New Construction Block Wall Problems 1

[mikeozo]

I am in the process of building a 2-story home that measures 30x44. My father and I have done all of the work by ourselves with the exception of digging the foundation. We have dome just about all types of construction prior to this, but never attempted a complete home.

I used 8" 3-core block for the foundation on top of a 8" deep x 20" footing. I reinforced the footing with 2 courses of 1/2" rebar.

The block wall is 12 blocks high (approximately 8' in height). The top course of block is filled with portland Cement.

We then built the frame of the house (2x6 construction) and installed the roof trusses. We also installed the 1/2" plywood to the all of first floor walls.

At this time, we decided there was enough weight on the foundation to resist any lateral forces that backfilling could produce. The front and sides of the house were only backfilled to about 6 block (48&quot high. However the back of the house was backfilled to 10 block (80&quot high due to the grade of the lot. Before we backfilled, we added about 1-2 feet of gravel for drainage over the french drains.

A few weeks ago, we borrowed a skidloader and backfilled. We then sheathed the upstairs walls, and also sheathed the roof. Yesterday we loaded the shingles up the roof and today we were working on soffit and fascia. I needed a pair of snips and they were in the basment. When I went down to the basement, I noticed that the back wall(which was backfilled to 80", had a horizontal crack running almost completely along the 44' of block wall. The seam was about 64" high. The wall apparently slightly buckled.

I do not know if this occured at the time of the backfill or later on. We have had a very rainy summer and the soil was slightly damp when we backfilled. I am not sure if this was the reason for the wall to buckle or not.

Another thing I thought might of caused this was the weight of the skidloader pushing down the soil against the wall. When my father backfilled, he drove the skidloader very close to the wall to level so we could place scaffolding along the foundation. I thought this might have pushed in the wall???

I am just looking for opinions of what mighthave caused this and also the best way to go about fixing this problem the correct way. I figure the backfill will need to be removed, the floor joists will need to be braced, and a new block wall will need to be put in. If I do this, I figure it will be best to backfill the entire trench with gravel.

If you know of any other better ways to go about this, I would greatly appreciate this. I have alot of money into this house, and I don't want further complicate the situation.

I have not installed any windows, drywall or plumbing yet so that is one positive.

Thanks for any advice you can offer.

 

[ishvaaag]

Well, it looks earth pressure or push (with water or skiploader help) has caused your wall fail in flexure. It also looks as if the resultant of the push has been a bit high and this combined with the restraint put by the structural elements atop the wall has placed the crack where it is. This high placement of the resultant may really indicate that compaction (or push against the wall) has been bigger at the top levels of the backfill.

Well, respect solutions ... Concrete Masonry Units are not the higher standard for basement walls, you know. In the area I live (Spain) this is unknown maybe since decades ago, and everything that is new and basement is reinforced concrete. The meaningful part of this to your problem is the word reinforced, for our walls are typically reinforced both faces, both horizontally and vertically by rebar.

Now, just sketching what may be done, you really may remove the backfill; with some skill one could attempt even to save the present wall by straightening it through push but well, this maybe beyond the skills of self-builders with limited means whilst keeping the risks of generating an accident low. Straightening is of course more a practice for ductile materials such steel than for masonry walls, and in the end you end with a wall with a horizontal crack that needs to be catered for and this is not easy through its complete thickness, so maybe building a wall anew may be safer and better, and even cheaper if not in money in effort. You could in any case "glue" the two parts to each side of the crack by working successively along small segments you would be piercing, even many of them at a time in a dashed line way, that is, keeping always enough vertical support. Also note that likely you will have as well to reseal with mortar in the same manner the base and top courses, since there must also be there cracks, as they are surely needed for the crack you see to form.

Now the question is that you need rebar placed at both sides of your "new" wall, and dowels to the foundation and top concrete member standing or to be made atop the wall. Again to produce holes where to place small vertical 8x8" columns you may act the dashed way, first some, then others. How many? Well, that would require a design, so I say here to you, the more, the better... and I won't venture more without making calculations. But think our walls use to have say one of your number 5 bars every 6" in the external side and elsewhere, both sides, both vertically and horizontally no less than one of your number 3 bars every 6", so see this is significative amount of rebar.

Also, if you make the wall anew and on masonry block work, do not forget to include rebar in the horizontal mortar joints; even special reinforcement with truss-like appearance is sold to this purpose. 1 truss every 2 joints might be OK.

But if you are ready to forfeit what done in the wall, consider doing a Reinforced Concrete wall. It will provide better performance on almost all accounts; and you better use the services of one able to do such design (and maybe also helping you on other issues that may develop) for these things have technical ways of deciding how much rebar to put, where etc., and they know according to the local practice.

 

[LPPE]

Sounds like a classic case of unreinforced masonry basement walls. There are many solutions to the problem, but next time use some vertical reinforcing in solidly grouted CMU cores.

 

[RDK]

It sounds like a classic pressure failure due to high lateral pressures on the block wall.

I would bet that the mid height crack splits into two cracks at each end. These will then go to the top and bottom at 45 degrees (more or less the crack will step along the block joints.)

The cause is due to moisture in the backfill (wet dirt will exert a larger lateral force than dry dirt) combined with the additional loading of the skid steer. The fact that the building was not complete means that there is less compressive load on the block so that the tensile forces due to the bending could crack the mortar joints.

The best solution is to remove the wall and replace it with a concrete basement. Block walls are no longer used in Canada because they tend to crack in just the manner you have described.


These cracks will typically be on the gable ends of a house. There is no floor or roof loading on these walls, only the self-weight of the wall. The driveway is also typically on the side of the house and the vehicle weights plus the lack on compressive loads causes cracks just as you have described.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[DriveThru]

You might consider adding screw anchor tiebacks to straighten the wall and then adding a concrete wall to the inside of the CMU wall. You would not have to take everything down that way. You did not say what the slope looks like behind the wall. Just a thought.

 

[LPPE]

Another possible solution is to install steel columns from the basement slab to the first floor (almost like butresses). Place the columns against the masonry, then grout solid between masonry and column at any out of plumb locations in the wall. That should stop your wall from moving.