Addition - Full Basement Foundation next to Slab Foundation
Addition - Full Basement Foundation next to Slab Foundation
(OP)
Hello - I am looking to put a full basement foundation (probably a PWF foundation) next to an existing cabin that has a slab foundation. How close can I excavate to the existing cabin? What are some things I need to consider for this project? Any help and additional information anyone can provide would be appreciated.





RE: Addition - Full Basement Foundation next to Slab Foundation
If you need/want to be closer, plan on a lot of professional help and increased construction costs.
RE: Addition - Full Basement Foundation next to Slab Foundation
As GeoPaveTraffic said, plan on professional help - before you start digging.
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
To do things more properly, you should have the cabin foundation loads and run a slope stability analysis. In any case, I think it's a bad idea to try to excavate close to and below a building with a shallow footing. I recommend you contact a geotechnical engineer in your area and hire a contractor with lots of insurance.
RE: Addition - Full Basement Foundation next to Slab Foundation
As a side comment, I would consider the proposed construction as simply too dangerous for the do-it-yourself-er. Hire a pro - the life you save may be your own.
(Put it another way: even with 23 years of design and construction experience with exactly the project you have described, I would not attempt that project.)
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Addition - Full Basement Foundation next to Slab Foundation
OSHA also says that, for a Type C soil, all simple slope excavations 20 feet or less in depth shall have a maximum allowable slope of 1 1/2:1.
For long term exposure, I would agree with Focht3 about the 2:1 slope. However, you won't often see a slope that flat for a temporary excavation, in sand or clay, unless the soil is very weak or there is a water condition.
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Addition - Full Basement Foundation next to Slab Foundation
The important idea here is that SkiOn1 should not be excavating up to and below the existing cabin structure without some type of underpinning or very stiff support wall. I asssume that most homeowners would want to have the new addition and basement immediately adjacent to the original structure. However, knowing that there should be some significant separation between the two structures could be sufficient to make a homeowner rethink the layout for the new addition.
SkiOn1 asked how close he can excavate to the existing cabin. Basically, the answer is, "You shoulnd't."
RE: Addition - Full Basement Foundation next to Slab Foundation
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
I agree that this may be a tricky site and that profes-sional help is needed. Still there may be a way, with professional guidance so that he doesn't have to go wild.
RE: Addition - Full Basement Foundation next to Slab Foundation
" unqualified" contractors try this.
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
I'm sure there people who have tried this without dropping a building. I would not try it.
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
RE: Addition - Full Basement Foundation next to Slab Foundation
Questions to ponder:
1. Will the excavated trench sides stand without shoring?
2. Will the excavated, unsupported trench be safe for a worker pouring concrete or setting blocks?
3. How will you transfer the load from the cabin foundation to the piers?
4. How many trenches do you expect to excavate at one time without damaging the cabin?
5. Can the existing cabin foundation accept, without damage, the point load at each block pier?
6. Will the 2 feet wide trench provide enough bearing area to support the cabin?
7. Who will go down into the unsupported trench to prepare (grade and compact) the subgrade for the concrete?
8. How will you retain the sandy soil 7 feet high between the block piers? This may not be necessary if the new basement is far enough away from the cabin and you can safely slope the excavation (which brings us back to your original question!).
RE: Addition - Full Basement Foundation next to Slab Foundation
You should just hire a local engineer to do a test hole and provide the required design drawings to a local contractor or yourself. But, inform the engineer what you are planning, say you will be doing the work over xxx time frame, then design details can be best fitted for situation at hand.
Trying things out without having the correct information is to dangerous, especially when lives and a building are at stake. This is not a small 10 by 10 ft tool shed.
regards
RE: Addition - Full Basement Foundation next to Slab Foundation