Crushed concrete backfill 1

[jam6444]

I have a CMU "box" of dimensions 13'x28' with walls roughly 5' high. I'd like to fill it 90 percent with crushed concrete &then top it with some 8910, let it compact, then pour a porch slab on top. Does this sound reasonable? Clay backfill would be a no no in this instance as the walls aren't filled solid and I'm afraid the pressures would be too great. The slab will be 5"-6" thick and be reinforced to account for loads and the eventual settling of the fill below.
The crushed concrete pieces are roughly golf ball to tennis ball in size and VERY VERY irregular in shape.

 

[civilperson]

Side wall pressures will be about the same with the angular aggregate,(crushed concrete) as with the soil. The Ko, will probably be less for the concrete fill but the unit weight will be higher giving comparable horizontal pressures. Why not fill both sides of the wall, thus no unbalanced pressures or reinforce and fill cells with grout. A #5 bar at 16" or 24" spacing centered in a grouted cell would have sufficient bending strength if correctly constructed, (doweled into the footing and horizontal wire ladders in alternate joints).
The six inch slab will have difficulty spanning thirteen feet if not engineered.

 

[jam6444]

Well, filling both sides is unfortunately not an option, and I've got a struct. eng. that's going to help with the slab design, so I guess the real sticking point is the infill.
the walls are already up BTW ,unfilled and unreinforced.

 

[concretemasonry]

Minimal vertical reinforcement such as suggested by civilperson is completely adequate, and will provide a degree of confidence considering the minimal wall height (5'). It is only necessary to fill the cores with steel, but it may be cheaper and more practical to fill the entire wall.

Do not attempt to calculate the wall stresses because the aspect ratio of the wall, questionable fixity and assumed density would lead to a very accurate guess based on unrealistic assumptions.

Treat the walls like a common shallow basement wall and use empirical design parameters to show that is is adequate. Under this assumption, even a hollow block wall probably would be adequate.

The obvious problem is the ability of the slab to carry the aticipated loads assuming to support from the concrete fill. This should be a very straight forward design.

Dick

 

[jam6444]

That's basically what it is, a shallow basement wall forming a box, with slab on top. I guess it would be a waste to fill all the cells and omit steel doweled to the ftg.

 

[concretemasonry]

jam6444 -

You hit it spot on!

It is a very basic, proven, garden variety structure with an unusual fill to make it interesting.

My 8' high basement is 12" unreinforced block with 6+ feet of clay/sand backfill and built intersecting a natural drainage pattern. No concrete slab for good top support. No cracks or problems for 30 years. When I tried to analyze it after I bought the home, it scared me, but I guess that is why there is a place for empirical design.

Dick

 

[emmgjld]

concretemasonry just hit on a big problem with smaller structures. Many things work well but, do not calculate well. The problem of residential basement wall design and performance is a continuing subject. This is vexing, from the standpoint of liability.