help what size footing for a 5ft wall help

[roermo]

Need some help for peace of mind.

Background. I am building a 34 deep by 28 ft wide garage. The back 12 feet or so will be dug into a hill side and the back wall will be about 5 to 6 feet into the hill.

My contactor is going to put 3foot x 2 foot footing where there is no wall.

He told this morning that the back wall does not need a footing 3 ft deep because the back wall will be below the frost line here in Hannibal MO by two feet. He told me it would only have to have 18 inch footing by 2 foot.

My question is this is he right. I would think a 3 ft deep footing would help hold the wall from tipping forward from the pressure of the dirt behind it. Of course we will use rebar every two foot across and every two foot up.

So what should the footing be when holding up a wall that has 5 feet of ground pushing on it.

Help he is starting this week.

Thanks Thanks Thanks

Roger

 

[oscer]

Sounds right to me although I'm not a contractor, as long as it drains behind the wall,and is backfilled with gravel a foot or so it shouldn't push.

 

[Dirthog]

That is how we have always done it here in Pa. the extra 18 will do very little to help the wall from pushing unless you put rebar down through every block and fill all the cores with cement. the other poster had it right the right drains and lots of gravel the dirt will not push the wall it mostly water that does that.

 

[Dusty]

I would put a 4" drain pipe at the bottom of the footing and run it around to a low spot. After the wall is completed, and before it is backfilled, I would tar the wall and put 6 mil. black plastic on top of the tar. This will help to keep any moisture from permeating the wall surface. If the building is going to be heated, put 2" Styrofoam board against the plastic. It will help to keep the heat inside the building. Make sure to backfill with porous material such as sand or gravel. Don't use material that has clay mixed in with it, because it will hold the water, rather than allowing it to drain to the bottom and out the drain pipe. There is a special fabric made specifically for drainage pipe that keeps the dirt from getting into the pipe. Use it also. Doesn't cost much and it is very worthwhile in the long run. A properly installed drainage system around a building costs very little in the overall construction costs, and the benefits are very worthwhile in the long run. Dusty

 

[CurlyDave]

So what should the footing be when holding up a wall that has 5 feet of ground pushing on it.

I have designed and built a number of retaining walls.

There are two criteria which must be satisfied. First, the wall must not tip or tilt from the pressure of the dirt behind it. Second, the wall must not slide from the pressure of the dirt behind it.

Five feet is high enough that you should get an engineer to look at it and design it. It is very easy, and should only cost a few hundred $.

This is a whole lot cheaper than having the wall (and building) fail from underdesign.

Drainage behind the wall greatly reduces the force on the wall and should definitely be done, but get an engineer to look at it before any concrete is poured & then insist that the contractor follow the plans. You have to be the inspector here.