Design Considerations for Underpinning of Structures Affected by Karst Using Hydraulically Driven Piles

Structural underpinning with hydraulically-driven steel pipe piles (underpins) is often employed for foundation remediation in order to address displacement related distress. This paper discusses thee (3) different underpinning approaches based on the subsurface mechanism(s) encountered at the subject site. First, a restorative underpinning approach is utilized to address the portion of the structure which has been structurally compromised by displacement. This approach is used to lift and re-level a structure that has experienced displacement related distress. This approach can be applied to a portion of or an entire structure. Second, a stabilitive underpinning approach is utilized to maintain a structure in its current state while preventing additional movement from occurring. This approach is not used to lift and relevel mainly to prevent collateral distress and damage. Lastly, a preventative underpinning approach is used to eliminate the potential for future movement to a structure from subsurface mechanism(s) by isolating the structure from the problematic soils which cannot be feasibly remediated with standard soil remediation techniques. This approach may take one of two forms. A structural preventative approach addresses only those load-bearing components of the structure while a comprehensive preventative approach addresses all portions of the structure susceptible to displacement. It is important to note that while underpinning is intended to address the structure and remediate displacement related distress, it is not intended to address or remediate the subsurface conditions responsible for the displacement of the structure.