Originally posted by @John Wanberg:
Aaron - what is the foundation made of, and do you know what type? Where is the water-table in the area? How is the condition of the foundation? All of these are pretty important questions... Most foundations (in my area at least) are called "strip footings." This is basically just a block of concrete poured directly into a trench, so that the weight of the house sits directly on "un-disturbed" soil. If this is the case with your house, then digging the crawl space down is not an issue... as long as you only do it in the center of the space, away from where the foundation is bearing on the un-disturbed soil. On the other hand, bigger buildings tend to be built on caissons with "grade beams" - so that the building itself sits directly on bedrock. As long as the bottom level is not a floating slab, you can dig to until the building is sitting on bare rock and not affect its stability.
In terms of water intrusion... this is related to the water table and grade of your yard (and soil type) more than the crawl space elevation. If you live in an area with a really high water table, so that digging down 6" causes a hole to fill with water, then maybe don't do it. On the other hand, if your yard drains back toward the house, you will get water intrusion from surface runoff. In most cases, water does not percolate very quickly through soil, so a normal rain-storm will inundate the top 6" of soil, and then the rest of the water will become surface runoff, going into rivers and eventually to the ocean. In soils with a high clay content, it can take months or even years for water to percolate even 1" (which is why clay is used to line landfills... keep that trash juice away from me!).
So I guess the answer is... it depends. Stay away from your foundation and any support columns, control water, and generally you will be fine.
Very detailed response. Thank you. The footer is poured concrete, not sure if it is original (~100 years old) or to what specs it was poured to, but the standard now is 8 inches deep and 16 inches wide, about 2 feet below grade. Then there is a course or maybe 2 courses of cinder block on top of that. The house was fully rehabbed about 12 years ago (down to the studs)...not sure why they didn't jack up the house and add another course of block...probably cutting costs.
The weight of the house sits on the footing which sits on undisturbed soil. I haven't been under there but I will definitely have the digging crew avoid getting too close to the footer/block foundation or near any piers that might be under there. The footprint of the house is pretty small - only 1400sf, 2 story and the house is about 22 feet wide.
Water table isn't close to the surface, though the house is only 15 feet above sea level and less than 1/2 mile from the water in the inlet to the ocean. The back yard really doesn't "drain in any direction...it fills up by a few inches when it rains and then just percolates into the ground in a matter of hours. My fear is that I might be encouraging water to run under the house, but I'm leaning toward believing that won't be an issue...
Thanks again for your feedback.
