Retaining Wall

[JonBrant]

I'm designing a 20' tall cantilevered retaining wall where the wall will only be 2' away from a very stable vertical hard rock cut. Crushed gravel will be filled in that 2' space. Since the gravel is only a 2' wide, how do I apply the gravel pressure on the wall? I think the typical triangular load pressure would be too conservative for this condition. Hope someone can help me.
Thanks.

 

[oldestguy]

If you go to the room "Geotech Other" you will see may comments about how I justify active pressure against a non-moving wall. That silo effect may apply for you.

http://www.eng-tips.com/viewthread.cfm?qid=393062

However, seepage of surface water may come into play some day for you.

Can you install any rock anchors to minimize all the grief with a wide footing for cantilevering support?

 

[Doctormo]

From a previous post:

Soil Engineering by Spangler and Handy, 4th Edition has exactly what you are looking for regarding a rock face. It is not covered in earlier editions though.

Full earth pressure usually occurs to a depth of approx. 2 x space between wall and rock face and then levels off assuming a certain friction between the backfill and rock face.

Note: Rock is never where it is supposed to be and this calculations is totally dependent on the space between wall and rock. If you assume 3 feet and it is actually 5 feet, the pressure calculation is off by quite a bit.

Note 2: some of this may not apply real well in a seismic zone where the pressures may increase due to a reduction in frictional factors.

 

[FixedEarth]

Over the design life of the retaining wall, the rock face may topple or move forward increasing the backfill earth pressure. So we can't rely on the active earth pressure just caused by the gravel. Talk with the soils engineer and the geologist. Some of these rock formations have out of plane dips which causes additional lateral force on the retaining wall.

http://www.soilstructure.com/

 

[Panars]

Assuming the rock face is stable and the cautions by FixedEarth do not apply, then the concept of silo pressure would apply.
I have used a procedure outlined in a journal article in ASCE's journal of geotechnical engineering.
Frydman, S. and Keissar, I. (1987). "Earth Pressure on Retaining Walls Near Rock Faces," J. Geot. Engrg., ASCE, 113(6), 586-599.

There has also been some good discussion about this subject over the years. Check these threads.
thread486-185817: Retaining Wall 2 ft from Rock
thread274-166357: vertical and lateral earth pressure - length req'd
thread507-193368: Parallel Retaining Walls