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Old 09-29-2009, 03:29 PM   #1
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I need to build a crawl space wall under an existing house.

There are two choices currently on the table:

1) USE CONCRETE BLOCKS 3 COURSES HIGH

Since this is an existing house, the concrete blocks do not lay out perfectly. The head joint varies considerably from one wall section to the next (but each section remains within a 1/8" - 3/4" margin.) The problem I see with laying out these blocks is getting the head joint measurements consistent along each wall. I think it is going to be a real pain trying to switch from 1/4 on one wall to 5/16 (or some other odd measurement) on another wall.

It seems it will be tough to get the blocks into place in the areas with vertical rebar since the house will not be jacked up much higher than it is (due to plumbing.)

2) BUILD FORMS AND POUR THE WALL

I don't like the idea of buying a bunch of plywood and 2Xs only to throw them away when I am done with them. This option also requires considerably more rebar. Seems to me this approach is wasteful and more expensive. Plus, it doesn't seem to me it will save any time since I will have to build all of the forms then tear them down.

This is a DIY project. I don't see myself mixing up enough concrete to fill an entire perimeter wall in one go. Also, a concrete truck cannot access my property effectively. It would also be cost prohibitive to hire a professional service.

BOTH of these options suffer from the fact it will be tough to get the concrete into the holes of the blocks or into the forms since this is an existing structure. There will only be about 3 inches between the bottom of the structure and the top of the new wall.

I can't decide. Any thoughts?

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Last edited by kemp; 09-29-2009 at 03:31 PM.
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Old 09-29-2009, 04:31 PM   #2
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Sounds like a challenging project. What I am not clear on is what supports your house in the area where you are planning to excavate the crawl space? Is there a dirt or concrete floor? Or are you planning to temporarily jack up and move the house while you do the excavation and place the blocks or concrete for the new wall? A sketch would help immensely.

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Old 09-29-2009, 05:20 PM   #3
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This may help you in deciding. Concrete ("Grout", in your case) is often placed using a "Trimmy" (might be spelled wrong).

This is no more than a chute made of plywood that hangs on the block. This one is shown about 5' long, and 10" high. But you could build one much smaller for your cramped area.

Also, rebar does not have to be continuous. If you have about a 20 diameter overlap, (diameter of the rebar... ie 1/2" rebar would overlap about 10") you will have more than sufficient strength. You just have to be sure to keep the two pieces of rebar close to each other... not more than an inch to an inch and a half apart. That is why we tie rebar with wire.

So, you could place the rebar in pieces, if you have to.

Contrary to popular belief, that rebar in walls and columns is NOT there to provide compressive vertical strength, but, rather, lateral support.

Just some thoughts for your consideration
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Old 09-29-2009, 05:34 PM   #4
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The existing foundation is spot piers. Over the years it has started failing due mostly to poor grading of the land around the house. The current spot piers are simply cinder blocks sitting on 15 X 15 X 2 pads. Some of them are not even below grade.

Since I have to repair the foundation, and upgrade it for a partial second story addition, plus re-do the grading, I wanted to go all the way and replace the entire concept of the foundation. That is where the two options originated.

Since the foundation was failing, we put jacking beams behind the existing piers and created a temporary foundation (just as strong as what was already there). So now, the jacking beams and temporary piers ARE the foundation. With them in place, I have plenty of room to trench the area needed for the new foundation.

After my last post, I remembered something: "Grade Beam Footing". Using a grade beam with spot piers might eliminate all the afore-mentioned problems. It might also take care of a few others, like existing septic and water lines.

Any thoughts?
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Old 09-29-2009, 05:37 PM   #5
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That "Trimmy" idea is a cool one.
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Old 09-29-2009, 05:41 PM   #6
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Quote:
Originally Posted by kemp View Post
That "Trimmy" idea is a cool one.
It's been around for a hundred years.
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Old 09-29-2009, 05:48 PM   #7
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Not too sure how grade beams would change anything if the piers are not carrying their loads properly even now? You'd still be using the very same supports. All new piers planned?
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Old 09-29-2009, 06:00 PM   #8
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This is what i mean with a grade beam. (picture attached i hope) With a substantially wide footer, it seems it would work to me. I realize grade beams are used to carry loads over unstable areas, but it seems to me, with piers spaced less than 5 foot apart, it would work almost identical to a continuous footer.

Oh, for those interested: 0-8 inches = Fine sandy loam, 8+ inches Clay. However, I have dug down further than 8 inches and saw no clay. I am sure that is some kind of average.
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Old 09-29-2009, 06:08 PM   #9
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I like the idea. But I have never liked the narowness of grade beams (usually 8") as opposed to the wide base of standard spread footings (often more than double the width of a grade beam)
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Old 09-29-2009, 06:17 PM   #10
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You are putting concentrated loads (the piers) on a continuous "rigid" beam that will remain that as long as the soil underneath it and the beam i strong enough. If not you will get cracks and end up with individual piers on separate short grade beams of pieces of a footing. - That is what will happen if you get one pier to cause a crack and shift the loads to the adjacent piers. Once a piers support fails, the load is shifted to the remaining piers. A common danger when dealing with concentrated loads on a beam with unknown soil properties. It can work, but not always.

I am not crying "wolf", but seem to get dragged into some strange failure problems.
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Old 09-29-2009, 06:30 PM   #11
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Your situation becomes a little more clear. I am guessing you are down in the Gulf Coast area, where pier and pad foundations are common. I am assuming that at some point you are going to discuss your plans with the local building inspector, since you are certainly going to need a permit,unless you are really out there where the buses don't go.

As for a grade beam, it is typically used as a support for columns. As such, the grade beam needs to be designed by an engineer, since it will need reinforcing for shear, bending, punching failure, and diagonal stress. Unless you are very confident of your design skills, you need some help for a grade beam.

As mentioned by a previous poster, if you are sitting on compressible soil (i.e. clay, silt, organics), you would need to excavate and remove all incompetent soil and replace with structural fill. If you are not familiar with the process, you should seek out some help from a local contractor who is, or perhaps from an architect or engineer. The replacement structural fill needs to be properly placed and compacted to support a grade beam, or for that matter a conventional spread footer.

By the way, you noted that the existing piers are failing due to poor grading. I am not sure what you believe the problem is, maybe water getting under the piers? However, if the soil is compressible or expansive, those piers were doomed to failure regardless of grading. I spent 8 months inspecting failed houses along the Gulf Coast after hurricanes Katrina and Rita, I can't tell you how may pier and beam houses had settled, cracked and worse due to poor soil, improper foundation preparation, overloaded piers, and improper drainage.
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Old 09-29-2009, 06:32 PM   #12
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If the grade beam is 18" wide and 8" deep with 2-wide, 3-deep rebar (6 total), would that not compensate for the "cracking apart" of the grade beam?

The grade beam would be just as strong as a continuous spread footer, but with concentrated loads.
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Old 09-29-2009, 06:44 PM   #13
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The mechanics of a grade beam of the size you describe is problematical. First of all, you need a minimum of 2 inches clear for the rebar from soil, and 1 inch clear from the top (more if your code requires it, some codes require 1-1/2 inch clear minimum. This puts the steel close to the neutral axis, where it performs no work. The center rebar (I assume that is what you have in mind when you say three deep) does no work at all, and can be eliminated.

Regardless, designing the rebar is normally done by a structural engineer, if you think you know how to do it, more power to you, but the three deep, two wide approach is a waste of expensive steel. In any case, the beam is no better than the soil it sits on, so you really need to take a hard look at the entire support system, not be entirely focused on the beam.
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Old 09-29-2009, 08:25 PM   #14
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I vote for the 3 courses of block layed on an adequate concrete footing.

Just remember, the footing is going to take a large amount of concrete also, especially if hand mixing.

The joint spacing is a non-issue here, there are many ways around that. If you think your going to be laying each block with an exactly equal head joint, that may be a sign that your in over your head.

Modifying CMU's to fit around vertical rebar isn't complicated by any means either.

I've been on quite a few underpin jobs in my life, & can tell you very few had that much (3") clearance betwwen the top course & the structure. This is a very do-able project, for the right kind of DIY'er.

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