Anytime one has to remove old foundations from below grade, hopefully the work was anticipated and accounted for as it is a costly and resource intense construction activity. Typically a minimum of 2 excavators, a hydraulic hammer attachment, and dump trucks are required. One will also typically need at least 2-3 operators and a laborer. One operator for the excavator with the hammer, one operator for the for the excavator that removes the material from the trench or hole, and an optional operator if a rubber loader is desired to move large amounts of material longer distances more efficiently. The laborer provides visual assistance to the operators.
As foundations are typically massive structures, they will need to be broken first then removed and loaded onto trucks for site removal. If the foundations were reinforced with steel this will only add time to the operation. The material is broken, hammered, and chipped by the excavator with the hydraulic hammer or breaker attachment and removed from the trench by the other excavator or backhoe.
As many foundations requiring removal are unforeseen, significant time and money can be added to a project and its schedule. Sometimes foundations encountered during construction can be left in place if they do not conflict with other construction items or lie far enough underneath future pavements or structures.