Friday, September 18, 2015

Construction -- Concrete Work - Footing Excavations

This is the first of at least a couple of posts on the construction of the footings, concrete walls and slab floor.  (Click on any photo to enlarge it.)

Earth Contact Walls
Previous  posts chronicled the amount of earth sheltering and specifically the north wall. In the end, the earth contact walls were poured in concrete which turns out to be a good decision but way beyond anything an inexperienced DIYer ought to tackle.  

Since the north wall will be supported internally by only one "T"-wall and most of it backfilled to a depth of 12', it had to be treated as a free-standing retaining wall.  The footing had to be 4' wide and 2' deep and the wall had to be 10" thick.  In addition, it had three deadmen extending northward 5' that were 10' thick and 8' high.  By turning the corner and extending the wall for 20' on the west side and 5' on the east side, the extensions also served as two more "deadmen". The final design was not that of the highly-paid structural engineer that stamped our plans but that of the experienced concrete contractor that we hired.

The engineer's design called for a 12" thick wall with an 8' footing and a fortune in #5 and 6 rebar mixed with the more common #4 rebar.  By using the deadmen and by overlapping the footing with the slab floor, as advocated by Jamie Schulte, the contractor, the wall is far more stable at much lower cost.  Our Building Director, the local permitting person, agreed.

Footing Excavations
Trenching for the north wall footing 2' deep and 4' wide
In terms of width and thickness, a different footing would be necessary for the concrete wall versus the other stick-built walls.  The footing for the concrete walls, as mentioned above, needed to be 4' wide and 2' thick.  The footing under the stick-built walls needed only to be 16" wide and 8" thick.  Due to the 2' thickness of the wider footing, the depth below floor level for the two kinds of footings was the same so all the trenching could be done at the same depth with a 4' wide backhoe bucket.  The wider footings under the concrete walls could then be poured directly against the earthen walls of the trench with minimal wood forming at the top.  For the narrower footing under the stick-built walls, the wide trench provided the room we needed to set and brace the wood forms.

Batter Boards
I used the house plans to set up batter boards, first, to guide Brian Hayes, our excavation
Mason lines and corner posts delineating stick-built walls
contractor, as he did the final grading and excavating for the solar collector then to guide the placement of the piers under the front wall of the house.  But their real value came when laying out the footings. Contrary to most of the references I studied, the batter boards had to be positioned abnormally far away from the action in order to be out of the way of the backhoe and concrete trucks, which was no big deal except for creating longer mason lines that could only be used accurately in the absence of any wind. (As an amateur, I did lose sight of the fact that the north batter boards would eventually be rendered moot by the concrete wall in front of them and would have to be relocated in front of the wall before the foundation walls could be properly sited.)

Identical trenches for wide and narrow footings; Brian uses
track loader to backfill pit between house and solar collector

I used taut mason lines running between boards and the equivalent of a plumb-bob to find the outside corners of intersecting walls and drove stakes under the plumb-bob and then a nail on top of the stakes to mark the intersections precisely.   I ran another line at the bottom of the trenches 5" inside the prospective 10" concrete walls and 4" inside the prospective 8" foundation walls under the stick-built walls to delineate the middle of the footings.  I then used marking paint alongside of the string to transfer the middle of the footings to the bottom of the trenches.

Leveling the Narrow Footings
The tolerance in footing height for the insulated concrete forms (ICFs) that we are using for the foundation walls under the stick-built walls is tighter than for concrete foundation walls poured in conventional forms -- actually a variance of only 1/4". Then, when the ICFs are set dead level on the footings and filled with concrete, the concrete could be finished flush with the smooth tops of the forms to give a perfectly level foundation throughout the house.

The reason for opting for footings poured in wood forms, as opposed to pouring directly into the trenches as is commonly done and was done for our wide footing, was for more precise leveling.  In order to accomplish it, I used the rotary laser as a guide for fastening a short piece of 1 x 4 to each corner post to delineate the exact height of the form boards -- therefore the final height of the footing itself.  Using straight boards and setting all of them exactly level with one another was extremely important in assuring a level foundation wall and, since the concrete floor would be screed level with the top of the foundation wall, assuring a level floor as well.

Parenthetically, l should add a caveat.  The decision was made later to increase the depth of the concrete slab from four inches to five to give it more strength.  The extra inch could be gained by either reducing the amount of gravel base under the slab from four to three inches or by increasing the height of the foundation wall by raising the footing by one inch. I elected the latter for reasons yet to be discussed in a subsequent post.

2 comments:

  1. It sounds like there are a lot of things that need to be done to ensure that a trench is properly installed. I had no idea that there was a way to actually take the time to trench an area on your own as well. The work that you did is actually very nicely done. I would be curious as to how long it took you to get this done? Thank you for sharing your progress.

    http://www.lancastertrenching.com

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  2. The DIY responsibility for accuracy lies with the absolutely critical batter boards. After that is was all about Brian the backhoe operator. As he dug, I used the rotating laser measuring rod to check depth. Most of the time, he was so adept that the laser was superfluous.

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As a do-it-selfer-in-training, I welcome your comments.