Using s.o.g. to Brace bottom of a foundation wall
Using s.o.g. to Brace bottom of a foundation wall
(OP)
I recently used a 4" thick s.o.g. to laterally brace the bottom of a cast-in-place concrete foundation wall. The s.o.g. only has wwf reinforcment for crack control in it. The foundation wall retains about 14' of earth, and was designed per the geotech engrs recommendations for an "at rest" earth pressure. The lateral force at the bottom of the wall due to the earth pressure is quite large (about 3.8 k/ft), due to the height of retained backfill.
Dead loads on the wall and footing ( including wall self-weight and earth above the projecting edge of the footing) are not enough to provide enough frictional resistance to counteract this lateral force (Let alone provide a safety factor against sliding). The frictional sliding resistance is only about 1.7 k/ft. The footing would need to get pretty big to catch enough backfill dead load to make any difference.
Basement walls in this building oppose each other in plan, so I used the slab-on-grade as a horizontal diaphragm, with the forces at the bottoms of the walls canceling each other.
The basement s.o.g. will be sawcut about 3/4" to provide relief of shrinkage stresses. That leaves about 3 1/4" thickness of slab as effectve. The compressive stress in the concrete may reach 55 psi +-. That didn't seem like much when I first checked this, but now I wonder. It may be possible that the lateral force going into the slab will load the concrete eccentrically, perhaps buckling it upwards, or otherwise damaging it. (Although, in many buildings I've worked on in the past with a similar detail, this has never been a problem.)
I have not thought about this concern much before but now I have revisited the design and wonder if my assumptions were correct.
Do any of you have any experience or comments on this condition?
Any comments greatly appreciated. Please help me sleep.
Dead loads on the wall and footing ( including wall self-weight and earth above the projecting edge of the footing) are not enough to provide enough frictional resistance to counteract this lateral force (Let alone provide a safety factor against sliding). The frictional sliding resistance is only about 1.7 k/ft. The footing would need to get pretty big to catch enough backfill dead load to make any difference.
Basement walls in this building oppose each other in plan, so I used the slab-on-grade as a horizontal diaphragm, with the forces at the bottoms of the walls canceling each other.
The basement s.o.g. will be sawcut about 3/4" to provide relief of shrinkage stresses. That leaves about 3 1/4" thickness of slab as effectve. The compressive stress in the concrete may reach 55 psi +-. That didn't seem like much when I first checked this, but now I wonder. It may be possible that the lateral force going into the slab will load the concrete eccentrically, perhaps buckling it upwards, or otherwise damaging it. (Although, in many buildings I've worked on in the past with a similar detail, this has never been a problem.)
I have not thought about this concern much before but now I have revisited the design and wonder if my assumptions were correct.
Do any of you have any experience or comments on this condition?
Any comments greatly appreciated. Please help me sleep.






RE: Using s.o.g. to Brace bottom of a foundation wall
In my experience, that involves typically 6" thick at least doubly reinforced s.o.g. I have never had a case where the push in the walls has damaged the s.o.g.
RE: Using s.o.g. to Brace bottom of a foundation wall
Also, I believe soil engineers are even more conservative than structural engineers, in general. Your 14' high wall needs only to deflect 0.168" (0.001H) in order for the lateral soil pressure to become active, not at rest. I believe the lateral soil pressure prescribed by the soils engineer is conservative.
RE: Using s.o.g. to Brace bottom of a foundation wall
RE: Using s.o.g. to Brace bottom of a foundation wall
RE: Using s.o.g. to Brace bottom of a foundation wall
1. Wouldn't adding a "shear key" to the retaining wall footing help? It may not take all the lateral force at the bottom of the wall, but it would help reduce the dependance on the slab.
2. Crack control joints parrallel to the wall would still resist the lateral force once they "slide" closed. Joints perpendicular would not have any bad effects.
3. Normally you would want a vapor barrier under a basement floor slab, in this situation you may not. However, you run the risk of damaged flooring materials without the vapor barrier. I would relook at the retaining wall to make it stand alone.
RE: Using s.o.g. to Brace bottom of a foundation wall
In order to buckle upward, the slab must overcome its own selfweight (this conservatively assumes there is a vapor barrier under the slab and there is no adhesion to the fill under the slab). Using the old rule of thumb that the force required to resist buckling is 2 percent of the axial force, you need 76 pounds of force to resist buckling, assuming a 3800 pound force on a one foot strip. The slab weighs 50 psf, so you can resist buckling every 1.52 feet along the length of the slab. It won't buckle upwards.
RE: Using s.o.g. to Brace bottom of a foundation wall
A shear key would help, or the footing could be made slightly deeper to provide some passive resistance. I have not made it common practice to use the SOG as a bracing element.
The S-O-G WILL be cut in the future to perform repairs. You don't know where those cuts will be and therefore don't know the effects on your bracing system.
Failures of the SOG is quite common when you combine the effects of poor compaction, freeze-thaw, unreinforced concrete... If the slab fails you loose you bracing system and then you will have two problems to rectify.
I would revisit the design and make it a stand alone system.
RE: Using s.o.g. to Brace bottom of a foundation wall
RE: Using s.o.g. to Brace bottom of a foundation wall
I agree that a shear key is not common practice, as the steel detailing and trenching is more difficult.
RE: Using s.o.g. to Brace bottom of a foundation wall
Fyi, the "at rest" lateral earth pressure the geotech engr prescribed for this wall was 56 psf. For a drained wall, it probably is conservative, as DaveAtkins remarked. If the wall does move, this can reduce to about 42 psf for the active case.
Usually don't use shear keys here, although one could definitely help if built right. Keep in mind this is a "basement" type wall and not a classic cantilevered retaining wall.
Thank you all again.
RE: Using s.o.g. to Brace bottom of a foundation wall