Basement Addition Next to Slab-on-grade house

[BobTheEngineer]

Hi, all. I’ve got to put a 2-story addition with a basement onto an existing slab-on-grade, 2 story home. The existing foundation is poured concrete, with a continuous footer, about 4 feet below grade. The house is 40 feet wide. The concern is stability of the existing structure during excavation and pour of the addition. I was wondering what the options are for shoring/underpinning, etc.

While I ‘d like to pour the new foundation essentially next to the existing one, I’d consider stepping out a bit if it would help lower the overall cost of the shoring requirements (e.g. if a sheet piling were driven next to the existing foundation, the new wall poured, and the sheet piling removed). The downside is the loss of basement volume as the location of the other wall is set. I’d be looking for the top of the new footer to be 6 feet below the top of the existing footer.

I don’t have any soil data at this point, I’d guess it’s a silty sand.

 

[SperlingPE]

Putting your basement away from the existing foundation will save money at the sacrifice of basement space. The distance can be determined by a local geotechnical.
Underpinning is time consuming, manual labor intensive, and somewhat risky. It should be done by experienced contractors. The loads are light for residential construction, but the existing foundations are usually "lightly" reinforced or not reinforced.

 

[DaveAtkins]

Here in Wisconsin, I know of a company that can drill small holes every so often next to a foundation wall, then install jacks that extend to the desired elevation below. Then the new wall is poured. I would think this would be expensive for a residential situation, however.

DaveAtkins

 

[jjeng2]

Most of the solutions I would use for this dont seem to make cost sense for a residential foundation. Moving the foundation wall seems to be the best idea. If this was a commercial bldg. I would use temporary sheet piling. A good foundation contractor may have some ideas. This is more of a means and methods of construction issue that a contractor may have more experience or better ideas with. In the past I have given a general guideline and emphasized that the contractor is responsible for shoring. You could require the contractor to hire a geotech eng. but again this is a small project.

 

[BigH]

Wasn't this posted - or a similar one about 6 months ago?