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Underpinning

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As we have been discussing, there a lot of reasons for the installation of
underpinning.  Increased building loads on eixisting footings, soils
bearing failures and movement of soils due to slope failure or creep.

We also agree, with Bruce Resnick, that the underpinning is usually the
best option for several reasons, age of building, condition of founding
soils and quality assurance.  We have installated new continuous grade beam
beneath the existing footing by using the A, B, C slot method, some pads
using the A and B slot method.  Installation of the new grade beam or pad
has not been a problem, contact with existing, force transfer, etc.  

But this method really does not help when the problem is with the founding
soils, back compaction in the fill, expansive, etc.  We then use a steel
"mini" pile system and doe not assume it will transfer any lateral loads
from either earthquake or soil movements.  For soil movements we install
laterally loaded drilled and cast in place reinforced concrete piles.  We
have installated the steel system or both the steel and concrete throughout
So. Cal., 10 in the CIty of LA last year and the first half of this.

Sasha Itsekson is also correct, the installation of any underpinning could
effect the soil-structure interaction and should be carefully examined. 
You alway have to examine the existing capacity of the existing footings
whether reinforced or not.  If the footing is assumed to be unreinforced,
the Code provides some guidance.

	Bill Warren, SE
	Newport Beach, CA