The most common repair to a porch foundation corrects subsidence of the underlying ground. Foundation support devices such as slab piers and helical slab piers shore up a slumping foundation by transferring the weight of the porch slab off the ineffective soil layer. This either compacts the soil level or reaches bedrock, providing support for the foundation. Additionally, you can repair a porch foundation with pressure grouting, which injects concrete-based grout into the soil voids beneath a foundation until level.
When the material under a foundation sinks, it weakens and eventually cracks the concrete if the foundation loses enough support. Other structural features, such as posts and the roof, also slump and may shear away from the house. If not repaired in a timely fashion, a cracked slab may erode to the point where it must be replaced, a more costly proposition than simple repair.
The subsidence of underlying foundation material depends largely on the specific terrain where the structure is located. Sandy, marshy and clay soils are all prone to slumping under weight, requiring additional load-bearing engineering to counteract the natural tendency to sink. Replacing such soil with rock fill is one method of shoring up a foundation prior to construction.