Quick CMU Question.

funkys79

I was hoping that someone may be able to give me some advice on a block foundation project that I am about to start on.

For this project, I am planning on using standard 8X8X16 concrete blocks for a foundation. Every 4 feet, the cavities will be fully grouted with 1/2" rebar embedded in the middle. Also, there will be ladder reinforcement embedded in the mortar for every other course. (Spaced 16")

The soil has a high clay content, so that does complicate matters. However, there will be drainage tile laid around both the exterior and interior of the footer. And of course, the exterior will be backfilled with gravel instead of the soil that will be removed during excavation. I will also be using filter fabric to prevent silt build up. In addition, the house would be on top of a bit of a hill, so all rain runoff would naturally more away from the house. So, after taking all of these precautions, I hoping that there will be minimal water issues.

So now that I have quickly gone over the projects specs, here are my questions.

1. I know that you are not supposed to use 8" thick concrete units for walls higher than 8 feet. (Basically a 12 course wall.) But I was wondering if there would be a problem with building a 13 course wall if I had 2 blocks above the finished grade instead of just one. Basically, think of it as building a standard 12 course wall and then adding one more row on the top. The reason that I am wondering if this would work is because the number of blocks that are underground would be the same number for a 12 course foundation. (11 blocks, about 7' 4") But I would end up with the benefits of a 13 course foundation. (The added head room for easier finishing in the future.) I was wondering if anyone has had any experience with this kind of situation before? Does this sound like it would work? Would I have to increase the amount of reinforcement in the wall? Would there be any issues with the vertical load? (The house itself) Or with the soil pressure?

2. What size footers would you recommend for this situation?

3. Finally, I was just curious which type of drainage tile you prefer? Have you had better luck with perforated PVC or corrugated tile? Also, how would you recommend tying the interior and exterior drainage tiles together? Method, (under footer, through footer, etc.) drainage tile size, spacing, etc.

Thank you everybody for your help.

concretemasonry

The thickness of the wall depends on the type of soil, the wall height and the amount of soil retailed behind the wall. There are tables in the IRC code to tell you what is required. I would go for 12" block for a 13 course basement, no matter what type of soil. - With this, you may be able to increase the steel spacing. A 20"x8" footing would be adequate for most any basement wall unless the wall does not have good lateral support (perpendicular joists or 3 bays of blocking between parallel joists) from the floor system above.

Since you are starting with little experience, you should get a permit. This will give you two things:

1. Protection from yourself and lack of experience in building basements.

2. An inspector that will inspect for you.

A basement is not as simple to build properly as most people think. This is evidenced by the number of problems down the road with DIY basements. A few short questions and answers can lead you in the wrong direction quickly.

A basement built in clay is very difficult, no matter which way the surface drains. you have a natural swimming pool you are building in.

The concept of using both interior and exterior drain tile is very good. Some contractors tie them together under the footing (between the outer and inner rock beds - not between pipes), while others will just drain the block cores into the rock bed around the inside drain tile.

Rigid PVC is much better than corrugated since it drains cleaner. Use solid (non-perforated) is you are draining to daylight.

__________________
Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.