I'm looking at a CMU basement wall that is exhibiting all the signs of cracking due to water/soil pressure outside the wall, but the trouble is there's an existing covered porch immediately outside the wall for about 6 ft out.  See attached section.  Any thoughts on why the wall is doing this?  The plants outside the porch are small and there's no visible distress in the stone patio from roots, etc.  Cracks are horizontal, midway up the wall, and for almost the whole length of the wall, about 1/8" wide to hairline.  There are step cracks at the corner of the wall going both up and down, also about 1/8" max.  The roof drains away to the other corner of the wall.  Definitely older cracks because the previous owners caulked several of them a while ago and the cracks don't appear to have moved since.

CMU walls suck.  It is probably hydro static thrust from soil and water pressure.

A horizontal crack that stair steps to the top and bottom of the wall at the corners is the perfect example of a failure of a two way plate, supported on three (or four) sides.  Probably an unreinforced CMU wall.  I'm guessing the cracks have been there a while.  They may not have moved much recently since the lateral pressure has been somewhat relieved by the movement of the wall.  I'd repair it.

Agree with IceNine.  The wall is probably unreinforced, or at least underreinforced.  I don't know why, but folks in the US seem to think that unreinforced masonry works for basement walls.

Agree w/ above.  A few other thoughts:

1.  The inward bowing with the horizontal and then end diagonals would indicate that the top of the wall was free to translate inward.  This means either the floor diaphragm was too flexible, or that the floor joists were parallel with the basement wall with a rotating exterior 2x plate.  If joists are parallel, perpendicular solid wood blocking is required to take the lateral thrust from the top of the wall and deliver it up into the floor diaphragm.

2.  Look for any sources of excess water getting to the soils outside the walls - downspouts that dump water right at the building edges or lack of drainage.

3.  The presence of the porch, and its weight, might be a surcharge of sorts on the wall and increase the lateral forces on it.

Tremors!

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com