Underpinning is typically required when an existing structure is experiencing or may potentially experience excessive settlement. Excessive foundation settlement occurs for many of the same reasons, despite the size or type of structure. A structure might be underlain by weak or compressible soils that compress excessively under the building’s weight. Competent soils may be weakened as they ravel into compromised underground utilities underlying a foundation. In addition, a foundation might be underlain by debris, which decomposes or consolidates over time. In karstic regions, sinkholes can cause a building to settle as the soils ravel into a void in the underlying bedrock. In other regions, buildings shift when soils expand and contract with changing moisture content.