Post a link to the original story.

They would have had to install piles through the slab if the piles were to extend beneath the entire footprint.

Or maybe they supported the perimeter on piles and didnt need to go beneath the slab. Installed them from the exterior.

There are many buildings which have had piles installed after the building was built. I have been involved in one such project a few months back.

You are changing a building which was meant to be ground bearing to now being supported on piles. The previous slab was not designed to perform in this way and the main issue is connecting the piles back to the slab. Often you need to install an additional reinforced slab above the old slab.

In the instance above the piles appear to be acting more like rigid inclusions than actual piles that transfer load to deeper stratum. This is because there is no connection between pile A and B.

Thank you very much for your very valuable response based on experience. The story is in ASIRI (2012) "Recommendations for the design, construction and control of rigid inclusion ground improvements" Page 27 to 28.

Installing piles and underpinning existing buildings was pretty common in the early 20th century. Buildings may have been constructed on soft and compressible soil, the loading of the building may have increased (adding floor levels or changing the use of the building), or shallow tunnels for subways were installed which would require the building to be either underpinned with hand excavated piers or driven and hand excavated piles. There is an incredible book on it called "Underpinning: Its Practice and Applications" by Prentis and White that talks about this (credit goes to PEinc for recommending this book in a thread from years ago). It's out of print but if you look hard enough you can find the original from 1917 and the updated 1950 PDF copies on the internet. Interestingly, the authors were responsible for underpinning the White House after it was deemed unsafe in the 40's due to differential settlement issues.

Fortunately, if you had to install piles inside an existing building today there's a chance you can use low clearance drill rigs to install them instead of digging by hand.

Thank you for the very Informative response MTNClimber!

How does the load get transferred from the old piles to the new piles if they are offset and there is no connection between them?

Two ways come to mind: install the new piles to a firm layer and wait for the building to "naturally" transfer the load via settlement or preload the new piles using hydraulic jacks.

Motor city/MTN - these are not actually piles, as in the primary aim is to transfer the load to a deeper stratum as piles do. The are acting as rigid inclusions(similar to stone columns (kinda but they dont bulge).

There of course is some load transfer but the design intent is also to benefit from the increased composite stiffness of the ground as a whole. This is something that piles dont do.

There is also no structural connection to the slab. If this was a new building placed on RI, there would be a load transfer platform (gravel raft essentially) to spread the load "evenly" onto the RI.

Sorry for the confusion, I haven’t had time to dig through the the internet for the story. It’s odd to call them piles if it’s ground improvement elements.

They are extremely similar to Piles in terms of What they are but in terms of behavior they are quite different.

@MotorCity The Piles, Both A and B, are within the Zone of Influence of the Structure, lets say 2B for a width of 10m, so being present within 20m (within the layers), they will naturally resist oncoming loads (of any kind vertical, lateral, etc. it's nature. Also, the denser medium always contributes more (concentration) than the less denser medium, ie. Concrete>Sand. when Concrete fails, more load goes to the sand, etc.

@MTNClimber The story is centered around people creating Rigid Inclusions without realizing they created Rigid Inclusions. In terms of element, they are exactly like piles but much smaller and they are NOT connected to the Base Plate of the structure. They have a layer called the "Load Transfer Platform" between them and the structure. It serves 2 purpose, 1) transfer the load between the soil and the RI's (unlike Piles which traditionally are meant to carry all the load) and 2) Absorb the Arching Effect from the Resistance of Rigid Inclusions. Rigid Inclusions when resisting load have an effect on the soil above them, i.e. imagine arches being formed in the soil above. If the RI are more efficient? The Arches will be more pronounced or larger. If the LTP is too thin, you will have bending in the slab and heave like effect on the surface. Since there distribution of load between the soil and the RI, this is a Soil Improvement Technique and also quite Cost Effective even proving to be cheaper than Stone Columns most of the time.

My confusion was in the story that, the structure was already built with piles extending to -10m let's say. Then they created New Piles from -10m till -20m while the structure was still Standing! In between the old piles and that in a way was fascinating.