Need some advice. Designing and building a one story addition with full basement. My existing structure has a basement with low ceilings. Constructed in the 1920’s in an area that at that time had a high water table. Existing structure has drains that lead to a sump pump in basement and I am the only home in the area that gets no water as a result. However, my familiy is growing . Need to build yet another addition. Would l;ike a full basement with 8 foot ceilings. That would require digging about 2 feet at minimum below and immedialy next to the existing footing. I am worried about undrermining the existing footings. The basement would run nearly the entire length of my home, and beside an addition with a crawl space. Is it feasable to dig for this new basement and what are the dangers of undermining my existing footings ? Any thoughts or suggestions are greatly appreciated. Appreciate your suggestions.
Edited 7/13/2005 6:22 pm ET by Manny
Replies
You are correct to think about the effect on your footings. You would probably need a local engineer familiar with the soil type etc in your area to really answer that one. Just based on the fact you had a relatively high water table and need a sump to keep the basement dry indicates to me going down another 2 feet might not be the best solution to your problem.
OTOH, I understand the desire for 8' to the joist bottoms in the basement. Nothing says 'basement room' faster to me than a low ceiling. And you're going to lose at least 4-6" for the drop ceiling anyway; so starting at a full 96" would be nice.
I'd suggest you look at the possibility of placing your addition footings at the same level as your existing ones; tie new drains into the existing drainage system etc; and then pour 8' walls anyway. This will give you a split-level effect on your first floor. You didn't describe your addition more than saying it would be as long as the house, so I can't judge from here how feasible that idea would be.
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
Thanks for the advise. I had not given much thought to the split level concept, becuase our house is a rather old colonial. But given the options it might work out. I think I will meke a few sample drawings. Thanks.
Manny
Lots of doodling now will pay off big-time later....
Dinosaur
A day may come when the courage of men fails,when we forsake our friends and break all bonds of fellowship...
But it is not this day.
The current addition that I am working on is similar to yours. After exposing the old foundation, I poured a footing and built a block wall the same hieght as the old foundation. Keep in mind that it is next to impossible to pour a wall along a one sided form so the block wall was built. The space in between then gets filled in with cement. Hope this is helpful.
Manny:
What you're describing is virtually exactly the same thing as for my addition, except the existing house is 1.5 storey and the addition is 2 storey. And we're nowhere near the permanent water table, but the soil is stiff clay till and very low permeability. Any water which falls on the surface would much rather drain into any backfill along side a basement than to drain away into the native soil.
Take what I have to say with the appropriate measure of salt, because I don't do concrete work for a living nor am I a structural engineer.
Building below the permanent water table is a very nasty job. Don't undertake it without an engineer's assistance and a skilled, experienced concrete worker involved. Getting a good, solid foundation in wet conditions is bad enough- in saturated conditions it can be a nightmare.
If you just have low permeability soils but are not near the permanent water table, then you have only really two choices if you need to go BELOW the existing footings with your new basement: a bench footing or underpinning.
For 2' depth, you're talking about a huge bench footing: it needs to be at least as wide as it is deep. Our neighbours did a huge bench like this and it actually worked out pretty well- they put deep storage closets along the whole basement wall adjoining the existing house, sitting on top of the bench. But if your idea is to create useful living space for your family and storage isn't your main problem, a bench wastes a huge amount of what you're trying to gain. Considering you have a lot of existing low-ceiling basement space which is probably not good for much except storage, a bench probably isn't your best option. But it's cheaper, easier and lower risk than the alternative.
Underpinning is a labourious process, and can damage your existing house if done wrong. Definitely not something that you want to undertake without thorough study and understanding of what's involved- even if you hire out the work. Some of the contractors I approached for quotes for underpinning my place scared the bejeezus out of me with their ignorance- probably a symptom of the building market up here. The good guys are too busy working for builders to bother with one-off homeowner work.
We underpinned the entire long wall (40') of the existing house 2'6" deep. The process consists of digging the excavation for the basement, leaving a fillet of soil against the existing footing at an angle no steeper than 45 degrees and no greater than the "angle of repose" of your soil. Google "angle of repose" and you'll find some information on the angle for various soil types- it gets very shallow for sand etc. Our local building department had standard details which spelled out the requirements pretty well, and our inspector was actually very helpful too. He had plenty of experience with the nasty local "soil".
You then excavate 3'-4' long trenches under the existing footing (no footing in my case- nasty surprise!), set forms and pour new footings the full width of the existing footing within 1-2" of the underside of the footing. You then dry-pack non-shrink grout into the space between the old footing and the new. After WAITING FOR THE CONCRETE AND GROUT TO CURE for a reasonable period, you excavate a second set of trenches and repeat the process. You then do it a third time, pouring the spaces between the first two sets and voila, your footings have basically moved down 2'. It's the "waiting" part that most people try to get away without doing, but rushing this job is risky.
Paying someone to do this is expensive, big time. For good reason- hand digging is hard work. I lost 15 pounds digging my underpinning. But I also have the satisfaction of knowing that my new underpinned wall that supports both my existing house and my addition is sitting on a good, solid foundation with twice the bearing area that the house sat on for 70 years prior- better than it was when I started.