A Monstrous Teeter-Totter
We recently completed a project that required some major shoring (temporary support) of some existing caissons, or drilled concrete pier foundations. Caissons are typically drilled vertically down through soil and socketed/embedded into bedrock. Caissons are very rigid elements that will support a lot of load, so generally caissons are arranged on site in such a way that they support several hundred thousand pounds.
Caissons are around for the long-haul of a project, but when required to excavate and actually undermine an existing caisson, some real engineering acrobatics have to take place to safely transfer the existing caisson load to new structural elements.
The method we used consisted of placing the vertical downward load onto the end of a pair of steel beams--180,000 lbs in this instance. Special attention was placed on the safe transfer of vertical load into the steel beams (for if the connection to steel cannot transmit 180,000 lbs, then the shoring is useless). A new foundation was placed under this new beam as close as possible to the existing caisson, and a second foundation placed as far away from the caisson as possible, thus increasing leverage and decreasing the uplift force at the end of the beam. The end of the beam furthest from the caisson used micropiles socketed some 15 feet into bedrock to tie the end of the beam down, resisting uplift.
For safe measure, and to protect the steel over time, the entire assembly was encased in concrete.
The end result was a sort of teeter-totter--but a static, stationary one!
Real life is seldom as neat as what is drawn on paper. Here's what the shoring looked like in the real world: