Sloping Surcharge on Concrete Retaining Walls

[bigmig]

I am in the process of proposing on a project built on a very steep hill side. The slope of the hill is so steep, that the up hill retaining walls will encounter the condition where the 'soil wedge' behind the wall is literally hundreds of yards long. In other words the slope of the hillside (beta) is the same as the slope of the slip plane of the soil wedge behind the wall. The two planes either don't intersect, or intersect so far up hill that the soil wedge is enormous.

I am referencing the Joseph Bowles book for guidance, but would appreciate anyone's input on designing for this condition.

I see walls of similar scenario on highways that abut steep, uphill slopes so I'm assuming AASHTO addresses this situation.

The statics approach for stability would produce wall designs that are impractical. Does a correct design account for wedge to mountain friction? Is a slope
stability study with an inclinometer the first place to start? Do you soil nail or tieback the upper portions of the wall?

The hill is covered with oak brush.

 

[Splitrings]

How steep is steep?

Attached is a Navy reference that may help.

 

[dcarr82775]

Are your soil properties too conservative? Is the current slope close to a F.S.=1 or even less (theoretically)? You could look at it as needing to provide retainage equivalent to the weight of soil removed to make the cut to at least kee. If you have to hold back an entire mountainside it becomes pointless to try

 

[FixedEarth]

Check with the geotechnical consultant. They may suggest a pile or pier supported retaining wall. The soils firm will do the slope stability analysis and based on that will recommend a minimum pile embedment & earth pressure values. Inclinometers will take several years before they tell you how much and how deep a movement is taking place. If your retained height is more than 10 ft, you may consider tieback wall instead of cantilever retaining wall on drilled piers & grade beams.

To give you a related example, I am now working on residence on a 20 foot high, 2:1 ascending slope. We are suggesting drilled piers and grade beam foundations. The embedment depth needed for the house is 12 ft to meet capacity and IBC setback requirement. But when I did the global static stability, I need 18 feet to be sufficiently below the failure plane. However, when I did the seismic case, I need a 22 ft embedment. So we are recommending 22 ft deep drilled pier.

In your case, the geotechnical firm will tell you what is the driving the available retaining wall options.

http://www.soilstructure.com/

 

[bigmig]

The slope is at least 20 degrees. When I calculate the angle of the slip plane using Bowles Chapter 11 equation 11-16 (solving for rho, the angle of the slip plane), the formula starts trying to square root a negative number. The steepest sloping surcharge this particular formula will accommodate is 18 degrees. And when I set my beta angle (hill slope) to 18 degrees, my slip plane angle is only 13 degrees. So in short, the soil wedge is bordered by two lines, the surface, and this slip plane. These lines are nearly parallel.

So as dcarr82775 pointed out, how do you hold back an entire mountain?

FixedEarth, thanks for the advice. I have contacted a geotech to get a sub proposal from them. My research has shown that the slope stability analysis and hardware installation seems to be key to the success of slopes this steep.