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Undermining Existing Foundation With Perched Water

bjaminu (Structural)
5 Apr 12 15:37
Hi,

I am building a small elevator addition onto an existing two story building with basement.  The existing foundations are bearing on a stratum of "running sand" that is 14" thick.  The water level is 6" below the bottom of footing.  Please see attached cross-section.  The sand is underlaid by stiff impervious clay.  The elevator pit of the addition is marginally within the bearing zone of the existing footing.  

The original design called for sheeting, strictly for OSHA purposes and helical anchor P=20k to support the existing footing.  We would have had to cut the exterior part of the existing footing and auger down with the anchor and then weld a shelf angle.  

Due to the stiff clay, we believe that we would get refusal before we got to the needed depth.  We tried to mass excavate to existing footing and then underpin, but when we discovered the running sand under the footing, we nixed that.

What are your thoughts of installing a cofferdam around the elevator pit area, to keep the perched water intact and then also using the sheeting to mitigate us undermining the existing footing?

Are there better solutions out there?  With regards to the elevator pit in a stratum of impervious clay with perched water above, would it be best to move the proposed drain tile to behind the cofferdam to only address any water that gets thru after completion and avoid dewatering and settling the existing area?  

Thanks for any help you could give!

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