Under pinning is a method that has been used for many years.  It is used to support a sagging or cracked footing and walls.  It can be used in most situations depending on the soil underneath each foundation.  Helical piers are used when the soil under the footing is unstable.  In most cases a structural engineer is brought in to laser the house and perform a soil bearing test.  The engineer determines how many piers are needed and also puts his seal on the blueprint at no cost to you.