like shown above where the live load is on the passive side of the wall, would I multiply the surcharge force by the at-rest or the passive pressure coefficient?

Also, what about the situation shown below. Would the lateral pressure be as shown below where the mound of soil above that is not being pushed on by the wall would act with an at-rest coefficient, and the soil below would act with the passive coefficient.

Thanks.

 

[r13]

You should ignore live load on the passive side of the retaining wall. Also, you shall at least include a case that there is no soil in front of the toe, unless you can ascertain there is no chance that the soil will be removed throughout the service life of the retaining wall. For normal operating condition, it was customary to ignore upper 1' soil for good practice.

 

[Quade999]

I'm looking at this from a standpoint of determining the reinforcement in the cut-off wall / toe. The passive pressure pushing on the toe would cause the greatest bending moment and shear in the cut-off wall.
When the wall moves I understand that the passive pressure is utilized as a resisting force and has an associated failure plane with the soil. But I'm not sure how the pressure distribution works when part of the pressure comes from external load or soil that isn't direction being compressed by the movement of the wall. I.e. when the wall moves it doesn't actively push on the live load, so would the live load surcharge force act with the passive pressure coefficient or at-rest?

 

[r13]

Below is a very conservative method to design the cut-off wall (turn-down).

Assume the turn-down is fixed at the bottom of the base slab. Since the soil at the toe is passive, so P[sub]R[/sub] must equal to the sliding force "V". Place P[sub]R[/sub] at the geometry center of the pressure diagram. Treating the equipment load as concentrate surcharge and calculate P[sub]H[/sub]. These are the design forces for the turn-down.

A concept is very helpful when getting confused about soil coefficient - We must use the given soil properties to calculate the driving force (active/at rest), but at the resisting side, use the passive soil properties for stability check only. When perform design, we have to bear in mind that the extent/intensity of the passive force is a direct response to the active/driving force, thus the magnitudes of the two must be equal but opposite to each other, so in this situation, the use of passive coefficient is incorrect.

 

[r13]

Forgot the surcharge. I opine either use ka, or k = P[sub]P[/sub]/V (P[sub]R[/sub]). The use of ka is based on the thought that, for a equilibrated system, adding weight on one side will push (driving) the wall tilt towards the other (resisting) side. In this case, the surcharge force is on the driving side. The latter is just by linear interpretation.

[Correction] The Ka above shall be replaced by Ko, as the wall is not possible to yield a sufficient amount to allow the development of active pressure. So coefficient for at rest condition shall be used.

 

[Kevin9679]

For INTERNAL STABILITY CALCULATIONS (reinforcement) ONLY:
Use a Bousinesq for a strip load surcharge. Many people suggest to use 2q in that equation.