Retaining Wall 2 ft from Rock 1

[sean2000]

I need to design a retaining wall 2 ft away from a vertical rock surface. Do I use the full 40 PSF equivalent fluid pressure (for granular fill and good drainage) or I can use a reduced load. I know theoretically fluid pressure is only related to height and not related to width or volume. But in reality if my wall is only 1 in. away from rock, the load should be very small.

Also, I need to dowel the footing into the rock to resist sliding force. Use #8 with non-shrinkage grout, what kind of design guide shuold I use. Contractor wanted to drill vertically in to rock underneath the footing, is that acceptable? The rebar is able to take the shear but the force concentration at the rock/concrete might crack them.

Thanks

 

[jdonville]

Sean2000,

What's the application? How competent is the rock?

Jeff

 

[sean2000]

Jeff, it's a retaining wall for a parking lot, Height varies from 10 ft to 24ft. Top of wall will not be restained. The rock is weathered sand stone. Also I am interested to know what if it's undisturbed solid rock? Thanks

 

[jdonville]

sean2000,

Could you give a more complete desciption of the situation? Why are you proposing a retaining wall if the rock face is stable?

I would think that a shotcrete facing with some sort of reinforcement into the rock face would be more appropriate, unless you have something that needs to bear on the top of the wall.

Please expand on the situation.

Jeff

 

[sean2000]

Jeff,

The site soil condition varies a lot, in some area there are just silty clay. So we need a long wall with different design considerations. Thanks

 

[Panars]

There is a journal article I have used in situations like this. It is ...
"Earth Pressure on Retaining Walls Near Rock Faces"
Sam Frydman and Israel Keissar, Journal of Geotechnical Engineering, ASCE, June 1987, Vol 113, No. 6

The pressure is less than the active earth pressure because the full active soil wedge cannot develop.

 

[BigH]

Further to Panars, there are also some threads about in the site which discuss silo effects and the like. Could check these out - plus google for silo pressures.

 

[jdonville]

sean2000,

Perhaps you should get a local geotech in your area to help you. Variable conditions warrant a professional opinion with local knowledge and experience.

Jeff

 

[civilperson]

If water can accumulate in the space between the rock and wall then full hydraulic pressurer could result. This is true for a fraction of an inch or many meters of width!

 

[sean2000]

Civilperson,

Perforated pipe and weep holes will be used to ensure good drainage, I think water will not be a issue. Please correct me if I'm wrong. thanks

Sean

 

[msquared48]

I have found that weep holes plug with time and are unreliable. If this is in an area of high rainfall or high groundwater, I would consider two perf pipes, one about halfway down and one low at the foundation, both daylighted and drained by gravity. Washed Gravel backfill. Should the one at the foundation ever become clogged, the midheight one will be functional.

Logically, simplistically considering a 45 degree slope failure/slip plane, if there is not the volume of soil to be retained, the lateral earth pressure should be able to be reduced, but that call is up to your geotech.

Mike McCann
McCann Engineering