retaining wall question

[Tahoebrian5]

I have a tiered retaining wall system. the upper and lower walls both retain 10' each with 12' between. I have the soil shear angle from the geotech report (32 degrees) and for vertical loads from the upper footing the influence line just hits the heel of the lower footing so does not really affect the lower wall design. My question is about the lateral soil reaction (passive soil resistance + soil friction) from the upper wall to resist the sliding force. If the upper wall is pushing on the soil laterally, does this reaction need to be distributed to the lower wall?

 

[hokie66]

The force has to go somewhere. If not to the lower wall, it would have to be tied back into the earth.

 

[FixedEarth]

Whether the upper wall will surcharge on the lower one, depends on 3 things: The setback distance as measured from the upper footing edge to the back of lower wall, The footing width for the upper Ret. Wall and the bearing pressure of the upper Ret. Wall. I did a quick calc based on a 4.5 ft wide upper footing and a 1500 psf bearing pressure & the upper 3.5 ft of your wall is surcharged double of the active earth pressure- See attached chart.

On your passive resistance region of the upper wall is also at the same zone of the active pressure of the lower wall (assuming the bottom of the upper footing is at same elev. of the top of stem of the lower wall). This will be too much stress on the lower wall. You could consider placing lean concrete below the upper wall footing so as to reduce the effect latral stress on the lower wall. Otherwise, you will need a tieback on the lower wall or support the lower wall on piers. A Geogrid wall will be specifically sensitive to these lateral stresses.

 

[ishvaaag]

From a classical viewpoint (other than seismic), you determine the active push on your lower wall just from the earth, water pressure AND surcharge. Classically the whole of the upper tiered wall would be being assimilated to one uniform surcharge (if the upper surface keeps horizontal); but you can differentiate on what happening at the footing part of the upper wall and the rest of the soil (that would identify the concentration of pressure under the upper footing). In the particular case as yours in which the inclined reactions of the upper footing fall behind the critical surface of the active wedge, no particular provision will be required for the push from the resultant of the horizontal component of the reaction of the upper footing.

If the case was one where the footing was definitely inciding within the active wedge, you can resource to graphical methods (method of Cullmann) to determine the active push; what in the Cullmann method were being vertical forces corresponding to the vertical surcharges, will be now -within the width of the upper footing- inclined forces from the reaction at such upper footing, but once drawn you can close the polygon and find the tentative push; and also the inclined reactions behind the critical surface can be counted this way, as segmental inclined forces, if you want such perfection; but for a conservative estimate of the active push and to simplify the work just the vertical weight behind the critical surface would be accounted. A number of active wedge limit tentatives would identify for you the worse active push. The process is amenable to both analytical programming (finding active push for one set of geometrical and material conditions) and even graphically-manually within Autocad -parametrical environment.

To summarize the maybe excessively complicated exposition of the previous paragraph, you may simply use the Cullman method by substituting the upper wall and its active wedge by their reaction of the remainder of the retaining system. This involves of course inclined forces on what is now the upper limit of the notional soil not retired by our abstraction.

It is anyway clear that by design it is the critical surface where the failure is expected to occur. Yet this doesn't mean we need to reinforce through the critical surface in every case, simply the forces to each side of it are in equilibrium for the analyized state.

You can see that at the heel of the upper wall the footing is pushing the free side towards the air side and the reaction of the active wedge on the standing soil behind the critical surface is pushing towards the mass of the soil, causing some tensile stress. So some tensile reinforcement is useful there.

It is also in the way of making reinforced earth work, each band of reinforcement being able to retain its tributary active push; and each reinforcing tier has to be soundly anchored through evaluated friction or deadman device well behind the critical wedge to be effective. So some reinforcing scheme under the upper footing as suggested by FixedEarth would be positive to a sound long life of the wall; but it may not be required.

 

[ishvaaag]

Two further commentaries.

First, to dismiss the forces acting against the soil at the critical wedge of the upper wall may lead to overconservatism in determining the active push, since they partially cancel the horizontal component of the pressures at the upper wall's footing. Overall the effect of the upper tier of soil mass and wall will be quite close to just a surcharge of upper soil mass, just a bit worse on the particulars of the upper retaining wall presence.

When referring to reinforced earth, to label the active push what restrained in a reinforced earth scheme would be incorrect; so check pro the proper way of determining the forces for the anchors in MSE in one specific check for such thing. These can also be seen as substituting the tensile stress developing in the elastic mass when settling with the surcharge applied; but then quite likely better commit ourselves to one entirely noncohesive and drained backfill, and add the watertable instantaneous pressure for adequate safety.

 

[BigH]

Draw the sections to scale and use the Culmann Graphical method to determine the load of the lower wall. (As was suggested within the text of ishvaaaag). See Terzaghi and Peck for procedures . . .

 

[Tahoebrian5]

Thanks to everyone for the help. I'm looking into the Culmann method.

 

[RFreund]

If this is a segmental wall, look at NCMA's segmental wall manual section 5.9.2.
If you want I can give you some more info however it is specific to segmental walls.
However you mentioned passive pressure so I would assume this is not a segmental wall.
For other types of walls, I would use the exposed height of the upper wall (q=H*soil density) as a surcharge for a boussinesq analysis and add these horizontal forces to the active pressures on the lower wall.

EIT