Addition - Full Basement Foundation next to Slab Foundation

[SkiOn1]

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.

 

[GeoPaveTraffic]

Generally, if you stay one foot horizontally away from the existing foundation for each foot vertically you are below the existing structure you are ok. This is only a general rule, sometimes you can be closer and sometimes you need to be further, but it usually works.

If you need/want to be closer, plan on a lot of professional help and increased construction costs.

 

[PEinc]

You haven't provided enough information for receiving a good answer. If you have problem soils and possibly a ground water condition, you may have more trouble than you bargained for. For example, is there rock below the existing cabin foundation but above the proposed basement level? What type of soil is the cabin founded on? Where is ground water? What are the cabin's foundation loads? How deep are you planning to go below the cabin foundation? What is the total depth from cabin floor to bottom of new basement foundations? Are you digging along a bearing or non-bearing wall?

As GeoPaveTraffic said, plan on professional help - before you start digging.

 

[SkiOn1]

Thanks for the information so far. To maybe help a little and give a bit more detail. I shouldn't have any ground water conditions to deal with and the soil is sandy. There should be no rock below the existing cabin foundation and above the proposed basement level. I'm planning to dig about 6 feet below the cabin's foundation. The total depth from cabin floor to bottom of new basement will be 8-9 feet. The digging would be done along a non-bearing wall. I hope this information helps.

 

[PEinc]

You will have about 8-9 feet of dirt trying to fall in toward the new basement when you excavate. OSHA would probably require at least a 1.5H:1V slope just to be safe for workers inside the excavation. If you intend to open cut the excavation (without underpinning the cabin), I would think you should step away from the top of cabin footing approximately 2 feet minimum horizontally before digging down on a 1.5H:1V (or flatter) slope. If the soils were stiffer clay soils, you might be able to do something else; but you said you have sandy soils. Also, any excavated slope should be protected from eroding due to rain or surface runoff. You don't want to undermine the cabin 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.

 

[Focht3]

I generally concur, except that the 1.5:1 (H:V) applies to clays, not sands. A 2:1 slope is more appropriate for that condition.

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?"

 

[PEinc]

According to OSHA, Type C soil means cohesive soil with an unconfined compressive strength of 0.5 tsf or less; or granular soils including gravel, sand, and silty loam; or submerged soil or soil from which water is freely seeping; or submerged rock that is not stable; or material in a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or steeper.

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.

 

[dirtguy4]

I agree with Focht3 on this on. For sand 2:1 is more appropriate. You may get away with 1.5:1 provided the entire slope is covered with plastic, well back from the top edge of the slope and well beyond the toe of the slope not allowing moisture to penetrate the slopes surfaces.

 

[Focht3]

I didn't say that 1.5:1 isn't allowed; only that 2:1 is more appropriate. I'm also considering the effects of time - most do-it-yourself-ers spend a lot of time scratching their heads (and other parts of their anatomy) trying to figure out what to do next. A two day job often stretches into two months. At least mine do - and they're nowhere near as ambitious as constructing a basement in sand...



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?"

 

[PEinc]

GeoPaveTraffic mentioned a 1:1 slope. I said that OSHA requires at least a 1.5:1 slope for a Type C soil. It is not hard to find reasons to classify a soil as Type C. I mentioned this minimum slope only for comparison with the previously mentioned 1:1 slope. If the dirt is very sandy, a 2:1 slope would definitely be better and, as dirtguy4 said, the slope may also need erosion protection.

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."

 

[Focht3]

AGREED.



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?"

 

[PEinc]

P.S. And, we haven't even discussed surcharge pressures from the existing cabin on the new basement wall.

 

[BigH]

At what level is the existing structure slab? If it is, say 3ft down, then the "excavation is only 5 to 6 ft deep. We know the soil is sandy - but is it gravelly sand, coarse sand or fine sand? How dense? If somewhat loose, is there a way he can drive in poles or planking adjacent to the existing structure and then do a lagging network behind using angled struts inside the excavation to support the poles? If the basement width is not very large, perhaps he can work out a way of staging with only a "small" width seen by the soil - i.e., do a leap-frogging method. He might be able to consider auger-cast piles as a tangent wall then use wales and struts as he excavates so he can get his wall in.

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.

 

[dicksewerrat]

Why don't you pick up the existing cabin, move it 20 feet away and put a basement under the old and new cabin. Or demolish and build new. Putting a full basement right next to the old slab is dangerous and will probably cave in the slab next to the new addition. I've seen too many
" unqualified" contractors try this.

 

[inaz]

How 'bout this. Dig a 6 feet deep, 12" wide trench between the two buildings, near the top of your proposed excavation. Backfill the trench with lean slurry (2-sack). After a few days, perform your basement excavation. The slurry wall should be sufficient for temporary support of soils below the existing buidlng. Maybe dig some perpendicular, wider trenches at any wall locations.

 

[PEinc]

Sorry, but that sounds like a recipe for disaster to me. I don't see how a 12 inch thick, unreinforced, lean concrete, slurry wall, without bracing, will support the lateral earth load and building surcharge. Also, if you add perpendicular, wider trenches where the other walls are located, how do you build the walls?

I'm sure there people who have tried this without dropping a building. I would not try it.

 

[inaz]

It wouldn't be a free standing slurry wall. It would be outside of his cut slope. Since there's no soils engineer involve, I figured he'll probably end up digging a steeper cut slope then OSHA requires, and the wall would help to prevent undermining the building if a portion of the cut slope sluffed off.

 

[SkiOn1]

First of all - I want to say thank you for all the replies. I have something else for all of you to think about. What would you think if I had someone dig out 2 foot trenches, 7 feet deep at regular intervals along the side of the cabin that the excavation will be done against. Then we would pour a 2 foot slab at the bottom of the trenches and use concrete block to build a tower up to the foundation. That way the foundation would be supported by concrete brick columns along the wall that would be excavated so even if the soil caves in when we dig the basement for the addition the existing structure will still be supported. Thoughts? ...and THANKS!

 

[PEinc]

Essentially this is underpinning.

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!).

 

[cdh61]

SkiOn1,

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