Adjacent footings - different depths

[Darren Emery]

2006 IRC

Existing full depth basement foundation - homeowner would like to place a 3 season porch next to the existing foundation, using a 12" wide trench foundation.

The bottom of the trench will be well above the footing depth for the basement.

Two questions:

1 - can the elevations of adjacent footings be different - without a step footing?

2- is pinning of adjacent footings required? If so, what code section, as I cannot find it!

Thanks for any input.

 

[pwood]

1. yes, must be below frost line and into undisturbed soil at a minimum.

2. pinning is not a code requirement.

 

[Darren Emery]

Thanks pwood -

My main concern on this one is that portion of the trench footing right next to the basement wall - say 2' and closer. This area was excavated and backfilled a few years ago when the orginal home was built. The majority of the trench will be dug in truely undistrubed soil, but what about that area right next to the foundation... how long does soil need to settle before it can be considered undisturbed?

 

[jim baird]

"...how long does soil need to settle before it can be considered undisturbed?..."

That question was put during the old BB.

My thinking is that the length of that maybe-not-as-undisturbed area would not be consequential.

I think the old one is archived somewhere, maybe worth a search. I remember on of the answers was expressed in thousands of years.

 

[Mark K]

This may be one of those areas that the IRC does not handle well. A new footing adjecent to a retaining wall can put added loads on the existing retaining wall which could potentially lead to failure. The deeper and farther away the new footing is the less your problem is.

 

[Yankee]

I would ask them to match the existing level and step it up

 

[jar546]

Match existing footer along foundation then step up only on undisturbed soil

 

[jim baird]

This may be one of those areas that the IRC does not handle well. A new footing adjecent to a retaining wall can put added loads on the existing retaining wall which could potentially lead to failure. The deeper and farther away the new footing is the less your problem is.

Good point, Mark. In my thinking I saw those footings as being contiguous somehow.

 

[akabob]

Unless the fill placed next to the basement foundation wall was engineered/compacted fill, in our jurisdiction it would never be permitted to support a new foundation... at least not without engineering. Also the surcharge placed on the adjacent existing foundation by the new foundation could be an issue depending on the existing wall construction and where the new footing landed next to it. An engineer should be involved there too.

We have a number of building pads and grading projects in our jurisdiction that were done in the "old days" without permits, inspections... and in most cases, without adequate compaction.

We've required some fills to be investigated even when we could verify they were there for 25 years or more, and they typically did not come up to the minimum compaction requirements. When this happens, builders are given the choice of placing the footings below the fill and into undisturbed soil or removing the fill and replacing it with engineered compacted fill that was tested by a licensed soils engineer as the fill lifts were brought up.

This policy of our as created a few unhappy contractors and property owners who bought a lot with what they thought was an acceptable building pad. Then we come along and make them test the fill and more times than not, the fill turns out to be unacceptable.

Unfortunately, before we went in this direction, we had several old building pads fail... one with a house that had to be demoed because of it.

We've found that some of the pads were created when the owner/contractor needed a place to bury stumps and trash when he first cleared the lot. Easier to bury that stuff than haul it off... and you get a building pad out of it. What a deal.

 

[peach]

good point aka... a 3 season porch is usually not heated, so one can argue so what... it's not much different than it was before the addition.

If it shares a roof with the existing house, it should be tied somehow... and that somehow usually means at the footing..not tied into the basement wall.

 

[RJJ]

How long does soil need to settle?

Depending on soil type 7 to 10 years if no mechanical compaction was done with prior back fill.

New footing pinned / added pressure? Both depend on the existing wall construction, depth of wall, type of wall, and unequal back fill in place against the wall. Also to be considered is the condition of the wall. Step cracks, bed cracks, spider cracks etc.

Easy fix: Go down to original footing depth. Any other arrangement needs design to be approved.

 

[peach]

compacting soil against an existing foundation has issues of it's own. However you do the roof is going to determine an awful lot.. particularly if wind, snow or seismic are issues in your area. A totally free standing structure is often times the best option. See it all the time in southern climates with swimming pool screen enclosures.