Stress Cutoff Solution

[EngMan40]

30 ft deep excavation is proposed for mat foundation of 50-story highrise right next to 100 years old 4-story stone building with 18" wide stone foundation, 7 ft below grade. Rock is 20 ft below the excavation (50 ft below ground). To minimize settlement of the stone building, geotechnical engineer requires rigid excavation support system that extends to rock that provide stress cutoff and will not cause lateral or vertical settlement of the stone building foundations. I would think underpinning all the way to rock and provide temporary tiebacks with required dewatering and shoring which may be difficult given the high permeability of the material below water table. Secant shaft wall with tiebacks which may serve as wall for the highrise. would the wall be able to function properly and handle the high rise type vertical load and provide lateral support for the old building footings. The mat of the highrise is 15 ft thick and will be poured about 6-12" off the property line.

PS. caissons for the highrise was out of the window.

 

[bowlingdanish]

Can you not just use pile a contig/secant wall and use it as a permanent foundation for the building? Two birds one stone. If it can key in to the rock it should achieve very high capacities I would have thought? Then specify a temporary shoring system during excavation to limit the piled wall movements.

PEinc - this is definitely a dumb question, but how do they actually get the piers directly underneath the existing footing? I understand in conventional underpinning for small depths the concrete is installed in a hit and miss manner so that only small parts are undermined at a time. But those piers look very long (and sqaure?). How does the excavation for the pier remain stable, what rig is used to excavate and how does it get under the footing? Also what is the point in post tensioning, would conventional RC drypacked not do the same thing? Similarly, if micropiles were used to underpin, how can they be installed under the footing?

Sorry for the long list of silly questions, but interesting stuff!

 

[PEinc]

EngMann, how much mat settlement do you expect? What is the bearing pressure under the mat? You are removing 30' of soil which could be a load reduction of about -30' x 120 pcf = -3600 psf.

BowlingDanish, checkout my web site for information on underpinning. Underpinning piers are hand excavated as vertical, rectangular shafts under the existing foundations. Check out my web site presentation on Causes of Building Collapses in Philadelphia. The segmental post-tensioned beam is installed in short sections so that you do not undermine too much of the existing wall footing at one time. Then, the short segments need to be post-tensioned into a continuous lintel beam that will bear on underpinning piers. Micropiles are not installed under the existing footings. This would required too large a hole beneath the footing to fit the drill rig. Micropiles are usually installed through the existing footing or foundation wall or adjacent to the foundation and then are attached. Search for info on "jacked pile underpinning." While not the same as micropiles, this will give you an idea of the trouble you will need to go through to get piles under an existing footing.

http://www.PeirceEngineering.com

 

[EngMan40]

PEinc - The mat is estimated to settle about 2-3 in. the preliminary loads are in the range of 15 ksf below the mat.
I don't think it would be 3,600 psf because you remove 30 ft of soil but pour 15 ft concrete mat so most likely to be 1.8 ksf and ok say 13.2 ksf! I am sure final loads will get higher. thinking about the micropile idea you have discussed above, it seems reasonable to extend the continuous underpinning down to some level and pick up the load with micropile but oh boy you need to make sure the micropiles don't pick up any load from the mat or they will snap like a toothpick.

 

[PEinc]

Maybe the new building needs 20 feet of ground modification below subgrade to reduce the 2 - 3 inches of predicted settlement? Or, more typically, a building that heavy would be on caissons (drilled shafts) instead of a very thick mat. Replacing the mat with caissons could mean less excavation, less dewatering, less concrete, less underpinning, no micropiles. I don't think I ever saw a new building with a mat that thick, except for a building's deeper core area that was not close to adjacent structures.

http://www.PeirceEngineering.com