Building stem wall

[braves25]

I have a typical slab on grade building that will have a 4' concrete stem wall. Looking at the design of the footing and wall and the earth pressures on the stem wall. Seems that sliding always governs and the footings get rather large if I include the water pressure to the at-rest pressure of the restrained stem wall design. Ko = 0.53, Moist unit weight of soil = 120 pcf, passive Kp = 2.77. Looking at the pressures (no weepholes or perf drains), pressure = 62.4 + (120-62.4)*.53 = 92.93. Just seems like overkill to have stone backfill, drains for this type of construction. Thoughts?

 

[msquared48]

Why would you have to worry about sliding if you have a slag-on-grade to resist any lateral forces due to soil pressure? I would be more concerned with overturning of the wall.

As for the gravel backfill with a foundation drain below the slab level - do it. You do not want water in your building, and without it, it WILL happen.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[braves25]

The slab on grade resists the top of the wall (therefore no overturning); however, the bottom of the wall/top of footing will have to resist some sliding forces correct (kickout at the bottom)?

 

[msquared48]

Only if there is no soil on one side.

If that is the case, I assume the slab is an interior slab and higher than the unfinished finish grade. That being true, you would not need any interior foundation drains if there are no springs under the slab. However, I would still put a foundation drain along the outside of the stemwall at the base of the stemwall footing.

The lateral kick at 35 psf/ft should only be 280#/ft for a four foot high retention. Depending on the nature of the soil structure, the slope of the ground, and the geometry of your foundation walls, a two foot burial should be able to handle that.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com