Foundation near old foundation surcharge?

[Kom3]

Hi,

Is there any issues with pouring a new foundation next to an old one? I don't think so but I was wondering if doing so would add a surcharge to the existing foundation causing it to potentially fail in soil bearing. I also found this excerpt in the building code but it is open to interpretation. Thanks.

 

[le99]

I think it will be less problematic if the soil pressures under both foundations are close, and remained under the allowable bearing pressure.

 

[MTNClimber]

Do you know what the foundations soils consist of?

 

[HTURKAK]

The following excerpt copy and paste from 2018 IBC Illustrated handbook..


1808.3.2 Surcharge. This new section in the 2015 IBC adds requirements pertaining to surcharge loads that could affect an adjacent structure. Although Chapter 33 covers requirements during construction, Chapter 18 had no specific provisions related to the
effects of permanent loads that could surcharge a neighboring structure. Fill or other surcharge loads are not permitted to be placed adjacent to a building or structure unless it is capable of withstanding the additional loads caused by the surcharge load. Existing footings or foundations that could be affected by an excavation must be protected from detrimental lateral or vertical movement and settlement. The exception allows minor grading for landscaping with limited grading heights using walk-behind equipment which does not
induce high forces against an adjacent foundation or wall when approved by the building official.

I think more than clear..

 

[phamENG]

You have to consider both vertical and lateral.

Lateral: if the new footing is at a higher elevation than the existing, it will impart a lateral surcharge on the existing. This will make the existing want to move away from the new. This is most severe in foundation walls, basement walls, and nearby retaining walls. This would require an in depth verification of the existing wall and possible reinforcing. But it can have a negative impact on some spread footings too, I suppose. General rule of thumb - when pouring a new footing next to an old one, put the new one at the same elevation.

Vertical: The existing footing has been there a while and the soils beneath it have consolidated with the weight. When you add a new footing adjacent to the old one, you're adding new vertical loads to the same soil. If the soils have much space left to consolidate, your new footing can cause additional settlement in that area. If the existing building had settled uniformly before, this new settlement is likely to be differential in nature and possibly damaging to the existing structure. This is best evaluated by a geotechnical engineer.