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Retaining Wall Questions

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Title: Retaining Wall Questions

List,

A few retaining wall related questions:

In the UBC 97’, the minimum setback to daylight from a footing is H/3. If the slope is > 45 degrees, the set back is the 1:1 slope from the bottom of slope.  In our area, soils engineers usually recommend a 5’-0” to daylight setback.  This seems to imply that the sliding force from retaining walls has completely dissipated in that distance.

Question: In a stacking, stepped retaining wall situation, if and when is it reasonable to disregard the sliding force from the upper wall on the lower wall?

In measuring the distance to daylight, the code reads the “face of the footing.”  In the case of a site retaining wall with a key, the key is typically designed to resist all the sliding forces. Also, simple statics would permit the key to be shifted anywhere along the footing.

Question: Since the key resists all sliding, isn’t it reasonable to presume the setback requirements should be measured from the face of the key? If so, it seems the key can be moved back along the footing until the minimum setback is achieved. In the case of the stacking retaining walls, depending upon the distance from the face of the key to the lower wall, the key could be placed back far enough no sliding force surcharges the lower wall.

In an effort to keep things focused, I am only looking at the effects of the sliding forces on stacking retaining walls.  Surcharges from the foundation bearing pressures, I believe I have a handle on those.

Thanks,

Jordan Denio