Underpinning is where we excavate under the existing foundation at predetermined points one at a time ,a new foundation is shuttered and poured under the existing foundation forming a pin this is repeated until the whole foundation is underpinned to the new level, by doing one pin at a time the weight of the structure is slowly transferred to the new foundation, this method can be used where a foundation is cracking, sinking ,or where the ground need to be excavated below the existing foundation level, to gain head height in basements,