Stepped Cantilever Retaining Wall

[Said the Sky]

Hello All,

looking for some engineering expertise on cantilever retaining wall with possible inclined backfill slope (still verifying with the architect that his drawings have a slope, most likely it does)

Question: how would you guys approach designing the two tier cantilever wall approach seen in my attached file. Would you design the higher cantilever wall as normal, and then add surcharge loading from the higher retaining wall, onto the lower one? If the higher one is sufficiently further away horizontally from the lower retaining wall, theoretically there would be no surcharge loading that would translate into additional horizontal forces on the lower retaining wall right and therefore can be designed as I normally would? If so what is the influence angle from the base of the higher wall to the lower wall, I've seen the numbers 2H:1V used quite often but I am unsure of where it comes from.

I also have a secondary issue, the retaining walls at the two sides of the home is quite tall (12'0" total height, retaining 8'0" of soil) is right by the property line so we cannot extend the footing under the soil to use the weight in the overturning calculations, so having issues just making everything work, wall probably has to be 10" thick with a footing about 7-8ft long, and if the soil is inclined the problem is further compounded.

TIA

 

[Said the Sky]

Bump

 

[EireChch]

your approach for the tiered walls is right, just apply the weight of the wall above on to the lower wall and design accordingly. You seem to have lots of embedment depth which is a help, is this correct, or just indicative?

You could look at using modular block walls like ALLAN block or keystone etc. Lots of good reference on line for them and how to tackle tiered walls.

Re you 2:1, its too conservative. The most accurate way is the rankine active wedge angle, which will probably be closer to 60 degrees. A lot of people just adopt 45 deg.

 

[Said the Sky]

your approach for the tiered walls is right, just apply the weight of the wall above on to the lower wall and design accordingly. You seem to have lots of embedment depth which is a help, is this correct, or just indicative?

Thanks for your input it’s helpful. The wall system is in the design phase and they are open to any modifications to make structural work, but of course keeping cost in mind. The higher wall ideally they would like to only have the footing down 4ft which is frost depth here, but can extend further down in order to not load the lower wall if the numbers don’t work.