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First, sorry if this has been covered. I searched the archives but did come up with a similar discussion. However, if I've missed one, I'd appreciate a link to it.
I'm building a T-type foundation for my 2 story garage. The foundation will consist of a stepped, rebar reinforced, poured concrete footer with reinforced, filled block (CMUs) walls with a reinforced concrete slab above. The site is sloping about 6 degrees and quite rocky which is one of the reasons the stepped footers makes sense.
My question concerns how I build my footings around the large slabs of bedrock that I've encountered. For the most part, is was able to excavate below my frost depth (24") in the trenches. However, about 25% of the area of the trenches contain outcroppings that extend up into the trench by about 1.5' on average (to a depth shallower than the frost depth). I need to build my footers around these outcroppings. BTW; the soil and rock outcroppings on my site are structurally sound and suitable as foundation support. I've consulted many references, including Taunton books, other books, the building codes, the local building inspector, engineering forums, and others. I'm encountering a lot of conflicting information. I want to appease the inspectors but I also want to build a bulletproof foundation. The three options are basically these:
Option 1. Per IBC, and my building inspector: isolate the rock from the footer using a thin sand layer (like 1/4") or 6 mil. poly sheet and pour the footing over and around the rock.
Option 2. Per some references, pin the rock to the footer using short sections of rebar, then pour around the rock. This seems to conflict with Building Codes that preclude steel reinforcement within concrete lying within 2" of soil (where the term "soil" apparently includes rock). Locally at least, engineers do not presribe pinning, maintaining that movement between foundation and rock should be allowed.
Option 3. A modified version of Option 1 but use a thicker layer of sand between rock and footer (more like 2"). The thought being that the sand will act as a "cushion" when differential settling occurs and more evenly distribute the force between ground and footer. The concept of differential settling is that the added weight of the building sitting on the footers will compress the soil more than the bedrock causing a stress concentraion in the footer near the transition between rock and soil underneath.
4. Others I haven't covered?
So, pin or don't pin? If not, how much sand should be used to ioslate the rock and footer? Or, do something else.
Please don't tell me to consult an engineer or geologist. I value the knowledge that registered professionals have to offer but like to temper that with insights from experienced professionals (yeah, you guys) about real-world observations. Unfortunately, the problems are usually buried and many foundations are over-built enough to compensate for design flaws.
Thanks in advance for any constructive discussion and advice that this prompts.